Ege 421-Lecture Notes

MOUNT KENYA UNIVERSITY DEPARTMENT OF EDUCATION

Geography of Natural Hazards

EGE 421

Course instructor : Mr. Koech H.K

8/23/2011

Natural hazard is a threat of a naturally occurring event that will have a negative effect on

humans-a natural disaster. A natural hazard is simply an event but when the threat they pose to

human population actually happens and harms humans, the event qualify to be a natural disaster.

Prepared by Koech H.K.

TOPIC ONE

Natural Hazards and Natural Disasters

A natural hazard is a threat of a naturally occurring event that will have a negative effect on

humans. This negative effect is what we call a natural disaster. In other words when the

hazardous threat actually happens and harms humans, we call the event a natural disaster.

Natural Hazards (and the resulting disasters) are the result of naturally occurring processes that

have operated throughout Earth's history.

Most hazardous processes are also Geologic Processes.

Geologic processes affect every human on the Earth all of the time, but are most

noticeable when they cause loss of life or property. If the process that poses the hazard

occurs and destroys human life or property, then a natural disaster has occurred. Among

the natural hazards and possible disasters to be considered are:

o Earthquakes

o Volcanic Eruptions

o Tsunami

o Landslides

o Subsidence

o Floods

o Droughts

o Hurricanes

o Tornadoes

o Asteroid Impacts

All of these processes have been operating throughout Earth history, but the processes

have become hazardous only because they negatively affect us as human beings.

Important Point - There would be no natural disasters if it were not for humans.

Without humans these are only natural events.

Risk is characteristic of the relationship between humans and geologic processes. We

all take risks everyday. The risk from natural hazards, while it cannot be eliminated, can,

in some cases be understood in such a way that we can minimize the hazard to humans,

and thus minimize the risk. To do this, we need to understand something about the

processes that operate, and understand the energy required for the process. Then, we can

develop an action to take to minimize the risk. Such minimization of risk is called

hazard mitigation.

Although humans can sometimes influence natural disasters (for example when poor

levee design results in a flood), other disasters that are directly generated by humans,

such as oil and toxic material spills, pollution, massive automobile or train wrecks,

airplane crashes, and human induced explosions, are considered technological disasters,

and will not be considered in this course, except when they occur as a secondary result of


a natural disaster.

Some of the questions we hope to answer for each possible natural disaster are:

o Where is each type of hazard likely to be present and why?

o What scientific principles govern the processes responsible for the disasters?

o How often do these hazards develop into disasters?

o How can each type of disaster be predicted and/or mitigated?

As discussed before, natural disasters are produced by processes that have been operating since

the Earth formed. Such processes are beneficial to us as humans because they are responsible

for things that make the Earth a habitable planet for life. For example:

Throughout Earth history, volcanism has been responsible for producing much of the

water present on the Earth's surface, and for producing the atmosphere.

Earthquakes are one of the processes responsible for the formation of mountain ranges

which direct water to flow downhill to form rivers and lakes.

Erosional processes’, including flooding, landslides, and windstorms replenishes soil

and helps sustain life.

Such processes are only considered hazardous when they adversely affect humans and their

activities (property).

Classification of Natural Hazards and Disasters

Natural Hazards and the natural disasters that result can be divided into several different

categories:

Geologic Hazards - These are the main subject of this course and include:

o Earthquakes

o Volcanic Eruptions

o Tsunami

o Landslides

o Floods

o Subsidence

o Impacts with space objects

Atmospheric Hazards - These are also natural hazards but processes operating in the

atmosphere are mainly responsible. They will also be considered in this course, and

include:

o Tropical Cyclones

o Tornadoes


o Droughts

o Severe Thunderstorms

o Lightening

Other Natural Hazards - These are hazards that may occur naturally, but don't fall in to

either of the categories above. They will not be considered to any great extent in this

course, but include:

o Insect infestations

o Disease epidemics

o Wildfires

Natural Hazards can also be divided into catastrophic/hazards, which have devastating

consequences to huge numbers of people, or have a worldwide effect, such as impacts with

large space objects, huge volcanic eruptions, world-wide disease epidemics, and world-wide

droughts. Such catastrophic hazards only have a small chance of occurring, but can have

devastating results if they do occur.

Natural Hazards can also be divided into rapid onset hazards (cataclysmic), such as Volcanic

Eruptions, Earthquakes, Flash floods, Landslides, Severe Thunderstorms, Lightening, and

wildfires, which develop with little warning and strike rapidly. Slow onset hazards

(continuing), like drought, insect infestations, and disease epidemics take years to develop

Anthropogenic Hazards

These are hazards that occur as a result of human interaction with the environment. They

include Technological Hazards, which occur due to exposure to hazardous substances, such as

radon, mercury, asbestos fibers, and coal dust. They also include other hazards that have

formed only through human interaction, such as acid rain, and contamination of the atmosphere

or surface waters with harmful substances, as well as the potential for human destruction of the

ozone layer and potential global warming

Effects of Hazards

Hazardous process of all types can have primary, secondary, and tertiary effects.

Primary Effects occur as a result of the process itself. For example water damage during a

flood or collapse of buildings during an earthquake, landslide, or hurricane.

Secondary Effects occur only because a primary effect has caused them. For exmple, fires

ignited as a result of earthquakes, disruption of electrical power and water service as a result of

an earthquake, flood, or hurricane or flooding caused by a landslide into a lake or river

Tertiary Effects are long-term effects that are set off as a result of a primary event. These

include things like loss of habitat caused by a flood, permanent changes in the position of river

channel caused by flood, crop failure caused by a volcanic eruption etc.


Vulnerability to Hazards and Disasters

Vulnerability refers the way a hazard or disaster will affect human life and property

Vulnerability to a given hazard depends on:

Proximity to a possible hazardous event

Population density in the area proximal to the event

Scientific understanding of the hazard

Public education and awareness of the hazard

Existence or non-existence of early-warning systems and lines of communication

Availability and readiness of emergency infrastructure

Construction styles and building codes

Cultural factors that influence public response to warnings

In general, less developed countries are more vulnerable to natural hazards than are

industrialized countries because of lack of understanding, education, infrastructure,

building codes, etc. Poverty also plays a role - since poverty leads to poor building structure,

increased population density, and lack of communication and infrastructure.

Human intervention in natural processes can also increase vulnerability by:

Development and habitation of lands susceptible to hazards, For example, building on

floodplains subject to floods, sea cliffs subject to landslides, coastlines subject to

hurricanes and floods, or volcanic slopes subject to volcanic eruptions.

Increasing the severity or frequency of a natural disaster. For example: overgrazing or

deforestation leading to more severe erosion (floods, landslides), mining groundwater

leading to subsidence, construction of roads on unstable slopes leading to landslides, or

even contributing to global warming, leading to more severe storms

Affluence can also play a role, since affluence often controls where habitation takes place, for

example along coastlines, or on volcanic slopes. Affluence also likely contributes to

global warming, since it is the affluent societies that burn the most fossil fuels adding CO2 to

the atmosphere.

Assessing Hazards and Risk

Hazard Assessment and Risk Assessment are two different concepts!

Hazard Assessment consists of determining the following

When and where hazardous processes have occurred in the past;

The severity of the physical effects of past hazardous processes (magnitude);

The frequency of occurrence of hazardous processes;


The likely effects of a process of a given magnitude if it were to occur now; and

Making all this information available in a form useful to planners and public officials

responsible for making decisions in event of a disaster

Risk Assessment involves not only the assessment of hazards from a scientific point of view,

but also the socio-economic impacts of a hazardous event. Risk is a statement of probability

that an event will cause x amount of damage, or a statement of the economic impact in

monetary terms that an event will cause. Risk assessment involves

hazard assessment, as above;

location of buildings, highways, and other infrastructure in the areas subject to hazards ;

potential exposure to the physical effects of a hazardous situation; and

The vulnerability of the community when subjected to the physical effects of the event.

Risk assessment aids decision makers and scientists to compare and evaluate potential hazards,

set priorities on what kinds of mitigation are possible, and set priorities on where to focus

resources and further study.

Prediction and Warning

Risk and vulnerability can sometimes be reduced if there is an adequate means of predicting a

hazardous event.

Prediction

Prediction involves:

A statement of probability that an event will occur based on scientific observation

Such observation usually involves monitoring of the process in order to identify some

kind of precursor event(s) - an anomalous small physical change that may be known to

lead to a more devastating event. - Examples:

o Hurricanes are known to pass through several stages of development: tropical

depression - tropical storm - hurricane. Once a tropical depression is identified,

monitoring allows meteorologists to predict how long the development will take

and the eventual path of the storm.

Volcanic eruptions are usually preceded by a sudden increase in the number of earthquakes

immediately below the volcano and changes in the chemical composition of the gases emitted

from a volcanic vent. If these are closely monitored, volcanic eruptions can be often be

predicted with reasonable accuracy

Frequency of Natural Disasters

Again, it is important to understand that natural disasters result from natural processes that

affect humans adversely.

First - Size Matters, For example:


Humans coexist with rivers all the time and benefit from them as a source of water and

transportation. Only when the volume of water in the river becomes greater than the

capacity of the stream channel is there a resulting disaster.

Small earthquakes occur all of the time with no adverse effects. Only large earthquakes

cause disasters.

Second – Location, location, location

For example:

A volcanic on an isolated uninhabited island will not result in a natural disaster.

A large earthquake in an unpopulated area will not result in a disaster.

A hurricane that makes landfall on a coast where few people live will not result in a

disaster.

So, of concern is the extent of the events that strike areas where humans live.

Thus, in natural hazards studies, it is important to understand the relationship between

frequency of an event and the size of the event. Size is often referred to a magnitude.

For just about any event, statistical analysis will reveal that larger events occur less frequently

than small events.

Statistical analysis of some types of events for specific locations allows one to determine the

return period or recurrence interval.

Examples:

Flood Frequency - For any river, high discharge events are rare. Large discharge events occur

much less frequently than small discharge events

Meteorite Impacts -

Although we as humans have not had the opportunity (fortunately) of observing large asteroid

or meteorite impacts, the data suggest that impacts of large asteroids (1 km or larger) occurs

only once every 10 million years.

Earthquakes –

Large earthquakes occur much less frequently than smaller earthquakes. Those with

magnitudes greater than 8.5 only occur once every 3 years on the

Is the Frequency of Natural Disasters Increasing?

Are natural disasters becoming more frequent as it seems from news reports of recent activity?

The short answer appears to be that yes, natural disasters are increasing in frequency. But, this

suggests some other important questions before we start making conclusions about the end of

the world:

1. Is the frequency of hazardous events increasing?


2. Why is the frequency of natural disasters increasing (what could explain the trend)?

First, Is the frequency of hazardous events increasing? This is much more difficult to answer

since natural events responsible for natural disasters have been occurring throughout the 4.5

billion year history of the Earth. Nevertheless, there is no evidence to suggest that hazardous

events are occurring more frequently.

What about global warming? There is evidence to suggest that weather related disasters are

becoming more frequent, compared to other disasters like earthquakes. For example, the

frequency of disasters from tropical cyclones and floods has been increasing; the frequency of

earthquakes has changed little. Although this is what we expect from global warming, there is

not yet enough statistical data to prove this right now.

Second, is there another explanation for the the frequency of natural disasters increasing? First

Consider the following facts:

Human population has been increasing at an exponential rate. With more people, vulnerability

increases because there are more people to be affected by otherwise natural events.

Human population is moving toward coastal areas. These are areas most vulnerable to natural

hazards such as tropical cyclones, tsunami, and, to some extent, earthquakes.

Our ability to communicate news of natural disasters has been increasing, especially since the

invention of the internet. Earlier in human history there may have been just as many disasters,

but there were few ways the news of such disasters could be communicated throughout the

world.

Meanwhile:

Deaths from natural disasters has decreased in developed countries and increased in developing

countries. What could explain this? Politics? Economics? Cultural Differences? Education?

The cost of natural disasters has been increasing in developed countries. What could explain

this? Economics?

This course is not about the political, cultural, or economic aspects of natural disasters. But

is about the science of natural hazards and disasters and how we can use our knowledge of

the scientific aspects of disasters to reduce the death and destruction caused by otherwise

natural events


TOPIC TWO

Earth Structure, Materials, Systems, and Cycles

The Earth in the Solar System

The Solar System

The Earth is one of nine planets (eight if Pluto is not considered a planet) in the solar

system

In addition to the planets, many smaller bodies called asteroids, comets, meteoroids are

present.

All objects in the solar system orbit around the Sun.

The four planets closest to the Sun (Mercury, Venus, Earth, and Mars) have high

densities because they are mostly composed of rock, and are called the Terrestrial

Planets.

The five planets outside the orbit of Mars (Jupiter, Saturn, Uranus, Neptune, and Pluto)

have low densities because they mostly composed of gases, and are called the Jovian

Planets.

Origin of the Solar System

Original Solar Nebula

Condensation of the Sun about 6 billion years ago

Condensation of the Planets about 4.5 billion years ago

Process is continuing today, although at a much slower rate.

The Planet Earth

Interior Structure of Earth

The Earth has a radius of about 6371 km, although it is about 22 km larger at equator than at

poles.

Density, (mass/volume), Temperature, and Pressure increase with depth in the Earth.

The Earth has a layered structure. This layering can be viewed in two different ways

i. Layers of different chemical composition and

ii. Layers of differing physical properties.


The compositional layering constitutes

Crust - variable thickness and composition

-Continental 10 - 70 km thick

-Oceanic 8 - 10 km thick

Mantle - 3488 km thick, made up of a rock called peridotite.

Core - 2883 km radius, made up of Iron (Fe) with some Nickel (Ni)

Layers of Differing Physical Properties

Lithosphere - about 100 km thick (up to 200 km thick beneath continents), very brittle,

easily fractures at low temperature.

Asthenosphere - about 250 km thick - solid rock, but soft and flows easily (ductile).

Mesosphere - about 2500 km thick, solid rock, but still capable of flowing.

Outer Core - 2250 km thick, Fe and Ni, liquid

Inner core - 1230 km radius, Fe and Ni, solid

All of the above is known from the way seismic waves (earthquake waves) pass through the

Earth.

Before we can begin to understand the causes and effects of natural hazards and disasters we

need to have some understanding of the materials that make up the Earth, the processes that act

on these materials, and the energy that controls the processes.

We start with the basic building blocks of rocks – i.e. the Minerals.

Minerals

The Earth is composed of rocks. Rocks are aggregates of minerals. Minerals are composed of

atoms. In order to understand rocks, we must first have an understanding of minerals.

Definition

A Mineral is a naturally formed (it forms in nature on its form) solid (it cannot be a liquid or a

gas), with a definite chemical composition (every time we see the same mineral it has the same


chemical composition that can be expressed by a chemical formula) and a characteristic

crystalline structure (atoms are arranged within the mineral in a specific ordered manner).

Examples

Glass - can be naturally formed (volcanic glass called obsidian), is a solid, its chemical

composition, however, is not always the same, and it does not have a crystalline structure

(individual atoms in a glass are arranged randomly similar to the arrangement in a liquid).

Thus, glass is not a mineral.

Ice - Is naturally formed, is solid, does have a definite chemical composition that can be

expressed by the formula H2O, and does have a definite crystalline structure when solid.

Thus, ice is a mineral. Liquid water is not since it is not solid and does not have a

crystalline structure.

Halite (salt) - Is naturally formed, is solid, does have a definite chemical composition that

can be expressed by the formula NaCl, and does have a definite crystalline structure, as

shown below. Thus halite is a mineral.

Important Minerals in the Earth's Crust

The varieties of minerals we see depend on the chemical elements available to form them. In the

Earth's crust the most abundant elements are as follows:

O, Oxygen 45.2% by weight

Si, Silicon 27.2%

Al, Aluminum 8.0%

Fe, Iron 5.8%

Ca, Calcium 5.1%

Mg, Magnesium 2.8%

Na, Sodium 2.3%

K, Potassium 1.7%

Ti ,Titanium 0.9%

H, Hydrogen 0.14%

Mn, Manganese 0.1%

P, Phosphorous 0.1%

Note that Carbon (one of the most abundant elements in life) is not among the top 12

Because of the limited number of elements present in the Earth's crust there are only about 3000

minerals known. Only 20 to 30 of these minerals are common. The most common minerals are

those based on Si and O: the Silicates. Silicates are based on SiO4 tetrahedron. 4 Oxygens

bonded to one silicon atom.


Formation of Minerals

Minerals are formed in nature by a variety of processes. Among them are:

Crystallization from melt - the process that results in igneous rocks.

Precipitation of water - the process that results in chemical sedimentary rocks.

Precipitation from living organisms - the process that results in biochemical sedimentary

rocks

Change to more stable state - the process that results in the formation of soil, through

weathering, and the formation of metamorphic rocks.

Precipitation from vapor. (Not common, but sometimes does occur around volcanic

vents)

Since each process leads to different minerals, we can identify the process by which minerals

form in nature. Each process has specific temperature and pressure conditions that can be

determined from laboratory experiments.

Important Minerals

For the purposes of this course, there are three minerals that are most important (others may be

introduced as needed) are:

Quartz - Chemical Formula SiO2. - Quartz is one of the primary minerals that originally

form by crystallization from a melt in igneous rocks. Although quartz is formed at

relatively high temperatures it is stable (does not break down or alter) at conditions

present near the Earth's surface. Thus quartz is a primary constituent of sand, soil, and

sedimentary rocks called sandstones.

Clay Minerals - Clay minerals are sheet silicates, thus they have a crystalline structure

that allows them to break easily along parallel sheets. Clay minerals form by alteration of

other minerals during the process of chemical weathering (alteration under conditions

present near the Earth's surface). Thus clay minerals are primary constituents of soils and

are also found in the sedimentary rock known as shale or mudstone.

Calcite - chemical formula CaCO3 (calcium carbonate). Calcite is easily dissolved in

water under slightly acidic conditions. Thus calcite can be precipitated directly from

water. Organisms can extract the Calcium and Carbonate ions from water to precipitate

their shells. Thus calcite is a primary constituent of chemical and biochemical

sedimentary rocks.


Rocks

Rocks are aggregates of minerals that are held together to form a consolidated mass. The three

general types of rocks are:

1. Igneous Rocks - rocks that result from crystallization from a melt - called magma.

If the crystallization takes place deep beneath the surface of the Earth they are called Plutonic

rocks. Examples include:

Granite - coarse textured rock consisting mostly of quartz, and feldspar with small

amounts of biotite and/or hornblende.

Gabbro - a coarse textured rock consisting mostly of pyroxenes, and plagioclase.

If the crystallization takes place on the surface of the Earth they are called volcanic rocks.

Examples include:

Rhyolite - a fine grained to glassy rock containing crystals of quartz, feldspar, and biotite

- chemically the same as granite.

Andesite - a fined grained rock containing crystals of pyroxene, plagioclase, and

sometimes hornblende.

Basalt - a fine grained rock containing crystals of olivine, pyroxene, and plagioclase -

chemically the same as a gabbro.

2. Sedimentary Rocks - rocks that form near the surface of the Earth through chemical

precipitation from water or by cementation of loose fragments (called sediment).

Clastic Sedimentary Rocks - result from the cementation of loose fragments of pre-

existing rock. The cementation occurs as a result of new minerals precipitating in the

space between grains. Clastic sedimentary rocks are classified on the basis of the size of

the fragments that makes up the rock

Name of particle Size range Loose sediment Consolidated rock

Boulder >256 mm Gravel

Cobble 256 mm Gravel

Bebble 64 mm Gravel

Sand 1/16 -2mm Sand Sandstone

Silt 1/256 - 1/16 mm Silt Siltstone

Clay <1/256 mm Clay Claystone, mudstone& shale

Chemical Sedimentary Rocks - result from direct chemical precipitation from surface

waters. This usually occurs as a result of evaporation which concentrates ions dissolved

in the water and results in the precipitation of minerals.

Biochemical Sedimentary Rocks - result from the chemical precipitation by living

organisms. The most common biochemical sedimentary rock is limestone, which is

composed of the shells of organisms, which are in turn composed mostly of the mineral

calcite.

3. Metamorphic Rocks - result when any kind of pre-existing rock is buried deep in the

Earth and subjected to high temperatures and pressures. Most metamorphic rocks show a

texture that shows an alignment of sheet silicate minerals, minerals like biotite and

muscovite that gives them a layered appearance and allows them to break easily along


nearly planar surfaces. Some common metamorphic rocks that we might encounter in this

course are:

Slate - a fine grained metamorphic rock consisting mostly of clay minerals that breaks

easily along smooth planar surfaces.

Schist - a coarser grained metamorphic rock consisting of quartz and micas that breaks

along irregular wavy surfaces.

Geologic Processes

A variety of processes act on and within the Earth - here we consider those responsible for

Natural Disasters

Melting - responsible for creating magmas that result in volcanism.

Deformation - responsible for earthquakes, volcanism, landslides, subsidence.

Isostatic Adjustment due to buoyancy - responsible for earthquakes, landslides,

subsidence.

Weathering - responsible for landslides, subsidence.

Erosion - responsible for landslides, subsidence, flooding.

Atmospheric Circulation - responsible for hurricanes, tornadoes, flooding.

Energy

All processes that occur on or within the Earth require energy. Energy can exist in many

different forms, and comes from a variety of sources. Natural disasters occur when there is a

sudden release of the energy near the surface of the Earth.

Forms of Energy

Energy may exist in many different forms, but can be converted between each of these forms

Gravitational Energy - Energy released when an object falls from higher elevations to

lower elevations. As the object falls the energy can be converted to kinetic energy

(energy of motion) or heat energy.

Heat Energy - Energy exhibited by moving atoms, the more heat energy an object has,

the higher its temperature. Heat energy can be converted to kinetic energy, as it is when

fuel is burned in an engine and sets the car in motion.

Chemical Energy - Energy released by breaking or forming chemical bonds. This type of

energy usually is converted to heat.

Radiant Energy - Energy carried by electromagnetic waves (light). Most of the Sun's

energy reaches the Earth in this form, and is converted to heat energy.

Nuclear Energy - Energy stored or released in binding of atoms together. Most of the

energy generated within the Earth comes from this source, and most is converted to heat

when it is released.

Elastic Energy (also called strain energy) - By deforming an elastic material (like rubber

bands, wood, and rocks) energy can be stored in the material. When this energy is

released it can be converted to kinetic energy and heat.

Electrical Energy - Energy produced by moving electrons through matter. Most of this

energy is generated by humans and is converted into heat energy to heat homes or water

or is converted to kinetic energy to drive air conditioners, vacuum cleaners, can openers,

etc.


Sources of Energy

The Earth has two basic sources of energy - that reaching the Earth from the Sun (Solar Energy)

and that reaching the surface of the Earth the Earth itself (Internal or Geothermal Energy).

Solar Energy - reaches the Earth in the form of radiant energy, and makes up 99.987% of the

energy received by the Earth.

About 40% is immediately reflected back into space by the atmosphere and oceans.

Some is converted to heat and is absorbed by the atmosphere, hydrosphere, and

lithosphere, but even this eventually escapes into space.

Some is absorbed by plants during photosynthesis and is stored in plants, used by

other organisms, or is stored in fossil fuels like coal and petroleum.

Solar Energy drives the water cycle, causing evaporation of the oceans and

circulation of the atmosphere, which allows rain to fall on the land and run downhill.

Thus solar energy is responsible for such natural disasters as severe weather, and

floods.

Internal Energy - is generated within or because of the Earth. It only amounts to about 0.013%

of the total energy reaching the Earth's surface, but is responsible for deformational events that

build mountains and cause earthquakes, for melting in the Earth to create magmas that result in

volcanism. Two source of internal energy are:

i. Radioactive Decay

Some elements like Uranium, Thorium, and Potassium have unstable isotopes that we say

are radioactive.

When a radioactive isotope decays to a more stable isotope, subatomic particles like

protons, neutrons, and electrons are expelled from the radioactive parent atom and are

slowed and absorbed by surrounding matter.

The energy of motion (kinetic energy) of these particles is converted to heat by the

collision of these particles with the surrounding matter.

Although radioactive isotopes like 235U (Uranium), 232U, 232Th (Thorium), and 40K

(potassium) are not very abundant in the Earth, They are sufficiently plentiful that large

amounts of heat are generated in the Earth.

ii. Conversion of Gravitational Energy

Gravity is the force of attraction between two bodies.

The force of gravity acts between the Sun, Earth, and Moon to create tidal forces, which

cause the Earth to bulge in the direction of the Moon. This bulging is kinetic energy,

which is converted to heat in the Earth.

Gravity has other energy effects near the surface of the Earth. All objects at the Earth's

surface are continually being pulled toward the center of the Earth by the force of

gravity.

When an object moves closer to the center of the Earth by falling, slipping, sliding, or

sinking, kinetic energy is released.

Some of the heat flowing out of the Earth is heat that has been produced by gravitational

compaction of the Earth which has caused matter to move closer to Earth's center.


Heat Transfer

Since much of the energy that reaches the Earth's surface eventually is converted to heat, it is

important to understand how heat can move through materials. Three basic modes of heat

transfer are possible

a. Conduction - atoms vibrate against each other and these vibrations move from high

temperature areas (rapid vibrations) to low temperature areas (slower vibrations). - Heat

from Earth's interior moves through the solid crust by this mode of heat transfer.

b. Convection - Heat moves with the material, thus the material must be able to move. The

mantle of the Earth appears to transfer heat by this method, and heat is transferred in the

atmosphere by this mode (causing atmospheric circulation).

c. Radiation - Heat moves with electromagnetic radiation (light) Heat from the Sun is

transferred by this mode, and thus radiative heat transfer is responsible for warming the

oceans and atmosphere, and for re-radiating heat back into space.

Time scale

Solar System began to form about 6 billion years ago and the Earth and other planets about 4.5

billion years ago and geologic processes have operated on the Earth ever since. Some of these

processes, like mountain building events expend energy on time scales of several hundred

million years, whereas others, like earthquakes, expend energy on time scales of a few seconds

(although the storage of energy for such an event may take hundreds or thousands of years).

Examining the time scales of various geologic and other processes reveals that those processes

that affect humans and that may be responsible for natural disasters occur on time scales less

than a few years.


Plate Tectonics

Much of what occurs near the surface of the Earth is due to interactions of the lithosphere with

the underlying asthenosphere. Most of these interactions are caused by plate tectonics. Plate

Tectonics is a theory developed in the late 1960s, to explain how the outer layers of the Earth

move and deform. The theory has caused a revolution in the way we think about the Earth. Since

the development of the plate tectonics theory, geologists have had to reexamine almost every

aspect of Geology. Plate tectonics has proven to be so useful that it can predict geologic events

and explain almost all aspects of what we see on the Earth. The theory states that the Earth's

lithosphere is divided into plates (about 100 km thick) that move around on top of the

asthenosphere. Continental crust is embedded within the lithospheric plates. The Plates move in

different directions, and meet each other at plate boundaries. The plates and their boundaries are

shown below.

Plate boundaries are important because plates interact at the boundaries and these are zones

where deformation of the Earth's lithosphere is taking place. Thus, plate boundaries are

important areas in understanding geologic hazards. Three types of plate boundaries occur:

A. Divergent Plate Boundaries - These are boundaries where plates move away from each

other, and where new oceanic crust and lithosphere are created. Magmas rising from the

underlying asthenosphere intrude and erupt beneath and at an oceanic ridge to create new

seafloor. This pushes the plates on either side away from each other in opposite directions.

The margin itself becomes uplifted to form oceanic ridges, which are also called spreading

centers, because oceanic lithosphere spreads away on each side of the boundary. While most

diverging plate boundaries occur at the oceanic ridges, sometimes continents are split apart

along zones called rift zones, where new oceanic lithosphere may eventually form.

Volcanism and earthquakes are common along diverging plate boundaries.


Figure 1: Diverging plate boundary

B. Convergent Plate Boundaries - These are boundaries where two plates move toward each

other. At such boundaries one of the plates must sink below the other in a process called

subduction. Two types of convergent boundaries are known.

a) Subduction Boundaries - These occur where either oceanic lithosphere subducts

beneath oceanic lithosphere (ocean-ocean convergence), or where oceanic lithosphere

subducts beneath continental lithosphere (ocean-continent convergence). Where the

two plates meet, an oceanic trench is formed on the seafloor, and this trench marks

the plate boundary. When two plates of oceanic lithosphere run into one another the

subducting plate is pushed to depths where it causes melting to occur. These melts

(magmas) rise to the surface to produce chains of islands known as island arcs. A

good example of an island arc is the Caribbean islands.

Figure 2: Ocean-continent convergence

When an plate made of oceanic lithosphere runs into a plate with continental

lithosphere, the plate with oceanic lithosphere subducts because it has a higher

density than continental lithosphere. Again, the subducted lithosphere is pushed to

depths where magmas are generated, and these magmas rise to the surface to produce,

in this case, a volcanic arc, on the continental margin. Good examples of this type of

volcanic arc are the Cascade Mountains of the northwestern U.S. and the Andes

mountains of South America.

b) Collision Boundaries - When two plates with continental lithosphere collide,

subduction ceases and a mountain range is formed by squeezing together and

uplifting the continental crust on both plates. The Himalayan mountains between


India and China where formed in this way, as were the Appalachian Mountains about

300 million years ago

Figure 3: Continent-continent convergence

Note that all convergent boundaries are zones of frequent and powerful earthquakes

C. Transform Fault Boundaries (shear plate) - When two plates slide past one another, the type of

boundary occurs along a transform fault. These are also zones of frequent and powerful

earthquakes, but generally not zones of volcanism. The famous San Andreas Fault of California is

an example of a transform fault, forming one part of the boundary between the Pacific Plate and

the North American Plate.

Why Does Plate Tectonics Occur?

Plate tectonics is driven by the internal energy of the Earth. Although there is some

debate among geoscientists as to the exact mechanism, most agree that motion of the

plates is ultimately driven by convection currents in the mantle.

Recall that convection is a means of heat transfer wherein the heat moves with the

material. It occurs when conduction is inefficient at transporting heat, particularly if the

material has a low thermal conductivity, like rocks.


Recall also that the Earth's asthenosphere is ductile, and therefore is likely to flow more

readily than the overlying lithosphere.

Thus, if the asthenosphere moves by convection, with rising currents carrying heat

toward the surface at the oceanic ridges, and, descending currents sinking at subduction

zones after loosing heat to the surface, then the brittle plates riding on top of the

convection cell will be forced to move over the surface, being in a sense, dragged along

by the moving asthenosphere.

Geologic Cycles

In reality separate Earth entities i.e. the atmosphere, hydrosphere, lithosphere, etc, interact with

each other continuously exchanging both matter and energy. This exchange of matter and energy

occurs on a cyclical basis, with both matter and energy cycling between various storage

reservoirs on various time scales. Because matter and energy is thus cycled, the various geologic

cycles play a large role in the development of natural disasters. We here look at a few of these

geologic cycles.

Hydrologic Cycle

Hydrologic cycle involves the movement of water throughout Earth systems. Water moves

between 7 main reservoirs:

i. the oceans

ii. the atmosphere where it moves in the clouds transported by winds

iii. glaciers and ice sheets

iv. surface lakes and streams

v. groundwater (water that moves in the pore spaces in rock beneath the surface)

vi. the biosphere, and

vii. the lithosphere, where it is held within the crystallographic structure of hydrous

(water bearing) minerals.

The ocean is by far the largest of these reservoirs with 97% of all water.

The main pathway by which water moves is through the atmosphere. Two main sources of

energy drive the cycle:

a. Solar energy causes evaporation of the surface waters and atmospheric circulation, and

b. Gravitational energy causes the water to flow back to oceans.

Residence time in each of the reservoirs is generally proportional to the size of the reservoir

with water residing in the oceans and glaciers for many thousands of years,

in groundwater for hundreds of years,

in surface waters for months,

In the atmosphere and biosphere for days.

Water may reside in the lithosphere for millions of years.


Biogeochemical Cycles

Although the hydrologic cycle involves the biosphere, only a small amount of the total water in

the system at any given time is in the biosphere. Other materials, for example Carbon and

Nitrogen have a much higher proportion of the total residing in the biosphere at any given time.

Cycles that involve the interactions between other reservoirs and the biosphere are often

considered differently because they involve biological processes like respiration, photosynthesis,

and decomposition (decay). These are referred to as biogeochemical cycles.

For instance the Carbon Cycle involves the cycling of Carbon between 4 major reservoirs:

a. Biosphere, where it is the major building block of life,

b. Lithosphere, where it is a component in carbonate minerals and rocks and fossil

fuels such as coal and petroleum,

c. Oceans, where it occurs as a dissolved ion in seawater, and

d. Atmosphere, where it occurs as Carbon Dioxide (CO2) gas.


In all reservoirs except the lithosphere, residence time is generally short, on the order of a few

years. Human burning of fossil fuels adds Carbon back to the atmosphere at a higher rate than

normal, and thus the concern for greenhouse gas warming induced by humans.

The Rock Cycle

The rock cycle involves cycling of elements between various types of rocks, and thus

mostly involves the lithosphere.

But, because materials such as water and Carbon cycle through the lithosphere, the rock

cycle overlaps with these other cycles.

The rock cycle involves the three types of rocks as reservoirs

a. igneous,

b. sedimentary, and

c. Metamorphic.

Chemical elements can reside in each type of rock, and geologic processes move these

elements into another type of rock.

Energy for the parts of the crustal cycle near the Earth's surface is solar and gravitational

energy (which control erosion and weathering), whereas

Energy that drives processes beneath the surface is geothermal and gravitational energy

(which control uplift, subsidence, melting, and metamorphism).

Uniformitarianism and Catastrophism

Prior to about 1850 most humans thought of the Earth as being a relatively young feature and

that processes and landforms that occur on the Earth were the result of catastrophic events (like

creation and the flood) that occurred very rapidly. But, careful observation of Earth process led

some, like James Hutton and Charles Lyell) to hypothesize that processes that one could observe

taking place at the present time had operated throughout the history of the planet. This led to the

development of the concept of uniformitarianism, often stated as "the present is the key to the


past". A more modern way of stating this principle is that since the laws of nature have operated

the same way throughout time, and all Earth processes must obey the laws of nature (i.e. the laws

of physics and chemistry). Initially one of the most difficult problems in applying this principle

to the Earth, was that an assumption was made that the rates of all geologic processes had been

the same throughout time. We know that the Earth is very old (4.5 billion years) and that it was

hotter near its birth than it is now. Thus, it is likely that the rates of some geologic processes

have changed throughout time. We also now recognize that there can in fact be catastrophic

events that occur infrequently that can cause very rapid changes in the Earth. Because these

catastrophic events occur infrequently, it is difficult to observe their effects, but if we can

recognize them, we still can see that even these infrequent catastrophic events follow the laws of

nature.


TOPIC THREE

Earthquakes: Causes and Measurements

Most earthquakes occur along zones where the Earth's crust is undergoing deformation.

Deformation results from plate tectonic forces and gravitational forces. The type of deformation

that takes place during an earthquake generally occurs along zones where rocks fracture to

produce faults. To better understand earthquakes, it is important we explore the process of

deformation of rocks and faulting.

Within the Earth rocks are constantly subjected to forces that tend to bend, twist, or fracture

them. When rocks bend, twist or fracture they are said to deform or strain (change shape or size).

The forces that cause deformation are referred to as stresses. To understand rock deformation we

must first explore stress and strain.

Stress and Strain

Stress is a force applied over an area. One type of stress that we are all used to is a uniform

stress, called pressure. A uniform stress is where the forces act equally from all directions. In the

Earth the pressure due to the weight of overlying rocks is a uniform stress and is referred to as

confining stress. If stress is not equal from all directions then the stress is a differential stress.

Three kinds of differential stress occur.

a. Tensional stress (or extensional stress), which stretches rock;

b. Compressional stress, which squeezes rock; and

c. Shear stress, which result in slippage and translation


Stages of Deformation

When a rock is subjected to increasing stress it changes its shape, size or volume. Such a change

in shape, size or volume is referred to as strain. When stress is applied to rock, the rock passes

through three successive stages of deformation.

a. Elastic Deformation -- wherein the strain is reversible.

b. Ductile Deformation -- wherein the strain is irreversible.

c. Fracture -- irreversible strain wherein the material breaks

We can divide materials into two classes that depend on their relative behavior under stress.

Brittle materials have a small to large region of elastic behavior, but only a small region

of ductile behavior before they fracture.

Ductile materials have a small region of elastic behavior and a large region of ductile

behavior before they fracture.

How a material behaves will depend on several factors. Among them are:

a). Temperature - At high temperature molecules and their bonds can stretch and move, thus

materials will behave in more ductile manner. At low Temperature, materials are brittle.

b). Confining Pressure - At high confining pressure materials are less likely to fracture

because the pressure of the surroundings tends to hinder the formation of fractures. At

low confining stress, material will be brittle and tend to fracture sooner.


c). Strain rate- Strain rate refers to the rate at which the deformation occurs (strain divided

by time). At high strain rates material tends to fracture. At low strain rates more time is

available for individual atoms to move and therefore ductile behavior is favored.

d). Composition - Some minerals, like quartz, olivine, and feldspars are very brittle. Others,

like clay minerals, micas, and calcite are more ductile. This is due to the chemical bond

types that hold them together. Thus, the mineralogical composition of the rock will be a

factor in determining the deformational behavior of the rock. Another aspect is presence

or absence of water. Water appears to weaken the chemical bonds and forms films around

mineral grains along which slippage can take place. Thus wet rock tends to behave in

ductile manner, while dry rocks tend to behave in brittle manner.

Evidence of Former Deformation

Evidence of deformation that has occurred in the past is very evident in crustal rocks. For

example, sedimentary layers and lava flows generally are deposited on a surface parallel to the

Earth's surface (nearly horizontal). Thus, when we see such layers inclined instead of horizontal,

evidence of an episode of deformation is present. To uniquely define the orientation of a planar

feature there is need to define two terms – strike and dip. For an inclined plane the strike is the

compass direction of any horizontal line on the plane. The dip is the angle between a horizontal

plane and the inclined plane, measured perpendicular to the direction of strike.

In recording strike and dip measurements on a geologic map, a symbol is used that has a long

line oriented parallel to the compass direction of the strike. A short tick mark is placed in the

center of the line on the side to which the inclined plane dips, and the angle of dip is recorded

next to the strike and dip symbol (Figure 4 ). For beds with a 900 dip (vertical) the short line

crosses the strike line and for beds with no dip (horizontal) a circle with a cross inside is used.


Figure 4: Strike and dip

Fracture of Brittle Rocks

Faults - Faults occur when brittle rocks fracture and there is an offset along the fracture. When

the offset is small, the displacement can be easily measured, but sometimes the displacement is

so large that it is difficult to measure.

Types of Faults

Faults can be divided into several different types depending on the direction of relative

displacement. Since faults are planar features, the concept of strike and dip also applies, and thus

the strike and dip of a fault plane can be measured. One division of faults is between dipslip

faults, where the displacement is measured along the dip direction of the fault, and strikeslip

faults where the displacement is horizontal, parallel to the strike of the fault.

Dip Slip Faults - Dip slip faults are faults that have an inclined fault plane and along

which the relative displacement or offset has occurred along the dip direction. Note that

in looking at the displacement on any fault we don't know which side actually moved or

if both sides moved, all we can determine is the relative sense of motion. For any inclined

fault plane we define the block above the fault as the hanging wall block and the block

below the fault as the footwall block.

Normal Faults – are faults that result from horizontal tensional stresses in brittle rocks and

where the hanging wall block has moved down relative to the footwall block.

Horsts & Grabens - Due to the tensional stress responsible for normal faults, they often occur in

a series, with adjacent faults dipping in opposite directions. In such a case the down-dropped

blocks form grabens and the uplifted blocks form horsts. In areas where tensional stress has

recently affected the crust, the grabens may form rift valleys and the uplifted horst blocks may

form linear mountain ranges. The East African Rift Valley is an example of an area where

continental extension has created such a rift. The basin and range province of the western U.S.

(Nevada, Utah, and Idaho) is also an area that has recently undergone crustal extension. In the

basin and range, the basins are elongated grabens that now form valleys, and the ranges are

uplifted horst blocks.


Reverse Faults - are faults that result from horizontal compressional stresses in brittle rocks,

where the hanging-wall block has moved up relative the footwall block.

A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 15°.

Thrust faults can have considerable displacement, measuring hundreds of kilometers, and can

result in older strata overlying younger strata.

Strike Slip Faults - are faults where the relative motion on the fault has taken place along

a horizontal direction. Such faults result from shear stresses acting in the crust. Strike slip

faults can be of two varieties, depending on the sense of displacement. To an observer

standing on one side of the fault and looking across the fault, if the block on the other

side has moved to the left, we say that the fault is a left-lateral strike-slip fault. If the

block on the other side has moved to the right, it is a right-lateral strike-slip fault. The

famous San Andreas Fault in California is an example of a right-lateral strike-slip fault.

Displacements on the San Andreas fault are estimated at over 600 km.


Transform-Faults are a special class of strike-slip faults. These are plate boundaries

along which two plates slide past one another in a horizontal manner. The most common

type of transform faults occur where oceanic ridges are offset. Note that the transform

fault only occurs between the two segments of the ridge. Outside of this area there is no

relative movement because blocks are moving in the same direction. These areas are

called fracture zones. The San Andreas fault in California is also a transform fault.

Earthquakes

Earthquakes occur when energy stored in elastically strained rocks is suddenly released. This

release of energy causes intense ground shaking in the area near the source of the earthquake and

sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can be

generated by bomb blasts, volcanic eruptions, and sudden slippage along faults. Earthquakes are

definitely a geologic hazard for those living in earthquake prone areas, but the seismic waves

generated by earthquakes are invaluable for studying the interior of the Earth.

Origin of Earthquakes

Most natural earthquakes are caused by sudden slippage along a fault zone. The elastic rebound

theory suggests that if slippage along a fault is hindered such that elastic strain energy builds up

in the deforming rocks on either side of the fault, when the slippage does occur, the energy

released causes an earthquake.

This theory was discovered by making measurements at a number of points across a fault. Prior

to an earthquake it was noted that the rocks adjacent to the fault were bending. These bends

disappeared after an earthquake suggesting that the energy stored in bending the rocks was

suddenly released during the earthquake.


Seismology: The Study of Earthquakes

When an earthquake occurs, the elastic energy is released sending out vibrations that travel

throughout the Earth. These vibrations are called seismic waves. The study of how seismic

waves behave in the Earth is called seismology.

Seismograms - Seismic waves travel through the Earth as vibrations. A seismometer is an

instrument used to record these vibrations, and the resulting graph that shows the vibrations is

called a seismogram. The seismometer must be able to move with the vibrations, yet part of it

must remain nearly stationary.

This is accomplished by isolating the recording device (like a pen) from the rest of the Earth

using the principal of inertia. For example, if the pen is attached to a large mass suspended by a

wire, the large mass moves less than the paper which is attached to the Earth, and on which the

record of the vibrations is made.

The source of an earthquake is called the focus, which is an exact location within the Earth were

seismic waves are generated by sudden release of stored elastic energy. The epicenter is the

point on the surface of the Earth directly above the focus.


Seismic waves emanating from the focus can travel in several ways, and thus there are several

different kinds of seismic waves.

Types of seismic waves

There are two types of seismic waves, body wave and surface waves.

Body Waves – emanate from the focus and travel in all directions through the body of the Earth.

They follow ray paths refracted by the varying density and modulus (stiffness) of the Earth's

interior. The density and modulus, in turn, vary according to temperature, composition, and

phase. This effect is similar to the refraction of light waves.

There are two types of body waves: P –waves and S-waves:

P - Waves P waves (primary waves) are compressional waves that are longitudinal in nature. P-waves are

pressure waves that are the initial set of waves produced by an earthquake. These waves can

travel through any type of material, and can travel at nearly twice the speed of S waves. In air,

they take the form of sound waves; hence they travel at the speed of sound. Typical speeds are

330 m/s in air, 1450 m/s in water and about 5000 m/s in granite.

𝑉𝑝 = [(𝐾 + 4/3µ)/𝜌]

Where, VP is the velocity of the P-wave, K is the incompressibility of the material, μ is the

rigidity of the material, and ρ is the density of the material.


P-waves behave as sound waves. They move through the material by compressing it, but after it

has been compressed it expands, so that the wave moves by compressing and expanding the

material as it travels. Thus the velocity of the P-wave depends on how easily the material can be

compressed (the incompressibility), how rigid the material is (the rigidity), and the density of the

material. P-waves have the highest velocity of all seismic waves and thus will reach all

seismographs first.

S-Waves-Secondary waves, also called shear waves

S waves (secondary waves) are shear waves that are transverse in nature. These waves typically

follow P waves during an earthquake and displace the ground perpendicular to the direction of

propagation. Depending on the propagational direction, the wave can take on different surface

characteristics; for example, in the case of horizontally polarized S waves, the ground moves

alternately to one side and then the other. S waves can travel only through solids, as fluids

(liquids and gases) do not support shear stresses. S waves are slower than P waves, and speeds

are typically around 60% of that of P waves in any given material.

Surface waves-Surface waves differ from body waves in that they do not travel through the

Earth, but instead travel along paths nearly parallel to the surface of the Earth. Surface waves

behave like S-waves in that they cause up and down and side to side movement as they pass, but

they travel slower than S-waves and do not travel through the body of the Earth. Surface waves

are often the cause of the most intense ground motion during an earthquake.

The record of an earthquake, a seismogram, as recorded by a seismometer, will be a plot of

vibrations versus time (Figure below). On the seismograph, time is marked at regular intervals,

so that we can determine the time of arrival of the first P-wave and the time of arrival of the first

S-wave. (Note again, that because P-waves have a higher velocity than S-waves, the P-waves

arrive at the seismographic station before the S-waves).

Locating the Epicenters of Earthquakes - To determine the location of an earthquake epicenter,

we need to have recorded a seismograph of the earthquake from at least three seismographic

stations at different distances from the epicenter. In addition, the time it takes for P-waves and S-

waves to travel through the Earth and arrive at a seismographic station. Such information has

been collected over the last 80 or so years, and is available as travel time curves. From the


seismographs at each station one determines the S-P interval (the difference in the time of arrival

of the first S-wave and the time of arrival of the first P-wave. Note that on the travel time curves,

the S-P interval increases with increasing distance from the epicenter.

Therefore the S-P interval tells us the distance to the epicenter from the seismographic station

where the earthquake was recorded. Thus at each station we can draw a circle on a map that has a

radius equal to the distance from the epicenter. Three such circles will intersect in a point that

locates the epicenter of the earthquake.


Magnitude of Earthquakes - Whenever a large destructive earthquake occurs in the world the

press immediately wants to know where the earthquake occurred and how big the earthquake

was. The size of an earthquake is usually given in terms of a scale called the Richter Magnitude.

Richter Magnitude is a scale of earthquake size developed by a seismologist named Charles

Richter. The Richter Magnitude involves measuring the amplitude (height) of the largest

recorded wave at a specific distance from the earthquake. While it is correct to say that for each

increase in 1 in the Richter Magnitude, there is a tenfold increase in amplitude of the wave, it is

incorrect to say that each increase of 1 in Richter Magnitude represents a tenfold increase in the

size of the Earthquake.

A better measure of the size of an earthquake is the amount of energy released by the

earthquake. While this is much more difficult to determine, Richter gave a means by

which the amount of energy released can be estimated:

𝐿𝑜𝑔 𝐸 = 11.8 + 1.5𝑀

Where Log refers to the logarithm to the base 10, E is the energy released in ergs, and M is the

Richter Magnitude. The table below show worked examples:

From these calculations you can see that each increase in 1 in Magnitude represents a 31 fold

increase in the amount of energy released. Thus, a magnitude 7 earthquake releases 31 times

more energy than a magnitude 6 earthquake. A magnitude 8 earthquake releases 31 x 31 or 961

times as much energy as a magnitude 6 earthquake.

The Hiroshima atomic bomb released an amount of energy equivalent to a magnitude 5.5

earthquake.

Note that the Richter scale is an open ended scale with no maximum or minimum. The

largest earthquakes are probably limited by rock strength, although meteorite impacts

could cause even larger earthquakes. The largest earthquakes so far recorded are the

Chile earthquake in 1960 with a Richter Magnitude of 8.5, and the Alaska (Good Friday)

earthquake of 1964 with a Richter Magnitude of 8.6.

Note that it usually takes more than one seismographic station to calculate the magnitude

of an earthquake. Thus you will hear initial estimates of earthquake magnitude

immediately after an earthquake and a final assigned magnitude for the same earthquake


that may differ from initial estimates, but is assigned after seismologists have had time to

evaluate the data from numerous seismographic stations.

Although the Richter Magnitude is the scale most commonly reported when referring to

the size of an earthquake, it has been found that for larger earthquakes a more accurate

measurement of size is the moment magnitude. The moment magnitude is a measure of

the amount of strain energy released by the earthquake as determined by measurements

of the shear strength of the rock and the area of the rupture surface that slipped during the

earthquake.

The moment magnitude for large earthquakes is usually greater than the Richter magnitude for

the same earthquake. For example the Richter magnitude for the 1964 Alaska earthquake is

usually reported as 8.6, whereas the moment magnitude for this earthquake is calculated at 9.2.

Sometimes a magnitude is reported for an earthquake and no specification is given as to which

magnitude (Richter or moment) is being reported. This obviously can cause confusion.

Table 1: Frequency of Earthquakes of different magnitude worlwide

Magnitude No. of earthquakes/year Description

>8.5 0.3 Great

8.0-8.4 1

7.5-7.9 3

Major 7.0-7.4 15

6.6-6.9 56

6.0-6.5 210 Destructive

5.0-5.9 800 Damaging

4.0-4.9 6200

Minor

3.0-3.9 49000

2.0-2.9 300000

0-1.9 700000

Modified Mercalli Intensity Scale

Note that the Richter magnitude scale results in one number for the size of the earthquake.

Maximum ground shaking will occur only in the area of the epicenter of the earthquake, but the

earthquake may be felt over a much larger area. Developed in the late 1800s, Modified Mercalli

Scale assesses the intensity of ground shaking and building damage over large areas.

The scale is applied after the earthquake by conducting surveys of people's response to

the intensity of ground shaking and destruction.

Thus, a given earthquake will have zones of different intensity all surrounding a zone of

maximum intensity.

The Modified Mercalli Scale is shown in the table below. Note that correspondence

between maximum intensity and Richter Scale magnitude only applies in the area around

the epicenter.

A given earthquake will have zones of different intensity all surrounding a zone of

maximum intensity.


Modified Mercalli intensity scale of 1931 Richter scale

equivalent

I. Not felt except by a very few under especially favourable circumstances.

II. Felt only be a few persons at rest, especially on upper floors of buildings. Delicately

suspended objects may swing.

III. Felt quite noticeably indoors, especially on upper floors of buildings but many people do

not recognize it as an earthquake. Standing motorcars may rock slightly. Vibration like

passing of truck. Duration estimated. IV. During the day felt indoors by many. Outdoors by few. At night some awakened. Dishes,

windows, doors disturbed, walls make cracking sound. Sensation like heavy truck striking

building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone, many awakened. Some dishes, windows, etc., broken. A few

instances of cracked plaster. Unstable objects overturned. Disturbances of trees, poles, and

other tall objects sometimes noticed. Pendulum clocks may stop.

VI. Felt by all, many frightened and run outdoors. Some heavy furniture moved a few

instances of fallen plaster or damaged chimneys. Damage slight.

VII. Everybody runs outdoors. Damage negligible in buildings of good design and construction,

slight to moderate in well-built ordinary structures, considerable in poorly built or badly

designed structures. Some chimneys broken. Noticed by persons driving motor cars.

VIII. Damage slight in specially designed structures, considerable in ordinary substantial buildings, with partial collapse, great in poorly built structures. Panel walls thrown out of

frame structures. Fall of chimneys, factory stacks, columns, monuments, and walls. Heavy

furniture overturned. Sand and mud ejected in small amounts. Changes in well water.

Persons driving motor cars disturbed.

IX. Damage considerable in specially designed structures. Well-designed structures thrown out

of plumb, great in substantial buildings, with partial collapse. Buildings shifted off

foundations. Ground cracked conspicuously. Underground pipes broken.

X. Some well-built wooden structures destroyed. Most masonry and frame structures with

foundations destroyed, ground badly cracked. Rails bent. Landslides considerable from

river banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks.

XI. Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in

soft ground. Rails bent greatly.

XII. Damage total. Practically all works of construction are damaged greatly or destroyed.

Waves seen on ground surface. Lines of sight and level are distorted. Objects are thrown

upward into the air.

The Mercalli Scale is very useful in examining the effects of an earthquake over a large

area, because it is responsive not only to the size of the earthquake as measured by the

Richter scale for areas near the epicenter, but also show the effects of the efficiency that

seismic waves are transmitted through different types of material near the Earth's surface.

The Mercalli Scale is also useful for determining the size of earthquakes that occurred

before the modern seismographic network was available (before there were

seismographic stations, it was not possible to assign a Richter Magnitude).

Earthquake Prediction and Control

Long-Term Forecasting


Long-term forecasting is based mainly on the knowledge of when and where earthquakes have

occurred in the past. Thus, knowledge of present tectonic setting, historical records, and

geological records are studied to determine locations and recurrence intervals of earthquakes.

Two methods of earthquake forecasting are being employed - paleoseismology and seismic gaps.

a. Paleoseismology - the study of prehistoric earthquakes. Through study of the offsets in

sedimentary layers near fault zones, it is often possible to determine recurrence intervals

of major earthquakes prior to historical records. If it is determined that earthquakes have

recurrence intervals of say 1 every 100 years, and there are no records of earthquakes in

the last 100 years, then a long-term forecast can be made and efforts can be undertaken to

reduce seismic risk.

b. Seismic gaps - A seismic gap is a zone along a tectonically active area where no

earthquakes have occurred recently, but it is known that elastic strain is building in the

rocks. If a seismic gap can be identified, then it might be an area expected to have a large

earthquake in the near future.

Short-Term Prediction

Short-term predication involves monitoring of processes that occur in the vicinity of earthquake

prone faults for activity that signify a coming earthquake.

Anomalous events or processes that may precede an earthquake are called precursor

events and might signal a coming earthquake.

Despite the array of possible precursor events that are possible to monitor, successful

short-term earthquake prediction has so far been difficult to obtain. This is likely because:

i. The processes that cause earthquakes occur deep beneath the surface and are difficult to

monitor.

ii. Earthquakes in different regions or along different faults all behave differently, thus no

consistent patterns have so far been recognized.

Among the precursor events that may be important are the following:

Ground Uplift and Tilting - Measurements taken in the vicinity of active faults sometimes

show that prior to an earthquake the ground is uplifted or tilts due to the swelling of rocks

caused by strain building on the fault. This may lead to the formation of numerous small

cracks (called microcracks). This cracking in the rocks may lead to small earthquakes

called foreshocks.

Foreshocks - Prior to a 1975 earthquake in China, the observation of numerous

foreshocks led to successful prediction of an earthquake and evacuation of the city of the

Haicheng. The magnitude 7.3 earthquake that occurred, destroyed half of the city of

about 100 million inhabitants, but resulted in only a few hundred deaths because of the

successful evacuation.

Water Level in Wells - As rocks become strained in the vicinity of a fault, changes in

pressure of the groundwater (water existing in the pore spaces and fractures in rocks)

occur. This may force the groundwater to move to higher or lower elevations, causing

changes in the water levels in wells.

Emission of Radon Gas - Radon is an inert gas that is produced by the radioactive decay

of uranium and other elements in rocks. Because Radon is inert, it does not combine with

other elements to form compounds, and thus remains in a crystal structure until some


event forces it out. Deformation resulting from strain may force the Radon out and lead to

emissions of Radon that show up in well water. The newly formed microcracks could

then serve as pathways for the Radon to escape into groundwater.

Changes in the Electrical Resistivity of Rocks - Electrical resistivity are the resistance to

the flow of electric current. In general rocks are poor conductors of electricity, but water

is more efficient a conducting electricity. If microcracks develop and groundwater is

forced into the cracks, this may cause the electrical resistivity to decrease (causing the

electrical conductivity to increase).

Unusual Radio Waves - Just prior to the Loma Prieta Earthquake of 1989, some

researchers reported observing unusual radio waves. Where these were generated and

why, is not yet known, but research is continuing.

Strange Animal Behavior - Prior to a magnitude 7.4 earthquake in Tanjin, China,

zookeepers reported unusual animal behavior. Snakes refusing to go into their holes,

swans refusing to go near water, pandas screaming, etc. This was the first systematic

study of this phenomenon prior to an earthquake. Although other attempts have been

made to repeat a prediction based on animal behavior, there have been no other

successful predictions.

Controlling Earthquakes

Although no attempts have yet been made to control earthquakes, earthquakes have been known

to be induced by human interaction with the Earth. This suggests that in the future earthquake

control may be possible.

Examples of human induced earthquakes

For ten years after construction of the Hoover Dam in Nevada blocking the Colorado

River to produce Lake Mead, over 600 earthquakes occurred, one with magnitude of 5

and 2 with magnitudes of 4.

In the late 1960s toxic waste injected into hazardous waste disposal wells at Rocky Flats,

near Denver apparently caused earthquakes to occur in a previously earthquake quiet

area. The focal depths of the quakes ranged between 4 and 8 km, just below the 3.8 km

deep wells.

Nuclear testing in Nevada set off thousands of aftershocks after the explosion of a 6.3

magnitude equivalent underground nuclear test. The largest aftershocks were about

magnitude 5.


TOPIC FOUR

Tsunami

Up until December of 2004, the phenomenon of tsunami was not on the minds of most of the

world's population. That changed on the morning of December 24, 2004 when an earthquake of

moment magnitude 9.1 occurred along the oceanic trench off the coast of Sumatra in Indonesia.

This large earthquake resulted in vertical displacement of the sea floor and generated a tsunami

that eventually killed about 230,000 people and affected the lives of several million people.

Although people living on the coastline near the epicenter of the earthquake had little time or

warning of the approaching tsunami, those living farther away along the coasts of Thailand, Sri

Lanka, India, and East Africa had plenty of time to move higher ground to escape. But, there was

no tsunami warning system in place in the Indian Ocean, and although other tsunami warning

centers attempted to provide a warning, there was no effective communication system in place.

Unfortunately, it has taken a disaster of great magnitude to point out the failings of the world's

scientific community and to educate almost every person on the planet about tsunami.

Even with heightened world awareness of tsunami, disasters still occur. On September 29, 2009,

earthquakes in the Samoa region of the southwest Pacific Ocean killed nearly 200 people, and as

a result of the Chilean earthquake of February, 2010, at least 50 casualties resulted from a

tsunami triggered by a moment magnitude 8.8 earthquake.

What is a Tsunami?

A tsunami is a very long-wavelength wave of water that is generated by sudden displacement of

the seafloor or disruption of any body of standing water. Tsunamis are sometimes called "seismic

sea waves", although they can be generated by mechanisms other than earthquakes. Tsunamis

have also been called "tidal waves", but this term should not be used because they are not in any

way related to the tides of the Earth. Because tsunami occur suddenly, often without warning,

they are extremely dangerous to coastal communities.

Physical Characteristics of Tsunami

All types of waves, including tsunami, have a wavelength, a wave height, an amplitude, a

frequency or period, and a velocity.

Wavelength is defined as the distance between two identical points on a wave (i.e.

between wave crests or wave troughs). Normal ocean waves have wavelengths of about

100 meters. Tsunami has much longer wavelengths, usually measured in kilometers and

up to 500 kilometers.

Wave height refers to the distance between the trough of the wave and the crest or peak

of the wave.

Wave amplitude - refers to the height of the wave above the still water line, usually this is

equal to 1/2 the wave height. Tsunami can have variable wave height and amplitude that

depends on water depth.

Wave frequency or period - is the amount of time it takes for one full wavelength to pass

a stationary point.

Wave velocity is the speed of the wave. Velocities of normal ocean waves are about 90

km/hr while tsunami have velocities up to 950 km/hr (about as fast as jet airplanes), and

thus move much more rapidly across ocean basins. The velocity of any wave is equal to

the wavelength divided by the wave period.


V= 𝜆/𝑃

Tsunamis are characterized as shallow-water waves. These are different from the waves which

are caused by the wind blowing across the ocean's surface. Wind-generated waves usually have

period (time between two successive waves) of five to twenty seconds and a wavelength of 100

to 200 meters. A tsunami can have a period in the range of ten minutes to two hours and

wavelengths greater than 500 km. A wave is characterized as a shallow-water wave when the

ratio of the water depth and wavelength is very small. The velocity of a shallow-water wave is

also equal to the square root of the product of the acceleration of gravity, g, (10m/sec2) and the

depth of the water, d.

𝑉 = 𝑔 ∗ 𝑑

The rate at which a wave loses its energy is inversely related to its wavelength. Since a tsunami

has a very large wavelength, it will lose little energy as it propagates. Thus, in very deep water, a

tsunami will travel at high speeds with little loss of energy. For example, when the ocean is 6100

m deep, a tsunami will travel about 890 km/hr, and thus can travel across the Pacific Ocean in

less than one day.

As a tsunami leaves the deep water of the open sea and arrives at the shallow waters near the

coast, it undergoes a transformation. Since the velocity of the tsunami is also related to the water

depth, as the depth of the water decreases, the velocity of the tsunami decreases. The change of

total energy of the tsunami, however, remains constant.

Furthermore, the period of the wave remains the same, and thus more water is forced between

the wave crests causing the height of the wave to increase. Because of this "shoaling" effect, a


tsunami that was imperceptible in deep water may grow to have wave heights of several meters

or more.

If the trough of the tsunami wave reaches the coast first, this causes a phenomenon called

drawdown, where it appears that sea level has dropped considerably. Drawdown is followed

immediately by the crest of the wave which can catch people observing the drawdown off guard.

When the crest of the wave hits, sea level rises (called run-up). Run-up is usually expressed in

meters above normal high tide. Run-ups from the same tsunami can be variable because of the

influence of the shapes of coastlines. One coastal area may see no damaging wave activity while

in another area destructive waves can be large and violent. The flooding of an area can extend

inland by 300 m or more, covering large areas of land with water and debris. Flooding tsunami

waves tend to carry loose objects and people out to sea when they retreat. Tsunami may reach a

maximum vertical height onshore above sea level, called a run-up height, of 30 meters. A notable

exception is the landslide generated tsunami in Lituya Bay, Alaska in 1958 which produced a 60

meter high wave.

Because the wavelengths and velocities of tsunami are so large, the period of such waves is also

large, and larger than normal ocean waves. Thus it may take several hours for successive crests

to reach the shore. (For a tsunami with a wavelength of 200 km traveling at 750 km/hr, the wave

period is about 16 minutes). Thus people are not safe after the passage of the first large wave, but

must wait several hours for all waves to pass. The first wave may not be the largest in the series

of waves. For example, in several different recent tsunami the first, third, and fifth waves were

the largest.

How Tsunami are Generated

There is an average of two destructive tsunami per year in the Pacific basin. Pacific wide

tsunamis are a rare phenomenon, occurring every 10 - 12 years on the average. Most of this

tsunami is generated by earthquakes that cause displacement of the seafloor. Also, tsunami can

be generated by volcanic eruptions, landslides, underwater explosions, and meteorite impacts.

Earthquakes

Earthquakes cause tsunami by causing a disturbance of the seafloor. Thus, earthquakes that occur

along coastlines or anywhere beneath the oceans can generate tsunami. The size of the tsunami is

usually related to the size of the earthquake, with larger tsunami generated by larger earthquakes.

But the sense of displacement is also important. Tsunamis are generally only formed when an

earthquake causes vertical displacement of the seafloor. Thus, tsunamis only occur if the fault

generating the earthquake has normal or reverse displacement. Because of this, most tsunami are

generated by earthquakes that occur along the subduction boundaries of plates, along the oceanic

trenches. Since the Pacific Ocean is surrounded by plate boundaries of this type, tsunamis are

frequently generated by earthquakes around the margins of the Pacific Ocean.

Examples of Tsunami generated by Earthquakes Although the December 2004 Indian Ocean

tsunami is by far the best well known and most deadly tsunami generated by earthquakes,March

27, 1964 - The Good Friday Earthquake in Alaska, September 2 etc

Volcanic Eruptions

Volcanoes that occur along coastal zones, like in Japan and island arcs throughout the world, can

cause several effects that might generate a tsunami. Explosive eruptions can rapidly emplace


pyroclastic flows into the water, landslides and debris avalanches produced by eruptions can

rapidly move into water, and collapse of volcanoes to form calderas can suddenly displace the

water. For instance the eruption of Krakatau in the Straights of Sunda, between Java and

Sumatra, in 1883 generated at least three tsunami that killed 36,417 people.

Landslides

Landslides moving into oceans, bays, or lakes can also generate tsunami. Most such landslides

are generated by earthquakes or volcanic eruptions. As previously mentioned, a large landslide

or debris avalanche fell into Lituya Bay, Alaska in 1958 causing a wave with a run-up of about

60 m as measured by a zone completely stripped of vegetation.

Underwater Explosions

Nuclear testing by the United States in the Marshall Islands in the 1940s and 1950s generated

tsunami.

Meteorite Impacts

While no historic examples of meteorite impacts are known to have produced a tsunami, the

apparent impact of a meteorite at the end of the Cretaceous Period, about 65 million years ago

near the tip of what is now the Yucatan Peninsula of Mexico, produced tsunami that left deposits

all along the Gulf coast of Mexico and the United States.

Mitigation of Risks and Hazards

The main damage from tsunami comes from the destructive nature of the waves

themselves.

Secondary effects include the debris acting as projectiles which then run into other

objects, erosion that can undermine the foundations of structures built along coastlines,

and fires that result from disruption of gas and electrical lines.

Tertiary effects include loss of crops and water and electrical systems which can lead to

famine and disease.

Within the last century, up until the December 2004 tsunami, there were 94 destructive tsunami

Prediction and Early Warning

For areas located at great distances from earthquakes that could potentially generate a

tsunami there is usually plenty of time for warnings to be sent and coastal areas

evacuated, even though tsunami travel at high velocities across the oceans. Hawaii is

good example of an area located far from most of the sources of tsunami, where early

warning is possible and has saved lives.

For earthquakes occurring anywhere on the subduction margins of the Pacific Ocean

there is a minimum of 4 hours of warning before a tsunami would strike any of the

Hawaiian Islands. The National Oceanic and Atmospheric Administration (NOAA) has

set up a Pacific warning system for areas in the Pacific Ocean, called the Pacific Tsunami

Warning Center consisting of an international network of seismographic stations, and

tidal stations around the Pacific basin.

Like all warning systems, the effectiveness of tsunami early warning depends strongly on local

authority's ability to determine that their is a danger, their ability to disseminate the information

to those potentially affected, and on the education of the public to heed the warnings and remove


TOPIC FIVE

Volcanoes, Magma, and Volcanic Eruptions

Volcanic eruptions are caused by magma (a mixture of liquid rock, crystals, and dissolved gas)

expelled onto the Earth's surface and therefore it is important that we understand the

characteristics of magma and how magmas form in the Earth.

Characteristics of Magma

Types of Magma

Types of magma are determined by chemical composition of the magma. Three general types are

recognized:

i. Basaltic magma - SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na

ii. Andesitic magma -- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K

iii. Rhyolitic magma -- SiO2 65-75%, low in Fe, Mg, Ca, high in K, Na

Gases in Magmas

At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a

separate vapor phase when pressure is decreased as magma rises toward the surface of the Earth

the same way carbonated beverages behaves at high pressure. The high pressure keeps the gas in

solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the

gas comes out of solution and forms a separate gas phase that seen as bubbles. Gas gives

magmas their explosive character, because volume of gas expands as pressure is reduced. The

compositions of the gases in magma are:

Mostly H2O (water vapor) & some CO2 (carbon IV oxide)

Minor amounts of Sulfur, Chlorine, and Fluorine gases

The amount of gas in magma is also related to the chemical composition of the magma. Rhyolitic

magmas usually have higher gas contents than basaltic magmas.

Temperature of Magmas

Temperature of magmas is difficult to measure (due to the danger involved), but laboratory

measurement and limited field observation indicate that the eruption temperature of various

magmas is as follows:

Basaltic magma - 1000 to 1200°C

Andesitic magma - 800 to 1000°C

Rhyolitic magma - 650 to 800°C.

Viscosity of Magmas

Viscosity is the resistance to flow (opposite of fluidity). Viscosity depends on primarily on the

composition of the magma, and temperature.

Higher SiO2 (silica) content magmas have higher viscosity than lower SiO2 content

magmas (viscosity increases with increasing SiO2 concentration in the magma).

Lower temperature magmas have higher viscosity than higher temperature magmas

(viscosity decreases with increasing temperature of the magma).

Thus, basaltic magmas tend to be fairly fluid (low viscosity), but their viscosity is still 10,000 to

100, 000 times more viscous than water. Rhyolitic magmas tend to have even higher viscosity,

ranging between 1 million and 100 million times more viscous than water. (Note that solids,

even though they appear solid have a viscosity, but it very high, measured as trillions time the


viscosity of water). Viscosity is an important property in determining the eruptive behavior of

magmas.

Magma type Solidified rock Chemical

composition

Temperature

(°C)

Viscosity Gas content

Basaltic Basalt 45-55 SiO2 %, high in

Fe, Mg, Ca, low in K, Na

1000 - 1200 Low Low

Andesitic Andesite 55-65 SiO2 %,

intermediate in

Fe, Mg, Ca, Na, K

800 - 1000 Intermediate Intermediate

Rhyolitic Rhyolite 65-75 SiO2 %, low in

Fe, Mg,

Ca, high in K, Na.

650 - 800 High High

How Magmas Form in the Earth

In order for magmas to form, some part of the Earth must get hot enough to melt the rocks

present. Under normal conditions, the geothermal gradient, which is how the temperature in the

Earth changes with depth or pressure, is not high enough to melt rocks, and thus with the

exception of the outer core, most of the Earth is solid. Thus, magmas form only under special

circumstances, and thus, volcanoes are only found on the Earth's surface in areas above where

these special circumstances occur.

As pressure increases in the Earth, the melting temperature changes as well. For pure minerals,

there are two general cases.

If the mineral contains no water (H2O) or carbon dioxide (CO2) and there is no water or

carbon dioxide present in the surroundings, then melting occurs at a single temperature at

any given pressure and increases with increasing pressure or depth in the Earth. This is

called dry melting.

If water or carbon dioxide are present within or surrounding the mineral, then melting

takes place at a single temperature at any given pressure, but first decreases with

increasing pressure

Since rocks are mixtures of minerals, they behave somewhat differently. Unlike minerals, rocks

do not melt at a single temperature, but instead melt over a range of temperatures. Thus, it is


possible to have partial melts, from which the liquid portion might be extracted to form magma.

The two general cases are:

1) Melting of dry rocks is similar to melting of dry minerals, melting temperatures increase

with increasing pressure, except there is a range of temperature over which there exists a

partial melt. The degree of partial melting can range from 0 to 100%.

2) Melting of wet rocks is similar to melting of wet minerals, except there is range of

temperature range over which partial melting occurs. Again, the temperature of beginning

of melting first decreases with increasing pressure or depth, then at high pressure or depth

the melting temperatures again begin to rise.

Origin of Basaltic Magma

Much evidence suggests that Basaltic magmas result from dry partial melting of mantle.

Basalts make up most of oceanic crust and only mantle underlies the crust.

Basalts contain minerals like olivine, pyroxene and plagioclase, none of which contain

water.

Basalts erupt non-explosively, indicating a low gas content and therefore low water

content.

Origin of Rhyolitic Magma

Most rhyolitic magma appears to result from wet melting of continental crust. The evidence for

this is:

Most rhyolites are found in areas of continental crust.

When most rhyolitic magma erupts from volcanoes it does so very explosively, indicating

high gas content.

Solidified rhyolite contains quartz, feldspar, hornblende, biotite, and muscovite. The

latter minerals contain water, indicating high water content.

Origin of Andesitic Magma

Andesitic magmas erupt in areas above subduction zones. This suggests a relationship between

the production of andesitic magma and subduction. Because the oceanic lithosphere is in contact

with ocean water there should be much water in the pore spaces of upper oceanic crustal rocks as

well as water contained within clay minerals that have settled to the sea floor. When this material

is subducted, it begins to heat up and water is driven off. If the water enters the overlying


sthenospheric mantle, it will lower its melting temperatures and thus melting will occur. This

melting will produce basaltic magmas with high water content.

Volcanic Eruptions

Magmas that are generated deep within the Earth begin to rise because they are less dense

than the surrounding solid rocks.

As they rise they may encounter a depth or pressure where the dissolved gas no longer

can be held in solution in the magma, and the gas begins to form a separate phase (i.e. it

makes bubbles just like in a bottle of carbonated beverage when the pressure is reduced).

When a gas bubble forms, it will also continue to grow in size as pressure is reduced and

more of the gas comes out of solution. In other words, the gas bubbles begin to expand.

If the liquid part of the magma has a low viscosity, then the gas can expand relatively

easily. When the magma reaches the Earth's surface, the gas bubble will simply burst, the

gas will easily expand to atmospheric pressure, and a non-explosive eruption will occur,

usually as a lava flow (Lava is the name we give to a magma when it on the surface of

the Earth).

If the liquid part of the magma has a high viscosity, then the gas will not be able to

expand very easily, and thus, pressure will build up inside of the gas bubble(s). When this

magma reaches the surface, the gas bubbles will have a high pressure inside, which will

cause them to burst explosively on reaching atmospheric pressure. This will cause an

explosive volcanic eruption.

Non-explosive Eruptions

Non explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to

andesitic magmas).

If the viscosity is low, non-explosive eruptions usually begin with fire fountains due to

release of dissolved gases.

Lava flows are produced on the surface, and these run like liquids down slope, along the

lowest areas they can find.

Lava flows produced by eruptions under water are called pillow lavas.

If the viscosity is high, but the gas content is low, then the lava will pile up over the vent

to produce a lava dome or volcanic dome.

Explosive Eruptions

Explosive eruptions are favored by high gas content and high viscosity (andesitic to rhyolitic

magmas).

Explosive bursting of bubbles will fragment the magma into clots of liquid that will cool

as they fall through the air. These solid particles become pyroclasts (meaning – hot

fragments) and tephra or volcanic ash, which refer to sand- sized or smaller fragments

Table 2: Tephra and pyroclastic rocks

Average Particle Size (mm) Unconsolidated Material (Tephra) Pyroclastic Rock

>64 Bombs or Blocks Agglomerate

2 - 64 Lapilli Lapilli Tuff

<2 Ash Ash Tuff


Blocks are angular fragments that were solid when ejected.

Bombs have an aerodynamic shape indicating they were liquid when ejected.

Bombs and lapilli that consist mostly of gas bubbles (vesicles) result in a low density

highly vesicular rock fragment called pumice.

Clouds of gas and tephra that rise above a volcano produce an eruption column that can rise up

to 45 km into the atmosphere. Eventually the tephra in the eruption column will be picked up by

the wind, carried for some distance, and then fall back to the surface as a tephra fall or ash fall.

If the gas pressure inside the magma is directed outward instead of upward, a lateral blast can

occur. When this occurs on the flanks of a lava dome, a pyroclastic flow called a glowing

avalanche or nuée ardentes (in French) can also result. Directed blasts often result from sudden

exposure of the magma by a landslide or collapse of a lava dome.

Types of Volcanic Eruptions

Volcanic eruptions, especially explosive ones, are very dynamic phenomena. That is the

behavior of the eruption is continually changing throughout the course of the eruption. This

makes it very difficult to classify volcanic eruptions. Nevertheless they can be classified

according to the principal types of behavior that they exhibit. An important point to remember,

however, is that during a given eruption the type of eruption may change between several

different types.

Hawaiian - These are eruptions of low viscosity basaltic magma. Gas discharge produces

a fire fountain that shoots incandescent lava up to 1 km above the vent. The lava, still

molten when it returns to the surface flows away down slope as a lava flow. Hawaiian

Eruptions are considered non-explosive eruptions. Very little pyroclastic material is

produced.

Strombolian - These eruptions are characterized by distinct blasts of basaltic to andesitic

magma from the vent. These blasts produce incandescent bombs that fall near the vent,

eventually building a small cone of tephra (cinder cone). Sometimes lava flows erupt


from vents low on the flanks of the small cones. Strombolian eruptions are considered

mildly explosive, and produce low elevation eruption columns and tephra fall deposits.

Vulcanian - These eruptions are characterized by sustained explosions of solidified or

highly viscous andesite or rhyolite magma from a the vent. Eruption columns can reach

several km above the vent, and often collapse to produce pyroclastic flows. Widespread

tephra falls are common. Vulcanian eruptions are considered very explosive.

Pelean - These eruptions result from the collapse of an andesitic or rhyolitic lava dome,

with or without a directed blast, to produce glowing avalanches or nuée ardentes, as a

type of pyroclastic flow known as a block-and-ash flow. Pelean eruptions are considered

violently explosive.

Plinian - These eruptions result from a sustained ejection of andesitic to rhyolitic magma

into eruption columns that may extend up to 45 km above the vent. Eruption columns

produce wide-spread fall deposits with thickness decreasing away from the vent, and may

exhibit eruption column collapse to produce pyroclastic flows. Plinian ash clouds can

circle the Earth in a matter of days. Plinian eruptions are considered violently explosive.

Phreatomagmatic - These eruptions are produced when magma comes in contact with

shallow groundwater causing the groundwater to flash to steam and be ejected along with

pre-existing fragments of the rock and tephra from the magma. Because the water

expands so rapidly, these eruptions are violently explosive although the distribution of

pyroclasts around the vent is much less than in a Plinian eruption.

Phreatic (also called steam blast eruptions) - result when magma encounters shallow

groundwater, flashing the groundwater to steam, which is explosively ejected along with

pre-exiting fragments of rock. No new magma reaches the surface.

Volcanic Landforms, Volcanoes and Plate Tectonics

Volcanic Landforms

Volcanic landforms are controlled by the geological processes that form them and act on them

after they have formed. Thus, a given volcanic landform will be characteristic of the types of

material it is made of, which in turn depends on the prior eruptive behavior of the volcano.

This section explores major volcanic landforms and how they are formed, and in some cases,

later modified.

Shield Volcanoes (Figure 5)

A shield volcano is characterized by gentle upper slopes (about 5°) and somewhat steeper

lower slopes (about 10°).

Figure 5: Cross section of a shied volcano


Shield volcanoes are composed almost entirely of relatively thin lava flows built up over

a central vent.

Most shields were formed by low viscosity basaltic magma that flows easily down slope

away form the summit vent.

The low viscosity of the magma allows the lava to travel down slope on a gentle slope,

but as it cools and its viscosity increases, its thickness builds up on the lower slopes

giving a somewhat steeper lower slope.

Most shield volcanoes have a roughly circular or oval shape in map view.

Very little pyroclastic material is found within a shield volcano, except near the eruptive

vents, where small amounts of pyroclastic material accumulate as a result of fire

fountaining events.

Shield volcanoes thus form by relatively non-explosive eruptions of low viscosity

basaltic magma.

Vents for most shield volcanoes are central vents, which are circular vents near the

summit.

Stratovolcanoes (Composite Volcanoes) (Figure 5b)

Have steeper slopes than shield volcanoes, with slopes of 6 to 10° low on the flanks to 30°

near the top.

The steep slope near the summit (peak) is due to partly thick, short viscous lava flows

that do not travel far down slope from the vent.

The gentler slopes near the base are due to accumulations of material eroded from the

volcano and to the accumulation of pyroclastic material.

Stratovolcanoes show inter-layering of lava flows and pyroclastic material explaining

why they are sometimes called composite volcanoes.

Lavas and pyroclastics are usually andesitic to rhyolitic in composition.

Due to the higher viscosity of magmas erupted from these volcanoes; they are usually

more explosive than shield volcanoes.

Stratovolcanoes sometimes have a crater at the summit that is formed by explosive

ejection of material from a central vent. Sometimes the craters have been filled in by lava

flows or lava domes, sometimes they are filled with glacial ice, and less commonly they

are filled with water.

Long periods of repose (times of inactivity) lasting for hundreds to thousands of years,

make this type of volcano particularly dangerous, since many times they have shown no

historic activity, and people are reluctant to heed warnings about possible eruptions.

Tephra Cones (Cinder Cones)

Tephra cones are small volume cones consisting predominantly of tephra that result from

strombolian eruptions. They usually consist of basaltic to andesitic material.

They are actually fall deposits that are built surrounding the eruptive vent.

Slopes of the cones are controlled by the angle of repose (angle of stable slope for loose

unconsolidated material) and are usually between about 25 and 35°

They show an internal layered structure due to varying intensities of the explosions that

deposit different sizes of pyroclastics.


On young cones, a depression at the top of the cone, called a crater, is evident, and

represents the area above the vent from which material was explosively ejected. Craters

are usually eroded away on older cones.

If lava flows are emitted from tephra cones, they are usually emitted from vents on the

flank or near the base of the cone during the later stages of eruption.

Cinder and tephra cones usually occur around summit vents and flank vents of

stratovolcanoes.

Example of cinder cone is Parícutin Volcano in Mexico.

Cinder cones often occur in groups, where tens to hundreds of cones are found in one

area.

Figure 6: Illustration of (a) stratovocanoes and (b) tephra cones

Maars

Maars result from phreatic or phreatomagmatic activity, wherein magma heats up

groundwater, pressure builds as the water turns to steam, and then the water and pre-

existing rock (and some new magma if the eruption is phreatomagmatic) are blasted out

of the ground to form a tephra cone with gentle slopes.

Parts of the crater walls eventually collapse back into the crater, the vent is filled with

loose material, and, if the crater still is deeper than the water table, the crater fills with

water to form a lake, the lake level coinciding with the water table.

Lava Domes (Volcanic Domes)

Volcanic Domes result from the extrusion of highly viscous, gas poor andesitic and

rhyolitic lava. Since the viscosity is so high, the lava does not flow away from the vent,

but instead piles up over the vent.

Blocks of nearly solid lava break off the outer surface of the dome and roll down its

flanks to form breccias around the margins of domes.

The surface of volcanic domes is generally very rough, with numerous spines that have

been pushed up by the magma from below.

Most dome eruptions are preceded by explosive eruptions of more gas rich magma,

producing a tephra cone into which the dome is extruded.

Volcanic domes can be extremely dangerous because they form unstable slopes that may

collapse to expose gas-rich viscous magma to atmospheric pressure. This can result in

lateral blasts or Pelean type pyroclastic flow (nuée ardentes) eruptions.

(a) (b)


Figure 7: An illustrattion of (a) Maars and (b) Lava Domes

Geysers, Fumaroles and Hot Springs

A fumarole is vent where gases, either from a magma body at depth, or steam from

heated groundwater, emerge at the surface of the Earth. Since most magmatic gas is H2O

vapor, and since heated groundwater will produce H2O vapor, fumaroles will only be

visible if the water condenses. (H2O vapor is invisible, unless droplets of liquid water

have condensed).

Hot springs or thermal springs are areas where hot water comes to the surface of the

Earth. Cool groundwater moves downward and is heated by a body of magma or hot

rock. A hot spring results if this hot water can find its way back to the surface, usually

along fault zones.

A geyser results if the hot spring has a plumbing system that allows for the accumulation

of steam from the boiling water. When the steam pressure builds so that it is higher than

the pressure of the overlying water in the system, the steam will move rapidly toward the

surface, causing the eruption of the overlying water.

Figure 8: An illustration of the formation of geyser/spring

Craters and Calderas

Craters are circular depressions, usually less than 1 km in diameter, that form as a result

of explosions that emit gases and tephra.

(a) (b)


Calderas are much larger depressions, circular to elliptical in shape, with diameters

ranging from 1 km to 50 km. Calderas form as a result of collapse of a volcanic structure.

The collapse results from evacuation of the underlying magma chamber.

Calderas are often enclosed depressions that collect rain water and snow melt, and thus

lakes often form within a caldera.

Examples of Calderas are: Crater Lake Caldera in southern Oregon, Yellowstone Caldera in

Wyoming, Long Valley Caldera in eastern California, and Valles Caldera in New Mexico etc

Resurgent Domes

After the formation of a caldera by collapse, magma is sometimes re-injected into the

area below the caldera. This can result in uplift of one or more areas within the caldera to

form a resurgent dome. Two such resurgent domes formed in the Yellowstone caldera.

If magma leaks back to the surface during this resurgent doming, then eruptions of small

volcanic domes can occur in the area of the resurgent domes.

Read assignment:

i. Craters and calderas

ii. Resurgent Domes

iii. Plateau Basalts or Flood Basalts

Volcanoes and Plate Tectonics

Global Distribution of Volcanoes

The conditions for volcanic activity to occur do not exist everywhere beneath the surface, and

thus volcanism does not occur everywhere. Volcanism therefore occurs based on four principal

settings.

1. Along divergent plate boundaries, such as Oceanic Ridges or spreading centers;

2. In areas of continental extension (that may become divergent plate boundaries in the

future);

3. Along converging plate boundaries where subduction is occurring; and

4. In areas called "hot spots" that are usually located in the interior of plates, away from the

plate margins.

Volcanic Hazards

Primary Effects of Volcanism

Lava Flows

Lava flows are common in Hawaiian and Strombolian type of eruptions, the least

explosive.

Although lava flows have been known to travel as fast as 64 km/hr, most are slower and

give people time to move out of the way.

Thus, in general, lava flows are most damaging to property, as they can destroy anything

in their path.

Control of lava flows has been attempted with limited success by bombing flow fronts to

attempt to divert the flow, and by spraying with water to cool the flow. The latter is

credited with saving the fishing harbor during a 1973 eruption of Heimaey in Iceland.

Violent Eruptions and Pyroclastic Activity

Pyroclastic activity is one of the most dangerous aspects of volcanism.


Hot pyroclastic flows cause death by suffocation and burning. They can travel so rapidly

that few humans can escape.

Lateral blasts knock down anything in their path; can drive flying debris through trees.

Tephra falls can cause the collapse of roofs and can affect areas far from the eruption.

Although tephra falls blanket an area like snow, they are far more destructive because

tephra deposits have a density more than twice that of snow and tephra deposits do not

melt like snow.

Tephra falls destroy vegetation, including crops, and can kill livestock that eat the ash

covered vegetation.

Tephra falls can cause loss of agricultural activity for years after an eruption, a secondary

or tertiary effect.

Poisonous Gas Emissions

Volcanoes emit gases that are often poisonous to living organisms. Among these

poisonous gases are: Hydrogen Chloride (HCl), Hydrogen Sulfide (H2S), Hydrogen

Fluoride (HF), and Carbon Dioxide (CO2).

For instance in 1984, CO2 gas escaping from the bottom of Lake Monoun, a crater lake in

the African country of Cameroon, killed 37 people, in 1986 an even larger CO2 gas

emission from Lake Nyos in Cameroon killed more than 1700 people and 3000 cattle.

Secondary and Tertiary Effects of Volcanism

Mudflows (Lahars)

Volcanoes can emit voluminous quantities of loose, unconsolidated tephra which become

deposited on the landscape. Such loose deposits are subject to rapid removal if they are

exposed to a source of water. The source of water can be derived by melting of snow or

ice during the eruption, emptying of crater lakes during an eruption, or rainfall that takes

place any time with no eruption.

Thus, mudflows can both accompany an eruption and occur many years after an eruption.

Mudflows are a mixture of water and sediment, they move rapidly down slope along

existing stream valleys, although they may easily top banks and flood out into

surrounding areas.

They have properties that vary between thick water and wet concrete, and can remove

anything in their paths like bridges, highways, houses, etc.

E.g. Mt. St. Helens eruption of May 18, 1980, mudflows were generated as a result of

snow melt on the volcano itself, and deposition of tephra in streams surrounding the

mountain.

Debris Avalanches and Debris Flows

Volcanic mountains tend to become over-steepened as a result of the addition of new

material over time as well due to inflation of the mountain as magma intrudes.

Over-steepened slopes may become gravitationally unstable, leading to a sudden slope

failure that result in landslides, debris slides or debris avalanches.

Examples: May 18, 1980 eruption of Mt. St. Helens, Washington, a debris avalanche was

triggered by a magnitude 5.0 earthquake. The avalanche removed the upper 500 m of the

mountain, and flowed into the Spirit Lake, raising its level about 40 m. It then moved to

the west filling the upper reaches of the North Fork of the Toutle River valley.


Debris avalanches, landslides, and debris flows do not necessarily occur accompanied by

a volcanic eruption. There are documented cases of such occurrences where no new

magma has been erupted.

Flooding

Drainage systems can become blocked by deposition of pyroclastic flows and lava flows.

Such blockage may create a temporary dam that could eventually fill with water and fail

resulting in floods downstream from the natural dam.

Volcanoes in cold climates can melt snow and glacial ice, rapidly releasing water into the

drainage system and possibly causing floods..

Tsunami

Debris avalanche events, landslides, caldera collapse events, and pyroclastic flows

entering a body of water may generate tsunami.

For instance the 1883 eruption of Krakatau volcano, in the straits of Sunda between Java

and Sumatra, several tsunami were generated by pyroclastic flows entering the sea and by

collapse accompanying caldera formation. The tsunami killed about 36,400 people, some

as far away from the volcano as 200 km.

Volcanic Earthquakes and Tremors

Earthquakes usually precede and accompany volcanic eruptions, as magma intrudes and

moves within the volcano.

Although most volcanic earthquakes are small, some are large enough to cause damage in

the area immediately surrounding the volcano, and some are large enough to trigger

landslides and debris avalanches, such as in the case of Mount St. Helens.

Volcanic Tremor (also called harmonic tremor) is a type of continuous rhythmic shaking

of the ground that is generated by magma moving underground.

Atmospheric Effects

Since large quantities of tephra and volcanic gases can be injected into the atmosphere,

volcanism can have a short-term effect on climate.

Volcanic ash can cause reflection of solar radiation, and thus can cause the temperatures

to be cooler for several years after a large eruption.

E.g. eruption of Tambora volcano in Indonesia in 1815, the year 1816) was called the

"year without summer". Snow fell in New England in July.

Volcanic gases like SO2 also reflect solar radiation. Eruptions in 1981 at El Chichón

Volcano, Mexico, and 1991 at Pinatubo, Philippines, ejected large quantities of SO2 into

the atmosphere. The effects of the El Chichón eruption were masked by a strong El Niño

in the year following the eruption, but Pinatubo caused a lowering of average temperature

by about 1°C for two years following the eruption.

Volcanic gases like CO2 are greenhouse gases which help keep heat in the atmosphere.

Famine and Disease

As noted above, tephra falls can cause extensive crop damage and kill livestock. This

can lead to famine.


Displacement of human populations, breakdown of sewerage and water systems, cut off

of other normal services can lead to disease for years after an eruption, especially if the

infrastructure is not in place to provide for rapid relief and recovery.

Predicting Volcanic Eruptions

Definitions of commonly used terminologies

Active Volcano - An active volcano is a volcano that has shown eruptive activity within recorded

history. Thus an active volcano need not be in eruption to be considered active.

Currently there are about 600 volcanoes on Earth considered to be active volcanoes and

each year 50 to 60 of volcanoes actually erupts.

Extinct Volcano - An extinct volcano is a volcano that has not shown any historic activity, is

usually deeply eroded, and shows no signs of recent activity. How old must a volcano be to be

considered extinct depends to a large degree on past activity? E.g. Yellowstone Caldera is about

600,000 years old and is deeply eroded. But fumorolic activity, hot springs, and geysers all point

to the fact that magma still exists beneath the surface. Thus, Yellowstone Caldera is not

considered extinct.

Other volcanoes that are deeply eroded, smaller, and much younger and show no

hydrothermal activity may be considered extinct.

Dormant Volcano - A dormant volcano (sleeping volcano) is somewhere between active and

extinct. A dormant volcano is one that has not shown eruptive activity within recorded history,

but shows geologic evidence of activity within the geologic recent past.

Because the lifetime of a volcano may be on the order of a million years, dormant

volcanoes can become active volcanoes all of sudden. These are perhaps the most

dangerous volcanoes because people living in the vicinity of a dormant volcano may not

understand the concept of geologic time, and there is no written record of activity. These

people are sometimes difficult to convince when a dormant volcano shows signs of

renewed activity.

Long - Term Forecasting and Volcanic Hazards Studies

Studies of the geologic history of a volcano are generally necessary to make an

assessment of the types of hazards posed by the volcano and the frequency at which these

types of hazards have occurred in the past. The best way to determine the future behavior

of a volcano is by studying its past behavior as revealed in the deposits produced by

ancient eruptions. Because volcanoes have such long lifetimes relative to human recorded

history, geologic studies are absolutely essential.

Once this information is available, geologists can then make forecasts concerning what

areas surrounding a volcano would be subject to the various kinds of activity should they

occur in a future eruption, and also make forecasts about the long - term likelihood or

probability of a volcanic eruption in the area.

During such studies, geologists examine sequences of layered deposits and lava flows.

Armed with knowledge about the characteristics of deposits left by various types of

eruptions, the past behavior of a volcano can be determined.

Using radiometric age dating of the deposits the past frequency of events can be

determined.


This information is then combined with knowledge about the present surface aspects of

the volcano to make volcanic hazards maps which can aid other scientists, public

officials, and the public at large to plan for evacuations, rescue and recovery in the event

that short-term prediction suggests another eruption.

Such hazards maps delineate zones of danger expected from the hazards: lava flows,

pyroclastic flows, tephra falls, mudflows, floods, etc.

Short - Term Prediction based on Volcanic Monitoring

Seismic Exploration and Monitoring – Since seismic waves are generated by both

earthquakes and explosions, and since S-waves cannot pass through liquids, arrays of

seismographs can be placed around a volcano and small explosions can be set off to

generate seismic waves. If a magma body exists beneath the volcano, then there will be

zone were no S-waves arrive (an S-wave shadow zone) that can be detected. Monitoring

the movement of the S-wave shadow zone can delineate the position and movement of

the magma body.

Changes in Magnetic Field - Rocks contain minerals such as magnetite that are

magnetic. Such magnetic minerals generate a magnetic field. However, above a

temperature called the Curie temperature, these magnetic minerals show no magnetism.

Thus, if a magma body enters a volcano, the body itself will show no magnetism, and if it

heats the surrounding rocks to temperatures greater than the Curie temperature (about

500°C for magnetite) the magnetic field over the volcano will be reduced. Thus, by

measuring changes in the magnetic field, the movement of magma can sometimes be

tracked.

Changes in Electrical Resistivity - Rocks have resistance to the flow of electrical

current which is highly dependent on temperature and water content. As magma moves

into a volcano this electrical resistivity will decrease. Making measurements of the

electrical resistivity by placing electrodes into the ground, may allow tracking of the

movement of magma.

Ground Deformation - As magma moves into a volcano, the structure may inflate. This

will cause deformation of the ground which can be monitored. Instruments like tilt meters

measure changes in the angle of the Earth's surface which are measured in micro-radians

0.00018°. Other instruments track changes in distance between several points on the

ground to monitor deformation.

Changes in Groundwater System - As magma enters a volcano it may cause changes in

the groundwater system, causing the water table to rise or fall and causing the

temperature of the water to increase. By monitoring the depth to the water table in wells

and the temperature of well water, spring water, or fumaroles, changes can be detected

that may signify a change in the behavior of the volcanic system.

Changes in Heat Flow - Heat is everywhere flowing out of the surface of the Earth. As

magma approaches the surface or as the temperature of groundwater increases, the

amount of surface heat flow will increase. Although these changes may be small they are

measured using infrared remote sensing.

Changes in Gas Compositions - The composition of gases emitted from volcanic vents

and fumaroles often changes just prior to an eruption. In general, increases in the

proportions of hydrogen chloride (HCl) and sulfur dioxide (SO2) are seen to increase

relative to the proportion of water vapor.


TOPIC SIX

Mass-Wasting and Mass-Wasting Processes

Mass-Wasting and its Human Impacts

Mass-Wasting is defined as the down slope movement of rock and regolith near the Earth's

surface mainly due to the force of gravity. Mass-wasting is an important part of the erosional

process, as it moves material from higher elevations to lower elevations where transporting

agents like streams and glaciers can then pick up the material and move it to even lower

elevations. Mass-wasting processes are occurring continuously on all slopes; some act very

slowly, others occur very suddenly, often with disastrous results. Any perceptible down slope

movement of rock or regolith is often referred to in general terms as a landslide. Landslides,

however, can be classified in a much more detailed way that reflects the mechanisms responsible

for the movement and the velocity at which the movement occurs.

As human populations expand and occupy more and more of the land surface, mass-wasting

processes become more likely to affect humans.

In a typical year in the United States, landslides cause over $2 billion in damages and 25 to 50

deaths. In other countries, especially less developed countries; the loss is usually higher because

of higher population densities, lack of zoning laws, lack of information about mass-wasting

hazards, and lack of emergency preparedness. Between 1969 and 1993, worldwide, landslides

caused an average of about 1550 deaths per year.

Knowledge about the relationships between local geology and mass-wasting processes can lead

to better planning that can reduce vulnerability to such hazards. Thus, this section will look at the

various types of mass-wasting processes, their underlying causes, factors that affect slope

stability, and what humans can do to reduce vulnerability and risk due to mass-wasting hazards.

Types of Mass-Wasting Processes

The down-slope movement of material, whether it be bedrock, regolith, or a mixture of these, is

commonly referred to as a landslide. All of these processes generally grade into one another, so

classification of such processes is somewhat difficult. Mass- wasting classification is divided

mass-wasting processes into two broad categories: Slope failure and sediment flow

Slope Failures - a sudden failure of the slope resulting in transport of debris downhill by sliding,

rolling, falling, or slumping.

Slumps (also called Rotational Slides)- types of slides wherein downward rotation of

rock or regolith occurs along a concave upward curved surface (rotational slides). The

upper surface of each slump block remains relatively undisturbed, as do the individual

blocks. They often form as a result of human activities, and thus are common along roads

where slopes have been over steepened during construction. They are also common along

river banks and sea coasts, where erosion has under-cut the slopes. Heavy rains and

earthquakes can also trigger slumps.


Falls - Rock falls occur when a piece of rock on a steep slope becomes dislodged and

falls down the slope. Debris falls are similar, except they involve a mixture of soil,

regolith, vegetation, and rocks. A rock fall may be a single rock or a mass of rocks and

the falling rocks can dislodge other rocks as they collide with the cliff. Because this

process involves the free fall of material, falls commonly occur where there are steep

cliffs. At the base of most cliffs is an accumulation of fallen material termed talus.

Slides (also called Translational Slides) -Rock slides and debris slides result when rocks

or debris slide down a pre-existing surface, such as a bedding plane, foliation surface, or

joint surface (joints are regularly spaced fractures in rock that result from expansion

during cooling or uplift of the rock mass). Piles of talus are common at the base of a rock

slide or debris slide. Slides differ from slumps in that there is no rotation of the sliding

rock mass along a curved surface

Sediment Flows -

Sediment flows occur when sufficient force is applied to rocks and regolith that they begin to

flow down slope. A sediment flow is a mixture of rock, and/or regolith with some water or air.

They can be broken into two types depending on the amount of water present.

i. Slurry Flows- are sediment flows that contain between about 20 and 40% water.

As the water content increases above about 40% slurry flows grade into streams.

Slurry flows are considered water-saturated flows.

ii. Granular Flows - are sediment flows that contain between 0 and 20% water. Note

that granular flows are possible with little or no water. Fluid-like behavior is

given these flows by mixing with air. Granular flows are not saturated with water.

Each of these classes of sediment flows can be further subdivided on the basis of the velocity at

which flowage occurs.

Slurry Flows

Solifluction -flowage at rates measured on the order of centimeters per year of regolith

containing water. Solifluction produces distinctive lobes on hill slopes. These occur in

areas where the soil remains saturated with water for long periods of time.

Debris Flows - these occur at higher velocities than solifluction, with velocities between

1 meter/yr and 100 meters/hr and often result from heavy rains causing saturation of the

soil and regolith with water. They sometimes start with slumps and then flow down hill

forming lobes with an irregular surface consisting of ridges and furrows.

Mudflows - these are a highly fluid, high velocity mixture of sediment and water that has

a consistency ranging between soup-like and wet concrete. They move at velocities

greater than 1 km/hr and tend to travel along valley floors. These usually result from

heavy rains in areas where there is an abundance of unconsolidated sediment that can be

picked up by streams. Thus after a heavy rain streams can turn into mudflows as they

pick up more and more loose sediment. Mudflows can travel for long distances over


gently sloping stream beds. Because of their high velocity and long distance of travel they

are potentially very dangerous. As we have seen, mudflows can also result from volcanic

eruptions that cause melting of snow or ice on the slopes of volcanoes, or draining of

crater lakes on volcanoes. Volcanic mudflows are often referred to as lahars. Some

lahars can be quite hot, if they are generated as a result of eruptions of hot tephra.

Granular Flows

Creep - the very slow, usually continuous movement of regolith down slope. Creep occurs

on almost all slopes, but the rates vary. Evidence for creep is often seen in bent trees,

offsets in roads and fences, and inclined utility poles.

Earthflows - are usually associated with heavy rains and move at velocities between

several cm/yr and 100s of m/day. They usually remain active for long periods of time.

They generally tend to be narrow tongue-like features that begin at a scarp or small cliff.

Grain Flows - usually form in relatively dry material, such as a sand dune, on a steep

slope. A small disturbance sends the dry unconsolidated grains moving rapidly down

slope.

Debris Avalanches - These are very high velocity flows of large volume mixtures of rock

and regolith that result from complete collapse of a mountainous slope. They move down

slope and then can travel for considerable distances along relatively gentle slopes. They

are often triggered by earthquakes and volcanic eruptions.

Mass-Wasting in Cold Climates

Mass-wasting in cold climates is governed by the fact that water is frozen as ice during long

periods of the year. Ice, although it is solid, does have the ability to flow, and freezing and

thawing cycles can also contribute to movement.

Frost Heaving - this process is large contributor to creep in cold climates. When water

saturated soils freeze, they expand, pushing rocks and boulders on the surface upward

perpendicular to the slope. When the soil thaws, the boulders move down vertically

resulting in a net down slope movement.

Gelifluction - Similar to solifluction, this process occurs when the upper layers of soil

thaw during the warmer months resulting in water saturated soil that moves down slope.

Rock Glaciers - a lobe of ice-cemented rock debris (mostly rocks with ice between the

blocks) that slowly moves downhill.

Subaqueous Mass-Wasting

Mass wasting processes also occur on steep slopes in the ocean basins. A slope failure can occur

due to over-accumulation of sediment on slope or in a submarine canyon, or could occur as a

result of a shock like an earthquake. Slumps, debris flows, and landslides are common.


Slope Stability, Triggering Events, Mass Wasting Hazards

Factors that Influence Slope Stability

Gravity

The main force responsible for mass wasting is gravity. Gravity is the force that acts everywhere

on the Earth's surface, pulling everything in a direction toward the center of the Earth. On a flat

surface the force of gravity acts downward. So long as the material remains on the flat surface it

will not move under the force of gravity.

On a slope, the force of gravity can be resolved into two components: a component acting

perpendicular to the slope and a component acting tangential to the slope.

The perpendicular component of gravity, gp, helps to hold the object in place on the

slope. The tangential component of gravity, gt, causes a shear stress parallel to the slope

that pulls the object in the down-slope direction parallel to the slope.

On a steeper slope, the shear stress or tangential component of gravity, gt, increases, and

the perpendicular component of gravity, gp, decreases.

The forces resisting movement down the slope are grouped under the term shear strength

which includes frictional resistance and cohesion among the particles that make up the

object.

When the sheer stress becomes greater than the combination of forces holding the object

on the slope, the object will move down-slope.

Alternatively, if the object consists of a collection of materials like soil, clay, sand, etc., if

the shear stress becomes greater than the cohesional forces holding the particles

together, the particles will separate and move or flow down-slope.

Thus, down-slope movement is favored by steeper slope angles which increase the shear stress,

and anything that reduces the shear strength, such as lowering the cohesion among the particles

or lowering the frictional resistance. This is often expressed as the safety factor, Fs, the ratio of

shear strength to shear stress.

𝐹𝑠 = 𝑆ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑛𝑔ℎ𝑡 𝑆ℎ𝑒𝑎𝑟 𝑠𝑡𝑟𝑒𝑠𝑠

If the safety factor becomes less than 1.0, slope failure is expected


The Role of Water

Although water is not always directly involved as the transporting medium in mass-wasting

processes, it does play an important role.

Water becomes important for several reasons

i. Addition of water from rainfall or snow melt adds weight to the slope. Water can seep

into the soil or rock and replace the air in the pore space or fractures. Since water is

heavier than air, this increases the weight of the soil. Weight is force, and force is stress

divided by area, so the stress increases and this can lead to slope instability.

ii. Water has the ability to change the angle of repose (the slope angle which is the stable

angle for the slope). Consider building a sand castle on the beach. If the sand is totally

dry, it is impossible to build a pile of sand with a steep face like a castle wall. If the sand

is somewhat wet, however, one can build a vertical wall. If the sand is too wet, then it

flows like a fluid and cannot remain in position as a wall.

Dry unconsolidated grains will form a pile with a slope angle determined by the angle of

repose. The angle of repose is the steepest angle at which a pile of unconsolidated grains

remains stable, and is controlled by the frictional contact between the grains. In general,

for dry materials the angle of repose increases with increasing grain size, but usually lies

between about 30 and 37o.

Slightly wet unconsolidated materials exhibit a very high angle of repose because surface

tension between the water and the solid grains tends to hold the grains in place.

When the material becomes saturated with water, the angle of repose is reduced to very

small values and the material tends to flow like a fluid. This is because the water gets

between the grains and eliminates grain to grain frictional contact.

iii. Water can be adsorbed or absorbed by minerals in the soil. Adsorption causes the

electronically polar water molecule to attach itself to the surface of the minerals.

Absorption causes the minerals to take the water molecules into their structure. By adding

water in this fashion, the weight of the soil or rock is increased. Furthermore, if

adsorption occurs then the surface frictional contact between mineral grains could be lost

resulting in a loss of cohesion, thus reducing the strength of the soil.

In general, wet clays have lower strength than dry clays, and thus adsorption of water leads to

reduced strength of clay-rich soils.

iv. Water can dissolve the mineral cements that hold grains together. If the cement is made

of calcite, gypsum, or halite, all of which are very soluble in water, water entering the soil

can dissolve this cement and thus reduce the cohesion between the mineral grains.

v. Liquefaction - Liquefaction occurs when loose sediment becomes oversaturated with

water and individual grains loose grain to grain contact with one another as water gets

between them. This can occur as a result of ground shaking, as well as during exploration

of earthquakes, or can occur as water is added as a result of heavy rainfall or melting of


ice or snow. It can also occur gradually by slow infiltration of water into loose sediments

and soils.

The amount of water necessary to transform the sediment or soil from a solid mass into a liquid

mass varies with the type of material. Clay bearing sediments in general require more water

because water is first absorbed onto the clay minerals, making them even more solid-like, then

further water is needed to lift the individual grains away from each other.

vi. Groundwater exists nearly everywhere beneath the surface of the earth. It is water that

fills the pore spaces between grains in rock or soil or fills fractures in the rock. The water

table is the surface that separates the saturated zone below, wherein all pore space is

filled with water from the unsaturated zone above. Changes in the level of the water table

occur due changes in rainfall. The water table tends to rise during wet seasons when more

water infiltrates into the system, and falls during dry seasons when less water infiltrates.

Such changes in the level of the water table can have effects on the factors (1 through 5)

discussed above.

vii. Another aspect of water that affects slope stability is fluid pressure. As soil and rock get

buried deeper in the earth, the grains can rearrange themselves to form a more compact

structure, but the pore water is constrained to occupy the same space. This can increase

the fluid pressure to a point where the water ends up supporting the weight of the

overlying rock mass. When this occurs, friction is reduced, and thus the shear strength

holding the material on the slope is also reduced, resulting in slope failure.

Triggering Events for the occurrence of mass-wasting

A mass-wasting event can occur any time a slope becomes unstable. Sometimes, as in the case of

creep or solifluction, the slope is unstable all of the time and the process is continuous. But other

times, triggering events can occur that cause a sudden instability to occur. This section explores

major triggering events for mass wasting. However it is important to note that a slope is very

close to instability, only a minor event may be necessary to cause a failure and disaster. This may

be something as simple as an ant removing the single grain of sand that holds the slope in place.

Shocks - A sudden shock, such as an earthquake may trigger slope instability. Minor

shocks like heavy trucks rambling down the road, trees blowing in the wind, or human

made explosions can also trigger mass-wasting events.

Slope Modification - Modification of a slope either by humans or by natural causes can

result in changing the slope angle so that it is no longer at the angle of repose. A mass-

wasting event can then restore the slope to its angle of repose.

Undercutting – streams eroding their banks or surf action along a coast can undercut a

slope making it unstable.

Changes in Hydrologic Characteristics - heavy rains can saturate regolith reducing grain

to grain contact and reducing the angle of repose, thus triggering a mass-wasting event.

Heavy rains can also saturate rock and increase its weight. Changes in the groundwater

system can increase or decrease fluid pressure in rock and also triggers mass-wasting

events.


Volcanic Eruptions - produce shocks like explosions and earthquakes. They can also

cause snow to melt or empty crater lakes, rapidly releasing large amounts of water that

can be mixed with regolith to reduce grain to grain contact and result in debris flows,

mudflows, and landslides.

Assessing and Mitigating Mass-Wasting Hazards

Mass-wasting events can be extremely hazardous and can result in extensive loss of life and

property. But, in most cases, areas that are prone to such hazards can be recognized with some

geologic knowledge, slopes can be stabilized or avoided, and warning systems can be put in

place to minimize such hazards.

Assessing the case histories of mass-wasting disasters, the conditions present then can tell

us that a hazardous condition existed prior to the event.

Because there is usually evidence in the form of distinctive deposits and geologic

structures left by recent mass wasting events, it is possible, if resources are available, to

construct maps of all areas prone to possible mass-wasting hazards. Planners can use such

hazards maps to make decisions about land use policies in such areas or steps can be

taken to stabilize slopes to attempt to prevent a disaster.

Short-term prediction of mass-wasting events is somewhat more problematical. For

earthquake triggered events, the same problems that are inherent in earthquake prediction

are present. Slope destabilization and undercutting triggered events require the constant

attention of those undertaking or observing the slopes, many of whom are not educated in

the problems inherent in such processes.

Mass-wasting hazards from volcanic eruptions can be predicted with the same degree of

certainty that volcanic eruptions can be predicted. However, the threat has to be realized

and warnings need to be heeded. Hydrologic conditions such as heavy precipitation can

be forecast with some certainty, and warnings can be issued to areas that might be

susceptible to mass-wasting processes caused by such conditions. Still, it is difficult of

know exactly which hill slope of the millions that exist will be vulnerable to an event

triggered by heavy rainfall.

Prevention and Mitigation

All slopes are susceptible to mass-wasting hazards if a triggering event occurs. Thus, all slopes

should be assessed for potential mass-wasting hazards. Mass-wasting events can sometimes be

avoided by employing engineering techniques to make the slope more stable. Among them are:

Steep slopes can be covered or sprayed with concrete covered with a wire mesh to

prevent rock falls.

Retaining walls could be built to stabilize a slope.

If the slope is made of highly fractured rock, rock bolts may be emplaced to hold the

slope together and prevent failure.


Drainage pipes could be inserted into the slope to more easily allow water to get out and

avoid increases in fluid pressure, the possibility of liquefaction, or increased weight due

to the addition of water.

Over-steepened slopes could be graded to reduce the slope to the natural angle of repose.

In mountain valleys subject to mudflows, plans could be made to rapidly lower levels of

water in human-made reservoirs to catch and trap the mudflows.

Some slopes, however, cannot be stabilized. In these cases, humans should avoid these areas or

use them for purposes that will not increase susceptibility of lives or property to mass-wasting

hazards.

Subsidence: Dissolution & Human Related Causes

Surface Subsidence and Collapse

Subsidence hazards involve either the sudden collapse of the ground to form a depression or the

slow subsidence or compaction of the sediments near the Earth's surface. Sudden collapse events

are rarely major disasters, certainly not anywhere near the scale of the earthquake, volcanic,

tsunami, or landslide disasters, but the slow subsidence of areas can cause as much economic

damage, although spread out over a longer period of time.

The most common type of sudden collapse is due to erosion of underground soil or rock caused

by leaking human-made sewer pipes or water mains. This rarely destroys large areas, but

commonly swallows up vehicles. The second most common type of sudden collapse involves

dissolution of carbonate rocks (limestones) beneath the surface.

Carbonate Dissolution and Karst Topography

Carbonate rocks such as limestone, composed mostly of the mineral calcite (CaCO3) are very

susceptible to dissolution by groundwater during the process of chemical weathering. Such

dissolution can result in systems of caves, sinkholes, and eventually to karst topography.

Dissolution

Water in the atmosphere can dissolve small amounts of carbon dioxide (CO2). These results in

rain water having a small amount of carbonic acid (H2CO3) when it falls on the Earth's surface.

As the water infiltrates into the groundwater system and encounters carbonate rocks like

limestone, it may start to dissolve the calcite in the limestone by the following chemical reaction:

which states that calcite reacts with carbonic acid to produce dissolved Calcium ion plus

dissolved Bicarbonate ion.

This reaction takes place as the water moves along fractures and other partings or openings in the

rock. This results in dissolution of much of the limestone if the reaction continues to take place

over a long period of time.


Caves & Cave Formation

Caves are large underground open spaces. If there are many interconnected chambers in a

cave system, it is called a cavern. Most caves are formed by the chemical dissolution

process described above, as a result of circulating groundwater. The dissolution begins

along fracture systems in the rock, widening the fractures and connecting them to other

fractures, until a cave is formed.

Most caves are thought to form near the water table (the surface below which all open

space in rock is filled with water), and thus the openings are initially filled with water.

After the water table is lowered due to changing geologic conditions, further seepage of

water into the now open cave system results in the deposition of stalactites (icicle like

stones) where the water drips into the cave. If water is absent from the floor of the cave

stalagmites form where the water drips on the floor of the cave. Both stalactites and

stalagmites are composed of newly precipitated calcite, initially dissolved from the

limestone above, carried in the groundwater, and re-precipitated when the water reaches a

low pressure area like a cave.

The rate at which caves form depends on such factors as the acidity of the water and the

velocity at which the water moves through the rock. Highly acidic water and high flow

velocity increase the rate of dissolution, and thus the rate at which a cave forms.

Sinkholes

A sinkhole is a large dissolution cavity that is open to the Earth's surface. Some sinkholes

form when the roofs of caves collapse, others can form at the surface by dissolving the

rock downward.

Sinkholes are common in areas underlain by limestone. For instance, Central Florida is

one small area of about 25 km2; over 1000 sinkholes have formed by collapse in recent

years.

Sinkholes may form as a result of lowering the water table by excessive pumping for

human use of the water.

Sinkholes may also form by slow enlargement of caverns by continued dissolution of the

limestone. This may no matter what the level of the water table.

When sinkholes collapse to expose the water table at the surface, the sinkhole will be

filled with water forming small circular lakes.

Although common in areas underlain by limestone, sinkholes can form in any area where

highly water soluble rocks occur close to the surface. Such rocks include rock salt made

of the mineral halite, and gypsum deposits, both of which easily dissolve in groundwater.

Karst Topography

In areas where highly water soluble materials lie close the surface, dissolution below the surface

can eventually lead to the formation of caverns and sinkholes. As the sinkholes begin to

coalesce, the surface topography will become chaotic, with many enclosed basins, and streams

that disappear into sinkholes, run underground and reappear at springs. Such a chaotic


topography is known as karst topography. Karst topography starts out as an area with many

sinkholes, but eventually, as weathering and dissolution of the underlying rock continue, the

ground surface may be lowered, and areas that have not undergone extensive dissolution stand

up as towering pillars above the surrounded terrain. The latter type of karst is called tower karst.

Removal of Solids and Mine Related Collapse

Humans can play a large role causing collapse of the surface. Mining activities that remove

material from below the surface can result in collapse if precautions are not taken to ensure that

the there is adequate support for the overlying rocks.

Removal of Salt-

Salt occurs beneath the surface in areas that were once below sea level in restricted basins

where extensive evaporation caused the concentration of salt in seawater to become so

high that the salt was precipitated on the bottom. Deposition of other sediments on top of

this salt resulted in low density salt underlying higher density sediments. Since salt is

rather ductile, it will flow upward toward the surface, and in many cases became

detached from the original layer of salt at depth to form what is called a salt dome. Since

the salt now occurs close to the surface, it can dissolve and collapse to form sinkholes.

The salt can also be mined to produce salt for human usage. One mining technique

involves injecting fluids into the salt to dissolve it. The fluids are then recovered and the

salt re-precipitated from the solutions. Such mining, because it dissolves large cavities in

the salt can lead to instability and collapse.

Coal Mining

Since mining often removes material from below the surface without dissolution, mining can

create voids that may become unstable and collapse.

Coal occurs beneath the surface as extensive layers called coal seams. These seams were

once swampy areas on the surface where much vegetation flourished, died, and became

buried before it could decay. Processes acting over long periods of geologic time have

turned dead vegetation into coal. Other useful substances are mined by digging tunnels in

rock, but in most mining techniques the useful substance occurs along narrow zones only

these enriched zones need to be removed.

In mining coal, however, all of the material is useful, so large masses of material are

removed. The technique used in coal mining is referred to as "room-and-pillar" mining.

The rooms are where the coal has been removed, and the pillars are left to support the

overlying rock. Sometimes, too few pillars are left, and the overlying rock collapses into

the mine. This is not only dangerous to the miners, but can also cause hazards to areas on

the surface where the collapse occurs

Underground fires in coal mines can also lead to collapse hazards. Fires can start by

spontaneous ignition of coal dust or methane gas released from the coal. Such fires are

difficult to extinguish, and often are left to burn for years. In Pennsylvania, for example,

coal mine fires have burned for more than 25 years. Burning of the coal results in


removal of the coal, and thus may undermine support for the roof of the mine resulting in

collapse.

Subsidence Caused by Fluid Withdrawal

We have seen how fluids (particularly water) in the subsurface can dissolve rock to undermine

support and cause collapse of the surface. This part explores the role of fluids in causing

subsidence. Any fluid that exists in the pore spaces or fractures of rock is under pressure due to

the weight of the overlying rock. So long as the pressure of the fluid is enough to support the

overlying rock, no subsidence at the surface will occur. But, if fluids are withdrawn from below

the surface, a decrease in fluid pressure may occur resulting in the removal of support and

possible collapse. The two most important fluids that occur beneath the surface are water (in the

form of groundwater) and petroleum (in the form of oil and natural gas). Both of these fluids

are often withdrawn for human use, and thus humans are often responsible for fluid withdrawal

related subsidence. But, such withdrawal can also occur by natural processes.

Groundwater

Groundwater occurs nearly everywhere below the surface of the Earth, where, as we have

said before, it fills the pore spaces and fractures in rock at levels below the water table.

The zone beneath the water table is called the saturated zone. Groundwater flows into the

saturated zone by percolation downward from rainfall on the surface. Surface bodies of

water, like streams, lakes, and swamps, are areas where the water table is exposed at the

surface. Springs are also areas where the water table is exposed at the surface. If one digs

or drills a well to intersect the water table, water will flow into the well and fill it to the

level of the water table. The level of the water table can change as a result of changing

amounts of input in the groundwater system (called recharge) and output from the

groundwater system (called discharge). Recharge takes place by water infiltrating down

from the surface. Discharge occurs as a result of outflow through surface bodies of water,

springs, and wells. During the wet season the water table is generally higher because

recharge exceeds discharge. During dry seasons the water table is depressed because

discharge exceeds recharge. Likewise, during periods of drought the water will be lower.

Groundwater moves through the saturated zone both downward and upward. The

downward flow occurs due to gravity and the upward flow occurs because fluids tend to

flow towards areas of lower pressure.

Subsidence can be caused by any process that results in lowering of the water table. So,

drought, dry seasons, and excessive withdrawal of groundwater by humans can cause the

water table to move to deeper levels and result in subsidence.

Most subsidence occurs as a result of hydro-compaction. Hydro-compaction occurs when

the sediments loose water. Since lowering of the water table involves loss of water, hydro

compaction often occurs.


Hydro compaction means that water absorbed on and within clay minerals are removed

by withdrawal or drying, and the clays shrink. Shrinkage of clays results in less volume,

so the surface will subside as the clays become more tightly compacted.

Hydro compaction also occurs when organic rich sediment like peat is subjected to loss

of water by draining for agricultural use.

Most hydro compaction is an elastic deformation process. Elastic deformation is

reversible, so that when the clays or peat dry out they compact, but when they become

wet once again, they expand. Compaction, however, can become inelastic, which is not

reversible. In such a case as the pores are closed by compaction, they cannot be restored

when new fluids are pumped in.

The rate of subsidence relative to rate of fluid withdrawal can sometimes show when

material passes from elastic compaction to inelastic compaction. If the rate of fluid

withdrawal is large yet the rate of subsidence is small, this is usually an indication of

elastic compaction. If, however, there is a large amount of subsidence with only small

amounts of fluid withdrawal, inelastic compaction is likely occurring.

Oil & Gas

Oil and Natural gas are both fluids that can exist in the pore spaces and fractures of rock, just like

water. When oil and natural gas are withdrawn from regions in the Earth near the surface, fluid

pressure provided by these fluids is reduced, With a reduction in fluid pressure, the pore spaces

begin to close and the sediment may start to compact resulting in subsidence of the surface.

Sinking Cities

Cities built on unconsolidated sediments consisting of clays, silt, peat, and sand are particularly

susceptible to subsidence. Such areas are common in delta areas, where rivers empty into the

oceans, along floodplains adjacent to rivers, and in coastal marsh lands. In such settings,

subsidence is a natural process. Sediments deposited by the rivers and oceans get buried, and the

weight of the overlying, newly deposited sediment, compacts the sediment and the material

subsides. Building cities in such areas aggravates the problem for several reasons.

a. Construction of buildings and streets adds weight to the region and further compacts the

sediment.

b. Often the areas have to be drained in order to be occupied. This results in lowering of the

water table and leads to hydrocompaction.

c. Often the groundwater is used as a source of water for both human consumption and

industrial use. This also results in lowering the water table and further hydrocompaction.

d. Levees and dams are often built to prevent or control flooding. This shuts off the natural

supply of new sediment to the area. In a natural setting sedimentation resulting from

floods helps replenish the sediment that subsides and thus builds new material over the

subsiding sediment, decreasing the overall rate of subsidence. When the sediment supply

is cut off, the replenishment does not occur and the rate of subsidence in enhanced.


Predicting and Mitigating Subsidence Hazards

The exact place and time of a disaster related to subsidence cannot usually be predicted with any

degree of certainty. This is true of both slow subsidence related to fluid withdrawal and sudden

subsidence related to sinkhole formation or mine collapse. Mitigation is the best approach to

these hazards. In an ideal world, all areas susceptible to such hazards would be well known and

actions would be taken to either avoid causing the problem if it is human related, or avoid

inhabitance of such areas if they are prone to natural subsidence.

For subsidence caused by sudden collapse of the ground to form sinkholes, several

measures can be taken. First, geologists can make maps of areas known to be underlain

by rocks like limestone, gypsum, or salt, that are susceptible to dissolution by fluids.

Based on knowledge of the areas, whether active dissolution is occurring or has occurred

in the recent past, and knowing something about the depth below the surface where these

features occur, hazard maps can be constructed.

Once these areas have been identified, detailed studies using drill holes, or ground

penetrating radar can be used to locate open cavities beneath the surface. These areas can

then be avoided when it comes time for decisions about land use.

In areas where there is a possibility of sudden collapse, one should be aware of any

cracks that form in the ground especially if the cracks start to form a circular or elliptical

pattern. Such ground cracking may be an indication that a collapse event is imminent.

In areas located above known mining operations or former mining operations, maps can

be constructed based on knowledge of the actual locations of open cavities beneath the

surface. Such maps can then be used as a guide for land use planning and formulation of

legislations governing mining in certain places e.g. mining beneath urban areas.

Where fluid withdrawal is the main cause of subsidence, information on the rate of fluid

withdrawal should be determined and combined with studies of the material in the

subsurface based on sampling with drill core methods. If subsidence is suspected or

observed, human activities can be modified to prevent further subsidence. For example

new sources of water can often be found, or waste water can be treated and pumped back

into the ground to help maintain the level of the water table, maintain fluid pressure, re-

hydrate hydro compacting clays and peat.


TOPIC SEVEN

The Ocean-Atmosphere System

The oceans and the atmosphere are the two large reservoirs of water in the Earth's hydrologic

cycle. The two systems are complexly linked to one another and are responsible for Earth's

weather and climate. The oceans help to regulate temperature in the lower part of the

atmosphere. The atmosphere is in large part responsible for the circulation of ocean water

through waves and currents.

Weather and Climate

Weather is the condition of the atmosphere at a particular time and place. It refers to such

conditions of the local atmosphere as temperature, atmospheric pressure, humidity (the amount

of water contained in the atmosphere), precipitation (rain, snow, sleet, & hail), and wind

velocity. Because the amount of heat in the atmosphere varies with location above the Earth's

surface, and because differing amounts of heat in different parts of the atmosphere control

atmospheric circulation, the atmosphere is in constant motion. Thus, weather is continually

changing in a complex and dynamic manner.

Climate refers to the average weather characteristics of a given region. Climate, although it does

change over longer periods of geologic time, is more stable over short periods of time like years

and centuries. The fact that the Earth has undergone fluctuation between ice ages and warmer

periods in the recent past (the last ice age ended about 10,000 years ago) is testament to the fact

that climate throughout the world as has been changing through time.

The Earth's weather and climate system represent complex interactions between the oceans, the

land, the sun, and the atmosphere. That these interactions are complex is evidence by the

difficulty meteorologists have in predicting weather on a daily basis. Understanding climate

change is even more difficult because humans have not been around long enough to record data

on the long term effects of these processes. Still, we do know that the main energy source for

changing weather patterns and climate is solar energy from the Sun.

The Atmosphere

Earth's atmosphere consists of a mixture of Nitrogen (N2) and Oxygen (O2). At the Earth's

surface, dry air is composed of about 79% Nitrogen, 20% Oxygen, and 1% Argon. It can also

contain up to 4% water vapor at saturation, but saturation depends on temperature. Relative

humidity is the term used to describe saturation with water vapor. When the relative humidity is

100%, the atmosphere is saturated with respect to water vapor, and precipitation results. Other

gases occur in the atmosphere in small amounts. Among the most important of these other gases

is Carbon Dioxide (CO2).

The atmosphere has a layered structure and each layer is defined on the basis of properties such

as pressure, temperature, and chemical composition.

The layer closest to the surface is called the troposphere, which extends to an altitude of

10 to 15 km. Temperature decreases upward in the troposphere to the tropopause (the

boundary between the troposphere and the next layer up, the stratosphere).

The troposphere contains about 90% of the mass of the atmosphere, including nearly all

of the water vapor. Weather is controlled mostly in the troposphere.


Solar Radiation and the Atmosphere

Radiation reaching the Earth from the Sun is electromagnetic radiation. Electromagnetic

radiation can be divided into different regions depending on wavelength. Note that visible light is

the part of the electromagnetic spectrum to which human eyes are sensitive. Earth receives all

wavelengths of solar radiation. But certain gases and other contaminants in the atmosphere have

different effects on different wavelengths of radiation.

.. Dry air is composed of about 79% Nitrogen, 20% oxygen, and 1% Argon. It also contains

water, 4% at saturation, but saturation depends on temperature. In addition trace gases have an

effect, the most important of which are the greenhouse gasses.

Greenhouse Gases

Energy coming from the Sun is carried by electromagnetic radiation. Some of this radiation is

reflected back into space by clouds and dust in the atmosphere. The rest reaches the surface of

the Earth, where again it is reflected by water and ice or absorbed by the atmosphere.

Greenhouse gases in the atmosphere absorb some of the longer wavelength (infrared) radiation

and keep some of it in the atmosphere. This keeps the atmospheric temperature relatively stable

so long as the concentration of greenhouse gases remains relatively stable, and thus, the

greenhouse gases are necessary for life to exist on Earth.

The most important green house gases are H2O (water vapor), CO2 (Carbon Dioxide), CH4

(methane), and Ozone. H2O is the most abundant greenhouse gas, but its concentration in the

atmosphere varies with temperature. The CO2 concentration in the atmosphere has been

increasing since the mid 1800s. The increase correlates well with burning of fossil fuels. Thus,

humans appear to have an effect. Methane concentration in the atmosphere has also been

increasing. Naturally this occurs due to decay of organic matter, the digestive processes of

organisms, and leaks from petroleum reservoirs. Humans have contributed through

domestication of animals, increased production of rice, and leaks from gas pipelines and

gasoline.

Volcanic Effects

Volcanoes produce several things that result in changing atmosphere and atmospheric

temperatures.

a. CO2 produced by volcanoes adds to the greenhouse gases and may result in warming of

the atmosphere.

b. Sulfur gases produced by volcanoes reflect low wavelength radiation back into space, and

thus result in cooling of the atmosphere.

c. Dust particles injected into the atmosphere by volcanoes reflect low wavelength radiation

back into space, and thus can result in cooling of the atmosphere.

d. Chlorine gases produced by volcanoes can contribute to ozone depletion in the upper

atmosphere.

The Mt. Pinatubo eruption in 1991 and El Chichón eruption in 1981 released large quantities of

dust and sulfur gases - resulted in short term cooling of atmosphere.


The Carbon Cycle

To understand whether or not humans are having an effect on atmospheric carbon

concentrations, it is important to explore how carbon moves through the environment. Carbon is

stored in four main reservoirs.

a). In the atmosphere as CO2 gas. From here it exchanges with seawater or water in the

atmosphere to return to the oceans, or exchanges with the biosphere by photosynthesis,

where it is extracted from the atmosphere by plants. CO2 returns to the atmosphere by

respiration from living organisms, from decay of dead organisms, from weathering of

rocks, from leakage of petroleum reservoirs, and from burning of fossil fuels by humans.

b). In the hydrosphere (oceans and surface waters) as dissolved CO2. From here it

precipitates to form chemical sedimentary rocks, or is taken up by organisms to enter the

biosphere. CO2 returns to the hydrosphere by dissolution of carbonate minerals in rocks

and shells, by respiration of living organisms, by reaction with the atmosphere, and by

input from streams and groundwater.

c). In the biosphere where it occurs as organic compounds in organisms. CO2 enters the

biosphere mainly through photosynthesis. From organisms it can return to the atmosphere

by respiration and by decay when organisms die, or it can become buried in the Earth.

d). In the Earth's lithosphere as carbonate minerals, graphite, coal, petroleum. From here it

can return to the atmosphere by weathering, volcanic eruptions, hot springs, or by human

extraction and burning to produce energy.

Global Warming

Average global temperatures vary with time as a result of many processes interacting with each

other. These interactions and the resulting variation in temperature can occur on a variety of time

scales ranging from yearly cycles to those with times measured in millions of years. Such

variation in global temperatures is difficult to understand because of the complexity of the

interactions and because accurate records of global temperature do not go back more than 100

years.

Effects of Global Warming

Among the effects of global warming are:

Global Precipitation changes - A warmer atmosphere leads to increased evaporation from

surface waters and results in higher amounts of precipitation. Equatorial regions will be

wetter than present, while interior portions of continents will become warmer and drier

than present.

Changes in vegetation patterns - because rainfall is distributed differently, vegetation will

have to adjust to the new conditions. Mid latitude regions become more drought prone,

while higher latitude regions become wetter and warmer, resulting in a shift in

agricultural patterns.

Increased storminess - A warmer, wetter atmosphere favors tropical storm development.

Tropical Cyclones will be stronger and more frequent.

Reduction of sea ice - Sea ice is greatly reduced due to higher temperatures at the high

latitudes, particularly in the northern hemisphere where there is more abundant sea ice.


Thawing of frozen ground (permafrost) - Currently much of the ground at high latitudes

remains frozen all year. Increased temperatures will cause much of this ground to thaw.

Organic compounds in the frozen ground will be subject to decay, releasing more

methane into the atmosphere and enhancing the greenhouse effect. Ecosystems and

human structures currently built on frozen ground will have to adjust.

Rise of sea level - Warming the oceans results in expansion of water and thus increases

the volume of water in the oceans. Along with melting of mountain glaciers and

reduction in sea ice, this will cause sea level to rise and flood coastal zones.

Changes in the hydrologic cycle - With new patterns of precipitation changes in stream

flow and groundwater level will be expected.

Climate Change

Because human history is so short compared to the time scales on which global climate change

occurs, we do not completely understand the causes. However, we can suggest a few reasons

why climates fluctuate.

Long term variations in climate (tens of millions of years) on a single continent are likely

caused by drifting continents. If a continent drifts toward the equator, the climate will

become warmer. If the continent drifts toward the poles, glaciations can occur on that

continent.

Short-term variations in climate are likely controlled by the amount of solar radiation

reaching the Earth. Among these are astronomical factors and atmospheric factors.

Astronomical Factors -

Variation in the eccentricity of the Earth's orbit around the sun has periods of about

400,000 years and 100,000 years.

Variation in the tilt of the Earth's axis has a period of about 41,000 years.

Variation in the way the Earth wobbles on its axis, called precession, has a period of

about 23,000 years.

The combined effects of these astronomical variations results in periodicities similar to

those observed for glacial - interglacial cycles.

Atmospheric Factors- the composition of the Earth's atmosphere can be gleaned from air

bubbles trapped in ice in the polar ice sheets. Studying drill core samples of such glacial ice and

their contained air bubbles reveals the following:

During past glaciations, the amount of CO2 and methane, both greenhouse gasses that

tend to cause global warming, were lower than during interglacial episodes.

During past glaciations, the amount of dust in the atmosphere was higher than during

interglacial periods, thus more heat was likely reflected from the Earth's atmosphere back

into space.

The problem in unraveling what this means comes from not being able to understand if

low greenhouse gas concentration and high dust content in the atmosphere caused the ice

ages or if these conditions were caused by the ice ages.

Changes in Oceanic Circulation - small changes in ocean circulation can amplify small

changes in temperature variation produced by astronomical factors.


Other factors

The energy output from the sun may fluctuate.

Large explosive volcanic eruptions can add significant quantities of dust to the

atmosphere reflecting solar radiation and resulting in global cooling.

Circulation in the Atmosphere

The troposphere undergoes circulation because of convection. Convection in the atmosphere is

mainly the result of the fact that more of the Sun's heat energy is received by parts of the Earth

near the Equator than at the poles. Thus air at the equator is heated reducing it’s the density.

Lower density causes the air to rise. At the top of the troposphere this air spreads toward the

poles.

Areas where warm air rises and cools are centers of low atmospheric pressure. In areas where

cold air descends back to the surface, pressure is higher and these are centers of high

atmospheric pressure.

The Coriolis Effect - Since the Earth is in fact rotating, atmospheric circulation patterns are

much more complex because of the Coriolis Effect. The Coriolis Effect causes any body that

moves on a rotating planet to turn to the right (clockwise) in the northern hemisphere and to the

left (counterclockwise) in the southern hemisphere. The effect is negligible at the equator and

increases both north and south toward the poles. The Coriolis Effect occurs because the Earth

rotates out from under all moving bodies like water, air, and even airplanes. Note that the

Coriolis effect depends on the initial direction of motion and not on the compass direction

Tropical Cyclones (Hurricanes) Tropical Cyclones are massive tropical cyclonic storm systems with winds exceeding 119 km/hr

(74 miles/hour). The same phenomena is given different names in different parts of the world. In

the Atlantic Ocean and eastern Pacific ocean they are called hurricanes. In the western Pacific

they are called typhoons, and in the southern hemisphere they are called cyclones. But, no

matter where they occur they represent the same process. Tropical cyclones are dangerous

because of their high winds, the storm surge produced as they approach a coast, and the severe

thunderstorms associated with them. Tropical Cyclones commonly develop in areas near, but

not at the equator

Tropical Cyclone Structure

Because the converging winds spiral inward toward the central low pressure area, the winds

rotate in a counterclockwise direction around the central low in the northern hemisphere

(clockwise in the southern hemisphere). As these winds spiral inward they draw in the

thunderclouds around the storm, creating the spiral rain bands that are clearly visible on satellite

images of the storm and as the winds converge toward the central core, they spiral upwards,

sending warm moist air upwards. As this air rises, it cools and releases its latent heat into the

atmosphere to add further energy to the storm. The winds spiraling around this central core

create the eye of the tropical cyclone and eventually spread out at high altitudes. Eventually, cool

air above the eye begins to sink into the central core. This dry descending air within the eye

gives the core a clear, cloud free sky, with little to no wind.


Since the main source of energy for the storm is the heat contained in the warm tropical and

subtropical oceans, if the storm moves over the land, it is cut off from its source of heat and will

rapidly begin to dissipate.

Winds spiraling counterclockwise (in the northern hemisphere) into the eye of the hurricane

achieve high velocities as they approach the low pressure of the eye. The velocity of these winds

is called the hurricane-wind velocity. The central low pressure center of the eye also moves

across the surface of the Earth as it is pushed by regional winds. The velocity at which the eye

moves across the surface is called the storm center velocity.

Tropical Cyclone Size

Since winds spiral inward toward the central low pressure area in the eye of a hurricane,

hurricane-wind velocity increases toward the eye. The distance outward from the eye to which

hurricane strength winds occur determines the size of the hurricane. Winds in the eye wall itself

have the highest velocity and this zone can extend outward from the center to distances of 16 to

40 km. Hurricane force winds (winds with velocities greater than 119 km/hr) can extend out to

120 km from the center of the storm. The largest tropical cyclone recorded, Typhoon Tip, had

gale force winds (54 km/hr) which extended out for 1100 km in radius in the Northwest Pacific

in 1979. Hurricane Katrina, in 2005, was a large hurricane with tropical storm force winds

extending outward from the eye about 320 km.

Hurricane Intensity and Frequency

Once a hurricane develops, the Saffir-Simpson Scale is used to classify a hurricane's intensity

and damage potential. Because a hurricane derives its energy from the warm ocean waters in the

topics and subtropics, hurricanes are more frequent in the late summer months e.g. in the Atlantic

ocean, they are more frequent in the months of August, September and October.

Hurricane Damage

Hurricanes cause damage as a result of the high winds, the storm surge, heavy rain, and

tornadoes that are often generated from the thunderstorms as they cross land areas. Strong winds

can cause damage to structures, vegetation, and crops. The collapse of structures can cause death.

The storm surge and associated flooding, however, is what is most responsible for casualties. For

instance extreme cases of storm surge casualties have occurred as recently as 1970 and 1990 in

Bangladesh and 2008 in Myanmar. Bangladesh is an area with high population density and with

over 30% of the land surface less than 6 m above sea level. In 1970 a cyclone struck Bangladesh

during the highest high tides (full moon). The storm surge was 7 m (23 ft.) high and resulted in

about 400,000 deaths. Another cyclone in 1990 created a storm surge 6 m high and resulted in

148,000 deaths. The May 2008 cyclone in Myanmar is estimated to have killed 138,000.

The amount of damage caused by a tropical cyclone is directly related to the intensity of the

storm, the duration of the storm, the angle at which it approaches the land, and the population

density along the coastline.

Predicting Hurricane Frequency and Intensity

Modern methods of weather forecasting involving satellites, radar, etc. allow accurate

tracking of the development and paths of hurricanes. In addition, computer models have


been developed to enable the prediction of storm surge levels given data on wind

velocity, wind distribution, and storm center velocity. Computer models have also been

developed to predict the paths the storms will take and have met with moderate success.

Accurate forecasting of storm tracks is more problematical because of the numerous

variables involved and the erratic paths hurricanes sometimes take.

Prediction of hurricane intensity (wind speed) is more problematic as too many factors

are involved. Hurricanes are continually changing their intensity as they evolve and move

into different environments. Without the ability to know which environmental factors are

going to change, it is very difficult to expect improvement on intensity forecasting.

Reducing Hurricane Damage

In terms of protection of human life, the best possible solution is to evacuate areas before

a hurricane and its associated storm surge reaches coastal areas.

Other measures can be undertaken to reduce hurricane damage as well.

Warning and Evacuation - With modern techniques of forecasting and tracking hurricane

paths, it is always possible to issue warnings about the probable locations that will be

affected by any given hurricane


TOPIC EIGHT

River Systems & Causes of Flooding

Throughout history humans have found it desirable to construct cities along streams. Streams are

sources of water for consumption, agriculture, and industry. Streams provide transportation

routes, energy, and a means of disposal of wastes. Stream valleys offer a relatively flat area for

construction. But, human populations that live along streams also have the disadvantage that the

flow of water in streams is never constant. High amounts of water flowing in streams often leads

to flooding, and flooding is one of the more common and costly types of natural disasters.

A flood results when a stream runs out of its confines and submerges surrounding areas.

In less developed countries, humans are particularly sensitive to flood casualties because

of high population density, absence of zoning regulations, lack of flood control, and lack

of emergency response infrastructure and early warning systems.

In industrialized countries the loss of life is usually lower because of flood control

structures, zoning regulations that prevent the habitation of seriously vulnerable lands,

and emergency preparedness. Still, property damage and disruption of life takes a great

toll, and despite flood control structures and land use planning, floods still do occur.

Causes of Flooding

From a geological perspective, floods are a natural consequence of stream flow in a continually

changing environment. Floods have been occurring throughout Earth history, and are expected so

long as the water cycle continues to run. Streams receive most of their water input from

precipitation, and the amount of precipitation falling in any given drainage basin varies from day

to day, year to year, and century to century.

The Role of Precipitation

Weather patterns determine the amount and location of rain and snowfall. Unfortunately the

amount and time over which precipitation occurs is not constant for any given area. Overall, the

water cycle is a balanced system. Water flowing into one part of the cycle (like streams) is

balanced by water flowing back to the ocean. But sometimes the amount flowing in to one area is

greater than the capacity of the system to hold it within natural confines. The result is a flood.

Combinations of factors along with exceptional precipitation can also lead to flooding. For

example, heavy snow melts, water saturated ground, unusually high tides, and drainage

modifications when combined with heavy rain can lead to flooding.

Coastal Flooding

Areas along coastlines become subject to flooding as a result of tsunamis, hurricanes (cyclonic

storms), and unusually high tides. In addition, long term processes like subsidence and rising sea

level as a result of global warming can lead to the encroachment of the sea on to the land.

Dam & Levee Failures

Dams occur as both natural and human constructed features. Natural dams are created by

volcanic events (lava flows and pyroclastic flows), landslides, or blockage by ice. Human

constructed dams are built for water storage, generation of electrical power, and flood control.

All types of dams may fail with the sudden release of water into the downstream drainage.

Stream Systems


A stream is a body of water that carries rock particles and dissolved ions and flows down slope

along a clearly defined path, called a channel. Thus streams may vary in width from a few

centimeters to several kilometers. Streams are important for several reasons

Streams carry most of the water that goes from the land to the sea, and thus are an

important part of the water cycle.

Streams carry billions of tons of sediment to lower elevations, and thus are one of the

main transporting mediums in the production of sedimentary rocks.

Streams carry dissolved ions, the products of chemical weathering, into the oceans and

thus make the sea salty.

Streams are a major part of the erosional process, working in conjunction with

weathering and mass wasting. Much of the surface landscape is controlled by stream

erosion, evident to anyone looking out of an airplane window.

Streams are a major source of water and transportation for the world's human population.

Most population centers are located next to streams.

Geometry and Dynamics of Stream Channels

The stream channel is the conduit for water being carried by the stream. The stream can

continually adjust its channel shape and path as the amount of water passing through the channel

changes. The volume of water passing any point on a stream is called the discharge.

Discharge is measured in units of volume/time (m3/sec).

� Cross Sectional Shape - varies with position in the stream and discharge. The deepest

parts of a channel occur where the stream velocity is the highest. Both width and depth

increase downstream because discharge increases downstream. As discharge increases

the cross sectional shape will change, with the stream becoming deeper and wider

Long Profile - a plot of elevation versus distance. Usually shows a steep gradient near the

source of the stream and a gentle gradient as the stream approaches its mouth.

Velocity – A stream's velocity depends on position in the stream channel, irregularities in

the stream channel caused by resistant rock, and stream gradient. The average velocity is

the time it takes a given particle of water to traverse a given distance. Stream flow can be

either laminar, in which all water molecules travel along similar parallel paths, or

turbulent, in which individual particles take irregular paths. Turbulent flow can keep

sediment in suspension longer than laminar flow and aids in erosion of the stream

bottom. Average linear velocity is generally greater in laminar flow than in turbulent

flow.

Discharge - The discharge of a stream is the amount of water passing any point in a given

time.

As the amount of water in a stream increases, the stream must adjust its velocity and cross

sectional area in order to form a balance. Discharge increases as more water is added through

rainfall, tributary streams, or from groundwater seeping into the stream. As discharge increases,

generally width, depth, and velocity of the stream also increase. Increasing the depth and width

of the stream may cause the stream to overflow is channel resulting in a flood.


Load - The rock particles and dissolved ions carried by the stream are the called the

stream's load. Stream load is divided into three parts:

Suspended Load - particles that are carried along with the water in the main part of the stream.

The size of these particles depends on their density and the velocity of the stream. Higher

velocity currents in the stream can carry larger and denser particles. The suspended load is what

gives most streams their muddy looking appearance and brown or red color.

Bed Load - coarser and denser particles that remain on the bed of the stream most of the time but

move by a process of saltation (jumping) as a result of collisions between particles and turbulent

eddies. Note that sediment can move between bed load and suspended load as the velocity of the

stream changes.

Dissolved Load - ions that have been introduced into the water by chemical weathering of rocks.

This load is invisible because the ions are dissolved in the water.Dissolved load consists mostly

of HCO-3 (bicarbonate ions), Ca

+2, SO4

-2, Cl

-, Na

+, Mg

+2, and K

+. These ions are eventually

carried to the oceans and give the oceans their salty character. Streams that have a deep

underground source generally have higher dissolved load than those whose source is on the

Earth's surface.

Channel Patterns

Straight Channels - Straight stream channels are rare. Where they do occur, the channel is

usually controlled by a linear zone of weakness in the underlying rock, like a fault or joint

system. Even in straight channel segments water flows in a sinuous fashion, with the deepest part

of the channel changing from near one bank to near the other. Velocity is highest in the zone

overlying the deepest part of the stream. In these areas, sediment is transported readily resulting

in pools. Where the velocity of the stream is low, sediment is deposited to form bars. The bank

closest to the zone of highest velocity is usually eroded and results in a cutbank.

Meandering Channels - Because of the velocity structure of a stream, and especially in streams

flowing over low gradients with easily eroded banks, straight channels will eventually erode into

meandering channels. Erosion will take place on the outer parts of the meander bends where the

velocity of the stream is highest. Sediment deposition will occur along the inner meander bends

where the velocity is low. Such deposition of sediment results in exposed bars, called point bars.

Because meandering streams are continually eroding on the outer meander bends and depositing

sediment along the inner meander bends, meandering stream channels tend to migrate back and

forth across their flood plain.

Braided Channels - In streams having highly variable discharge and easily eroded banks,

sediment gets deposited to form bars and islands that are exposed during periods of low

discharge. In such a stream the water flows in a braided pattern around the islands and bars,

dividing and reuniting as it flows downstream. Such a channel is termed a braided channel.

During periods of high discharge, the entire stream channel may contain water with the islands

covered to become submerged bars. During such high discharge, some of the islands could

erode, but the sediment would be re-deposited as the discharge decreases, forming new islands or

submerged bars. Islands may become resistant to erosion if they become inhabited by vegetation.

Flooding Hazards, Prediction & Human Intervention

Hazards associated with flooding can be divided into primary hazards that occur due to contact

with water, secondary effects that occur because of the flooding, such as disruption of services,


health impacts such as famine and disease, and tertiary effects such as changes in the position of

river channels.

Primary Effects

The primary effects of floods are those due to direct contact with the flood waters. Water

velocities tend to be high in floods hence as discharge increases velocity increases.

With higher velocities, streams are able to transport larger particles as suspended load.

Such large particles include not only rocks and sediment, but, during a flood, could

include such large objects as automobiles, houses and bridges.

Massive amounts of erosion can be accomplished by flood waters. Such erosion can

undermine bridge structures, levees, and buildings causing their collapse.

Water entering human built structures cause water damage. Even with minor flooding of

homes, furniture is ruined, floors and walls are damaged, and anything that comes in

contact with the water is likely to be damaged or lost. Flooding of automobiles usually

results in damage that cannot easily be repaired.

The high velocity of flood waters allows the water to carry more sediment as suspended

load. When the flood waters retreat, velocity is generally much lower and sediment is

deposited. After retreat of the floodwaters everything is usually covered with a thick

layer of stream deposited mud, including the interior of buildings.

Flooding of farmland usually results in crop loss. Livestock, pets, and other animals are

often carried away and drown.

Humans that get caught in the high velocity flood waters are often drowned by the water.

Floodwaters can concentrate garbage, debris, and toxic pollutants that can cause the

secondary effects of health hazards.

Secondary and Tertiary Effects

Secondary effects are those that occur because of the primary effects and tertiary effects are the

long term changes that take place. Among the secondary effects of a flood are:

Disruption of services -

� Drinking water supplies may become polluted, especially if sewerage treatment plants

are flooded. This may result in disease and other health effects, especially in under

developed countries.

Gas and electrical service may be disrupted.

Transportation systems may be disrupted, resulting in shortages of food and clean-up

supplies. In under developed countries food shortages often lead to starvation.

Long - term effects (tertiary effects)-

Location of river channels may change as the result of flooding; new channels develop,

leaving the old channels dry.

Sediment deposited by flooding may destroy farm land (although silt deposited by

floodwaters could also help to increase agricultural productivity).

Jobs may be lost due to the disruption of services, destruction of business, etc. (although

jobs may be gained in the construction industry to help rebuild or repair flood damage).

Insurance rates may increase.

Corruption may result from misuse of relief funds.

Destruction of wildlife habitat.


Predicting River Flooding

Floods can be such devastating disasters that anyone can be affected at almost anytime. When

water falls on the surface of the Earth, it has to go somewhere. To reduce the risk due to floods,

three main approaches are taken to flood prediction.

a. Statistical studies can be undertaken to attempt to determine the probability and

frequency of high discharges of streams that cause flooding;

b. Floods can be modeled and maps can be made to determine the extent of possible

flooding when it occurs in the future; and

c. Since the main causes of flooding are abnormal amounts of rainfall and sudden thawing

of snow or ice, storms and snow levels can be monitored to provide short-term flood

prediction.

Frequency of Flooding

If data is available for discharge of the stream over an extended period of time, such data allows

statistical analysis to determine how often a given discharge or stage of a river is expected. From

this analysis a recurrence interval can be determined and a probability calculated for the

likelihood of a given discharge in the stream for any year. The data needed to perform this

analysis are the yearly maximum discharge of a stream from one gaging station over a long

enough period of time.

Flood Hazard Mapping

Food hazard mapping is used to determine the areas susceptible to flooding when discharge of a

stream exceeds the bank-full stage. Using historical data on river stages and discharge of

previous floods, along with topographic data, maps can be constructed to show areas expected to

be covered with floodwaters for various discharges or stages.

Reduction of Vulnerability

With a better understanding of the behavior of streams, the probability of flooding, and areas

likely to be flooded during high discharge, certain measures can be undertaken to reduce

vulnerability to flooding. Among the non-structural measures are:

Floodplain zoning - Laws can be passed that restrict construction and habitation of

floodplains. Instead floodplains can be zoned for agricultural use, recreation, or other

uses wherein lives and property are not endangered when flood waters re-occupy the

floodplain.

Floodplain building codes - Structures that are allowed within the floodplain could be

restricted those that can withstand the high velocity of flood waters and are high enough

off the ground to reduce risk of contact with water.

Floodplain buyout programs - In areas that have been recently flooded, it may be more

cost effective for the government, which usually pays for flood damage either through

subsidized flood insurance or direct disaster relief, to buy the rights to the land rather than

pay the cost of reconstruction and then have to pay again the next time the river floods.


Mortgage limitations - Lending institutions could refuse to give loans to buy or construct

dwellings or businesses in flood prone areas.

Factors that Affect Flooding

The main factors that cause flooding are heavy rainfall, sudden or heavy snow melt, and dam

failure. All of these factors can suddenly increase discharge of water into streams, within

streams, and out of streams. Furthermore, the discharge causes the river to rise above flood stage

water runs onto the floodplain.

Heavy Rain

When rain falls on the surface of the Earth, some of the water is evaporated and returns to the

atmosphere, some of it infiltrates the soil and moves downward into the groundwater system, and

some is intercepted by depressions and vegetation. What remains on the surface of the Earth and

eventually flows into streams is called runoff. In general, then:

Runoff = Precipitation - Infiltration - Interception - Evaporation

Evaporation tends to be the least of these quantities, particularly over short periods of time, and

thus precipitation, infiltration, and interception are the most important variables that determine

runoff and eventual discharge into streams.

Rainfall Distribution-If rainfall is heavier than normal in a particular area and infiltration,

interception, and evaporation are low then runoff can be high and the likelihood of flooding will

increase. Heavy rainfall can be depicted on maps that show curves of equal rainfall. Such curves

are called isohyets, and the resulting maps are called isohyetal maps.

Lag Time - The time difference between when heavy precipitation occurs and when weak

discharge occurs in the streams draining an area is called lag time. Lag time depends on such

factors as the amount of time over which the rain falls and the amount of infiltration and

interception that takes place along the path to a stream.

If the amount of rain is high over a short time period, lag time is short.

If the amount of rain is high over a longer time period, lag time is longer.

Lack of infiltration and interception reduce lag time

Upstream flooding and flash floods

In areas where large amounts of rain fall over a short period of time within a small area, streams

in the local area may flood, with little or no effect on areas downstream. Such floods are referred

to as upstream floods. In such floods, water rises quickly and flows away quickly after the storm

has passed. Lag times are measured in days.

Flash floods occur when the rate of infiltration is low and heavy rains occur over a short period

of time. They are upstream floods with very little lag time (lag times may be only a few hours).

Because they come with little warning, flash floods are the most dangerous to human lives.

Downstream flooding

If large amounts of rain fall over an extended period of time over a large region, downstream

floods may occur. Lag times are usually longer as tributary streams continually increase the


discharge into larger streams. Such floods extend over long periods of time and affect the larger

streams as well as tributary streams. Water levels rise slowly and dissipate slowly.

Infiltration-Infiltration is controlled by how readily the water can seep into the soil, be absorbed

by the soil, and work its way down to the water table. Several factors determine the rate of

infiltration:

Extent of water saturation of the soil-If the soil is already saturated with water and the

water table has risen as a result of rainfall prior to a heavy storm, then little further water

can infiltrate the soil, and the rate of infiltration will be highly decreased.

Vegetation cover-Vegetation can aid infiltration by slowing the flow of water over the

surface and providing passageways along root systems for water to enter the soil. In

desert regions or areas that have recently been deforested either by fires or humans,

infiltration will be reduced, thus increasing the rate of runoff and decreasing the lag time.

Soil types (dependent on climate)- Different soil types have different capacities to absorb

moisture. Soil type is to a large extent dependent on climate. For example a type of soil

that forms in dry, desert-like environments has a thin layer of poorly developed soil

overlying a crust of caliche. Caliche is calcium carbonate that has precipitated out of

water infiltrating though the thin soil. The caliche zone acts as an impermeable layer

though which water can only penetrate with difficulty. Such soils in deserts, combined

with the lack of vegetation make flash flooding in desert areas more common.

Frozen ground- If the ground is frozen little water can penetrate. Thus rainfall after a

period of cold temperatures may not be able to infiltrate through the frozen ground.

Human construction- Humans tend to pave the Earth with such things as parking lots,

highways, sidewalks, and plazas that prevent infiltration of water into the soil.

Furthermore they tend to channel the water into storm sewer systems and concrete lined

drainages, all of which increase runoff and decrease infiltration.

Levee Failures

Natural levees are constructed as a result of flooding. But, natural levees tend to be relatively

low and do not offer much protection from large discharge because they can easily be

overtopped. Human made levees are much higher and are constructed to prevent flooding from

high discharges on the River. Most levees are constructed of piles of dirt (rock and soil) with a

concrete cover on the river side of the levee. Such levees often give a false sense of security for

those living on the floodplain the levee was built to protect, because failure of such levees can

lead to flooding, either because discharge can become great enough to overtop the levees or the

levees can become weakened and fail. Levees can fail for three main reasons.

a. Overtopping of levees

If high discharge in the river leads to a river stage that is higher than any point on a levee, the

water will overtop the levee and start to flow onto the floodplain. Because the initial gradient

from the river to flood plain is relatively high, the velocity of the stream as it overtops the levee

will be high. High velocities can result in high rates of erosion, and thus the levee that is initially

overtopped will soon become eroded and a channel through the levee will soon be created.

b. Undercutting and slumping of levee


Higher discharge in the river will lead to higher velocities with the stream trying to increase its

width and depth. Higher velocities can lead to higher rates of erosion along the inner parts of

levees and thus lead to undercutting and slumping of the levee into the river. Heavy rainfall or

seepage into the levee from the river can increase fluid pressure in the levee and lead to slumping

on the outer parts of the levee. If the slumps grow to the top of the levee, large sections of the

levee may slump onto the floodplain and lower the elevation of the top of the levee, allowing it

to be more easily overtopped.

c. Buildup of fluid pressure beneath levees

Increasing levels of water in the river will cause the water table in the levee to River Flooding

rise. This will also increase fluid pressure and may result in water being pushed through the

levee to rise as springs on the surrounding flood plains. If a high rate of flow is developed due to

the increased fluid pressures, then a high velocity pathway to the flood plain may develop and

undermine the levee causing its collapse and failure.

REFERENCES

Schramm, D., & Dries, R. 1986: Natural Hazards: Causes and effects. Disaster Management

Centre, University of Wilconsin-Madison

Mccracken, D., Newcomb, L.H. & Warmbrod, J.R. 2009: Handling Natural Hazards; Prentice

Hall

Burton, J. 1968: The Environment a Hazard: Oxford University Press. New York

Ege 421-Lecture Notes

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