Plate Tectonics Engineering Geology

Earth‟s crust is composed of numerous plates, which move due to

convection processes in the mantle.

Theory of plate tectonics

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The lithosphere is broken up into interconnected slabs that geologists call

plates. Plate tectonics is the theory that describes how these plates move

about and interact with each other at their boundaries.

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What are Tectonic Plates?

The Earth’s crust consists of about a dozen large slabs of rock, or PLATES, that the continents

and oceans rest on. These tectonic plates can move centimeters per year (1-15 cm per year)

Tectonic plates are also called lithospheric plates because the crust and the upper-most

mantle make up a sub-layer of the earth called the lithosphere. The plates can move about

because the uppermost mantle, or the asthenosphere, is partially molten and possesses a physical

property called plasticity, allowing the strong, rigid plates of the crust to move over the weaker,

softer asthenosphere.

The word TECTONICS is of Greek origin and it means “to build.” The word “tectonism” refers

to the deformation of the lithosphere. This deformation most notably includes mountain building.

Plates and relative plate motion.

Modified after NOAA

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Earth‟s Sublayers

Lithosphere: This layer combines the rigid crust plus the upper-most mantle. (Greek: Rock)

Asthenosphere: Partially molten part of upper mantle (Greek: weak). Tectonic plates are able

to move about on top of the softer, partially molten asthenosphere.

The outer-most layers of the earth.

McGraw Hill/ Glencoe, 1st ed., pg. 142.

What are Tectonic Plates? (continued)

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There are 8 primary plates on the planet (some time Indo-Australian

Plate is considered as a single plate), and they comprise of the

majority of the World's continents' landmass, along with most of the

surface area of the World's Ocean's.

The secondary plates (7) are smaller in size than the primary plates,

and they do not cover any substantial landmass, apart from the Arabian

Plate.

There are a further group of smaller plates, often called tertiary plates,

which are the disappearing remains of much larger ancient plates that

are now on the edges of our major plates, plus some micro-plates,

many of whom will be widely-considered as a part of a primary or

secondary plate on maps and in scientific publications.

Major Tectonic Plates in the world:

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Major Tectonic Plates in the world:

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Primary Tectonic Plates Secondary Tectonic Plates

1. African Plate 1. Arabian Plate

2. Antarctic Plate 2. Caribbean Plate

3. Australian Plate 3. Cocos Plate

4. Eurasian Plate 4. Juan de Fuca Plate

5. Indian Plate 5. Nazca Plate

6. North American Plate 6. Philippine Sea Plate

7. Pacific Plate 7. Scotia Plate

8. South American Plate

List of Major Tectonic Plates :

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Plate Tectonics: The Beginning

At the beginning of the 20th Century, scientists realized that that they could not explain many ofthe Earth’s structures and processes with a single theory. Many scientific hypotheses weredeveloped to try and support the conflicting observations. One hypotheses was continental drift,

which was proposed by Alfred Wegener in a series of papers from 1910 to 1928.

The principal thought of continental drift theory is that the continents are situated on slabs ofrock, or plates, and they have drifted across the surface of the Earth over time; however, originally,they were all joined together as a huge super-continent at one time.

In the 1960’s, the theory ofcontinental drift was combined withthe theory of sea-floor spreading tocreate the theory of plate tectonics.

Alfred Lothar Wegener (1880-1930)

Background

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Additional evidence supporting the continental drift theory:

Fossils of the same plant (Glossopteris) found in Australia, India, Antarctica and

South America.

Fossils of same reptile (Mesosaurus) found in Africa and South America. This animal

could not have swum across the existing Atlantic Ocean

Glacial deposits found in current warm climates and warm climate plant fossils found

in what is now the Arctic (Paleoclimate Indicators).

Nearly identical rock formations found on the east coast of U.S. and the west coast of

Europe and eastern South America and western Africa.

Coal reserve found under ice cap of Antarctica.

The idea for Wegener's theory was sparked by

his observation of the nearly perfect “fit” of the

South American and African continents.

The “fit” of two continents.

Plate Tectonics: The

Beginning

412

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Continents FIT together

like the pieces of a puzzle

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Fossil evidence- Glossopteris & Mesosaurus

Paleoclimate Indicators

Glacial till of the same age is found in southern Africa, South America, India and

Australia-areas that it would be very difficult to explain the occurrence of glaciation.

At the same time, large coal deposits were formed from tropical swamps in

N.America and Europe.

Rock Type and Structural Similarities

We find similar rock types on continents on opposite sides of the Atlantic

Ocean.

Similar, age, structure and rock types are found in the Appalachian Mtns.

(N.A.) and mountains in Scotland and Scandinavia.

There are three basic ways that plates interact with one another. Each of these plate

boundaries has the potential to create different geological features.

Plate Boundaries

1. When plates collide with each other = Convergent boundary

2. When plates separate from each other = Divergent boundary

3. When plates slide along side each other = Transform boundary

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The tectonic plates and plate boundaries.

McGraw Hill/Glencoe, 1st ed., pg 143

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1. Convergent Boundary:

Ocean-Continent Collision Because the oceanic crust is more dense than continental crust, when these two

collide, the continental crust rides up over the oceanic crust and the oceanic crust is

bent down and sub ducted beneath the continental crust. This is called a subduction

zone, where the old oceanic crust is dragged downward and “recycled.”

Deep-sea trenches are created at subduction zones. Trenches are narrow, deep

troughs parallel to the edge of a continent or island arc. They typically have slopes

of 4-5 degrees, and they are often 8-10 km deep. The deepest spots on earth are

found in oceanic trenches. The Mariana Trench is the deepest ocean depth at 11 km

(35,798 ft) below sea level.

Figure depicting oceanic crust

subducting beneath continental

crust, creating volcanoes on the

land surface above, and a deep-

sea trench off of the coast.

Credit: U.S. Geological Survey

Department of the Interior/USGS36

If two continental plates collide, mountain building usually takes place because they

are both relatively low in density.

Earthquake activity at these boundaries is common; however, because igneous

activity is different from ocean-continent collisions, volcanoes are rare.

Convergent Boundary:

Continent-Continent Collision

The Himalaya mountains are still forming today as the Ind-Australian Plate collides with the Eurasian Plate

Examples: The Himalayan and the Appalachian mountain chains.

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Convergent Boundary:

Ocean-Ocean Collision If 2 oceanic plates collide, the older, denser one is subducted downward into the

mantle and a chain of volcanic islands can form, called a volcanic arc.

Example: Mariana Islands (Mariana Trench). It is deeper than the earth’s tallest

mountain is tall. Mariana Trench: 11,000 meters deep. Mt. Everest: 8850 meters

high.

The interaction of the descending oceanic plate causes incredible amounts of

stress between the plates. This usually causes frequent earthquakes along the

top of the descending plate known as the “Benioff Zone.” The focii of Benioff

earthquakes can be as deep as 700 km below sea level.

Oceanic/oceanic collision

resulting in a chain of island

arcs.

Credit: U.S. Geological Survey

Department of the Interior/USGSBenioff Zone

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At a divergent boundary, two oceanic plates pull apart from each other through a

process called sea-floor spreading.

Sea-floor spreading was proposed by Harry Hess in the early 1960’s.

Hess proposed that hot magma rises from the asthenosphere and up into existing

ocean crust through fractures. The crust spreads apart making room for new magma

to flow up through it. The magma cools, forming new sea floor and resulting in a

build-up of basaltic rock around the crack, which is called a mid-ocean ridge.

2. Divergent Boundary: Sea-floor Spreading

New material is constantly being created. This is the opposite of a convergent

boundary, where material is constantly being destroyed.

Sea-floor spreading at an oceanic divergent boundary.

Modified after McGraw Hill/ Glencoe, 1st ed., pg. 138 (with permission)

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3. Transform Boundary When two plates slide past each other moving in different directions or the same

direction, it is termed a transform boundary and is characterized by a transform

fault and earthquake activity.

An example of a transform fault is the San Andreas Fault in California. Here the

North American Plate joins the Pacific Plate. The difference in plate motion along

the contact (fault) leads to a buildup of strain energy that sometimes slips releasing

a huge amount of energy and causing an earthquake.

An aerial photo of the San Andreas

fault line. McGraw Hill/Glencoe, 1st ed., pg.

146 (with permission)

Movement between the 2 plates at the San Andreas

Transform Fault. McGraw Hill/Glencoe, 1st ed., pg. 146 (with

permission). 2041

Tectonic Plate „s

Boundaries

Earthquake

epicenters

(1963-1998)

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Earthquakes

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Introduction

Earthquake is a quaking or shaking of theground caused by the sudden release of energystored in the rocks beneath the earth’s surface

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What causes earthquakes?Tectonic plates move past each other causing stress.

Stress causes the rock to deform

– Plastic deformation – does not cause earthquakes

– Elastic deformation – rock stretches then reaches a

breaking point, releasing energy.

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Energy released and propagates in all

directions as seismic waves causing

earthquakes

Focus = location of initial slip on the fault; where the earthquake originates

Epicenter = spot on Earth’s surface directly above the focus 47

Body Waves• Seismic waves that travel through the earth’s

interior, spreading outward from the focus in all

directions

• P (primary) wave: a compressional (or longitudinal)

wave in which rock vibrates back and forth parallel

to the direction of wave propagation; can travel

through rock, gas, or liquid

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Body Waves (cont.)

• S (secondary/shear) waves: a slower, transverse

wave that travels through near surface rocks at 2 to

5 kilometers per second; the rock vibrates

perpendicular to the direction of wave propagation;

can travel through rock but not gas or liquid

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Surface Waves

surface waves: seismic waves that travel on the earth’s

surface away from the epicenter

Long waves: waves that have no vertical displacement;

they move side to side in a horizontal plane that is

perpendicular to the direction the wave is traveling or

propagating; do not travel through liquids; because of

horizontal movement the waves tend to knock buildings

off their foundation

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Seismic Waves

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How do scientists calculate how far a location

is from the epicenter of an earthquake?

• Scientists calculate the difference betweenarrival times of the P waves and S waves

• The further away an earthquake is, thegreater the time between the arrival of the Pwaves and the S waves

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Measuring Earthquakes

• Seismograph: a recording device that produces apermanent record of earth motion detected by aseismometer, usually in the form of a wiggly line drawnon a moving strip of paper

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Seismogram Printout

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Magnitude -- measure of energy released during earthquake.

There are several different ways to measure magnitude.

Most common magnitude measure is Richter Magnitude, named for the renowned seismologist, Charles Richter.

Richter Magnitude

Measure amplitude of largest S wave on seismograph record.

Take into account distance between seismograph & epicenter.

Richter Scale

Logarithmic numerical (NOT a physical) scale

Increasing one whole unit on Richter Scale represents 10 times greater magnitude.

Going up one whole unit on Richter Scale represents about a 30 times greater release of energy.

Intensity

Intensity refers to the amount of damage done in an earthquake

Mercalli Scale is used to express damage

Determining the magnitude of an earthquake

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Richter Scale

Measures the energy released by

fault movement

related to the maximum amplitude of

the wave measured from the

seismogram

Logarithmic-scale

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6

31.5 times energy

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992 times more energy!!

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Effects of Earthquakes

• ground motion

• fire

• landslides

• liquefaction (a special type of ground failure)

• permanent displacement of the land surface

• aftershocks: small earthquakes that follow the main shock

• tsunami

Liquefaction of soil by a 1964 earthquake in Niigata, Japan, causedearthquake-resistant apartment buildings to topple over intact. An example of permanent displacement of the land surface - fence

compressed by ground movement, Gallatin County, Montana, 1959.58

TsunamisTsunamis (seismic sea waves): huge ocean

wave produced by displacement of the sea floor

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A tsunami is a series of

ocean waves with very long

wavelengths (typically

hundreds of kilometers)

caused by large-scale

disturbances (earthquake) of

the ocean

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World Distribution of Earthquakes

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Pancake-Style Collapse — 1985 Mexico City Quake

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Timber-Frame Buildings in Turkey

Landslide from Seattle

Earthquake, 1965

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Effects of Soil Liquefaction,

Japanese Quakes (Nigata), 1964

Sand Boils after Loma Prieta

Earthquake

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Damage due to

the Tsunami in

2004, Sri Lanka

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Japan earthquake,

March 2011

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Summary-Earthquake

1. Earthquakes generate waves that travel throughthe earth

2. Earthquakes occur when rocks slip along faults

3. Magnitude and Intensity

4. Seismic waves are used to map the earth’s interior

5. Predicting earthquakes is not yet possible

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Plate Tectonics Engineering Geology

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