Earthquakes and Volcanoes - Coach Shannon's...
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UNIT SIX: Earth’s Structure Chapter 20 Earthquakes and Volcanoes
Transcript of Earthquakes and Volcanoes - Coach Shannon's...
UNIT SIX: Earth’s Structure
Chapter 20
Earthquakes and
Volcanoes
Chapter Twenty: Earthquakes and
Volcanoes
20.1 Earthquakes
20.1 Learning Goals
Describe the plate interactions that causes
earthquakes.
Discuss the types of information scientists
learn from seismograms.
Compare and contrast the scales used to
measure and describe earthquakes.
20.1 Earthquakes and plate boundaries
Earthquakes commonly
occur at the boundaries
of lithospheric plates.
This is because plate
boundaries tend to be
zones of seismic
activity.
20.1 Earthquakes
The second longest ever
recorded earthquake
occurred in 1964 in
Alaska and lasted for
four minutes.
During an earthquake,
strong shaking makes
the ground move up and
down and back and forth.
20.1 When an earthquake occurs
An earthquake is the movement of Earth’s crust resulting from the release of built-up potential energybetween two stuck lithospheric plates.
20.1 Stick-slip motion
An earthquake is a
form of stick-slip
motion.
Stick-slip motion can
be compared to a
stuck door.
20.1 Stick-slip motion
Three conditions are needed for stick-
slip motion:
1. Two objects that are touching each other
where at least one of the objects can
move.
2. A force, or forces, that will cause the
movement.
3. Friction strong enough to temporarily
keep the movement from starting.
20.1 When an earthquake occurs
The point below the surface where the
rock breaks is called the earthquake
focus.
20.1 When an earthquake occurs
As soon as the rock breaks, there is
movement along the broken surface
causing a split in the surface called a
fault.
20.1 What causes earthquakes? The seismic waves from an earthquake are
usually strongest at the epicenter, the point on the surface right above the focus.
20.1 Lithospheric plates have many sections
A moving line of grocery carts is a good analogy of a movinglithospheric plate.
Although a plate may be moving as a single unit, its boundaries act like they were made of manysmall sections like the line of carts.
20.1 Lithospheric plates have many sections
A lithospheric plate may be thousands of km across.
It takes a long time for movement on oneend of the plate to affect a section further away.
20.1 Seismic waves Seismic waves are recorded and measured by
an instrument called a seismograph.
Seismic waves inside Earth are called bodywaves.
The two main types of body waves are P-waves and S-waves.
20.1 Seismic waves
After an earthquake occurs, the firstseismic waves recorded will be P-waves.
S-waves are recorded next, followed by the surface waves.
20.1 Seismic waves
Seismic waves radiate
from the focus after the
earthquake.
Three seismic stations can
accurately determine the
times of body wave arrival.
The larger the difference in
arrival time, the farther the
epicenter is from the
station.
20.1 Seismic waves
In a quarter-mile race,
the track is so short
that fast and slow cars
are often just fractions
of a second apart.
In a long race, like the
Indianapolis 500, the
cars might be minutes
apart.
The time difference
between slow and fast
cars is related to the
length of the race track.
20.1 Measuring earthquakes
The Richter scale ranks earthquakes
according to their magnitude of the seismic
waves recorded on a seismograph.
20.1 Measuring earthquakes
The Moment Magnitude scale rates the total
energy released by an earthquake.
The numbers on this scale combine energy
ratings and descriptions of rock movements.
Seismologists tend to use the more
descriptive Moment Magnitude scale to
distinguish between strong earthquakes.
20.1 Measuring earthquakes
The Modified Mercalli scale has 12descriptive categories.
Each category is a rating of the damageexperienced by buildings, the ground, and people.
Chapter Twenty: Earthquakes and
Volcanoes
20.2 Volcanoes
20.2 Learning Goals
Identify locations where volcanoes are most
likely to form.
Explain the factors involved in volcanic
eruptions.
Evaluate the features of different types of
volcanoes.
20.2 Where you find volcanoes
A volcano is a site where melted rock and other materials from Earth’s mantle are released.
Mount St. Helens is a type of volcano called a composite volcano (also known as a stratovolcano).
20.2 Where you find volcanoes
What is the Ring of Fire?About half of the active surface volcanoes on Earth occur along the shores of the Pacific Ocean.
This region is called the “Ring ofFire.”
20.2 Where you find volcanoes
The Ring of Fire is found where the oceanic crust of the Pacific Plate is subducting under nearby plates.
Most volcanoes are located along plateboundaries.
Volcanoes, like those in Hawaii, are also present along divergent boundaries and within plates.
20.2 What is a volcano?
During an eruption, melted rock called
magma leaves the magma chamber and
moves up the conduit. The magma
leaves the conduit at the vent.
Magma is called lava after it leaves the
vent.
20.2 The life of a volcano
An active volcano is the most vigorous kind of
volcano.
Active volcanoes are erupting or have erupted
recently, and are expected to erupt again in the
near future.
A dormant volcano is a quiet volcano.
Dormant volcanoes are not active now, but may
become active again in the future.
20.2 The life of a volcano
Devil’s Tower and
Ship Rock are
examples of extinct
volcanic “necks.”
As the volcano erodes, a core of solid magma
gets exposed by erosion.
20.2 What makes magma?
There are two ways to
make rock melt.
One way is to reduce
the pressure.
20.2 What makes magma?
The other way is to
mix water with the
hot rock.
The conditions
needed to melt rock
are very special
and exist inside our
planet.
20.2 Volcanoes vary
The shapes of volcanoes depend on
the composition of the magma that
formed them.
Volcanoes can look like wide, flat
mounds (shield volcanoes), like tall
cones (composite volcanoes), or like a
heap of rock bits (cinder cones).
20.2 Volcanoes vary
The quantity of dissolved gases affects
how explosive the eruption will be.
20.2 Volcanoes at divergent boundaries
Can you name an oceanic
ridge formed at diverging
plates?
Mid-ocean ridges occur underwater at divergingplate boundaries.
When lava oozes out at a mid-ocean ridge, it immediately hits cold seawater, forming a crust.
20.2 Volcanoes at divergent boundaries
On land, basaltic lava flows
like spilled syrup.
Underwater, oozing lava hits
cold seawater and air fills a
solid lava skin like a balloon.
When geologists find pillow
lava on land, they know that
there was once a midocean
ridge nearby.
20.2 Volcanoes at divergent boundaries
Iceland is separating along the Mid-Atlantic Ridge.
Similarly, Ethiopiais the site of the East African Riftzone.
Due to the separation of plates at these locations, each is
intensely volcanic.
20.2 Volcanic islands chains and mantle plumes
Volcanic islands form
when mantle plumes
bring material from deep
within the lower mantle
under an ocean.
The top of an active
mantle plume is called a
hot spot.
20.2 Volcanic chains
1. As the plate moves, it carries the volcanic
island away from the active hot spot.
2. Without the hot spot to supply magma, the
volcano becomes extinct.
3. The hot spot begins to form a new volcano
beside the old one.
4. The result is a volcanic island chain.
20.2 Shield and composite volcanoes
Low silica magma produces a shield
volcano.
20.2 Shield and composite volcanoes
Because low-silica,
basaltic magma is
runny, it can’t build up
a tall, cone-shaped
volcano.
12.2 Volcanoes with low silica magma
When low silica magma has
high levels of dissolved
gas, gas bubbles out as it
reaches the volcano vent.
The effect is identical to
shaking a soda bottle to
produce a shower of soda.
High-gas magma produces a spectacular fire fountain.
20.2 Shield and composite volcanoes
A tall cone, or
composite volcano
is a tall cone
formed by layers of
lava and ash.
20.2 Silica rich magma
During the upward migration, minerals in magma begin to crystallize.
As the mineralscrystallize, the silica increases in concentration.
20.2 Silica rich magma
Compare and contrast shield and composite
volcanoes using the data below:
20.2 Dissolved gas and cinder cones
If silica-rich magma
contains high levels
of dissolved gas,
pressure usually
builds inside a
volcano.
20.2 Dissolved gas and cinder cones The lava bits filled with gas bubbles break
apart as the dissolved gas expands.
The gas-filled fragments cool to produce
pumice and ash.
20.2 Cinder cones
A cinder cone, a
third type of
volcano, is not the
result of flowing
lava.
Imagine a volcano
that ejects a lot of
gas with only small
bits of lava.
Chapter Twenty: Earthquakes and
Volcanoes
20.3 Igneous Rocks
20.3 Learning Goals
Explain how igneous rocks are formed.
Distinguish intrusive and extrusive igneous
rocks.
Describe the relationship between
crystallization in rocks and where the rocks
are formed.
20.3 Igneous Rocks
Igneous rocks begin to form when rock melts in Earth’s mantle.
A good way to describe hot, solid mantle rock is that it is like stiff putty that takes millions of years to move.
Decreased pressure and the addition of water lower the melting temperature of mantle rock so that it melts.
20.3 How are igneous rocks formed?
The wide variety of igneous rocks result
from changes that take place after
magma forms.
As melted rock cools, minerals in
magma or lava form crystals that can be
large, small, or microscopic depending
on the rate at which cooling takes place.
20.3 Crystals in igneous rocks
Minerals in magma or lava form crystals as the melted rock cools.
Crystallization is the process by which crystals form and grow in size.
20.3 Crystals in igneous rocks
Often called volcanic
glass, obsidian is an
igneous rock that lacks
crystals.
Which Oregon
volcano produced the
Big Obsidian Flow?
20.3 Interpreting igneous rocks
The general color of
igneous rocks is a
measure of the kinds
of minerals present.
20.3 Interpreting igneous rocks
Depending on the rate of cooling, the crystals might be invisible, small, or large.
Pegmatite cooled slowly and formed large, visible crystals.
20.3 Comparing igneous rocks
An igneous rock that
forms above Earth’s
surface is called an
extrusive rock.
An igneous rock that
forms within Earth’s
crust is called an
intrusive rock. Which rocks are more likely
to have large crystals?
20.3 Comparing igneous rocks
Can you classify other igneous rocks such as rhyolite or andecite?
2004 Indian Ocean:
The Indian Ocean tsunami may be one of the
deadliest natural disasters in modern history.
Officials believe over 275,000 people died.
Scientists used data from around the world to
revise the earthquake to a 9.3 magnitude.
Earthquake and
Tsunami
REMEMBER……………This is the end of the “Freshman Science”
course material.
We will complete chapter 20 by Friday.
We will have a 4-5 week unit on Ecology
(biology) that you will need for next year.
We will still have a final exam in Freshman
Science, and a great review before the test.
