Download Structural Geology and Plate Tectonics Sections 21.1-21.6

Structural Geology and
Plate Tectonics
Physical Science II
Chapter 21 in 13th edition, Chapter 22 in 12th edition
Introduction - Geology

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
Geology is the study of planet Earth – its composition,
structure, processes, and history.
In this chapter we will look at the basic structure of the
Earth’s interior.
We will see how plate tectonics is the primary mover of
the Earth’s outer shell, and how it is related to mountain
building, continental drift, earthquakes, volcanoes, and
seafloor spreading.
Intro
The Earth’s Interior Structure


Despite many remarkable scientific advances, most of the
Earth’s interior is still unreachable.
Ideas on the composition and structure of the Earth’s
interior come from
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Earthquake body waves
Meteorites – thought to be similar to composition of Earth
Spacecraft measurements of gravity and magnetic variations
High P & T laboratory experiments on rocks
Section 21.1
Earthquake Waves Help
Reveal the Earth’s Interior

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During an earthquake, some of the vibrations travel
completely through the interior of the Earth, providing
scientists with significant information.
Scientists can use the speed and direction of these waves
to identify the types of materials through which they
move.
Section 21.1
Seismic Waves
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Earthquakes produce seismic waves of two basic types:
surface waves and body waves.
Surface waves, as their name implies, travel within the
upper few kilometers of the Earth’s surface.


Surface waves are responsible for most surface earthquake
damage.
Body waves are transmitted in all directions through the
interior of the Earth.
Section 21.1
Body Waves

Two types of body waves can be distinguished by their
type of motion and speed:

P (primary) waves are compressional in nature.

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Particles move back and forth (longitudinal compressional) in the
same direction as the wave is traveling.
S (secondary) waves are transverse in nature.

Particles move at right angles to the direction of wave movement.
Section 21.1
P and S Waves
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P and S waves have two other important differences:
S waves will only travel through solids, P waves can travel
through solids, liquids, and gases.
P waves travel faster than S waves and arrive earlier at
the seismic station.

The difference in arrival times between the P and S waves
allow geophysicists to determine the focus of the earthquake.
Section 21.1
Body Waves - Refraction

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The velocity of body waves is dependent upon the density
of the interior of the Earth.
The Earth’s interior generally increases in density with
depth, therefore body waves will curve and refract.
Geophysicists use the travel velocity, the refraction
pattern, and the travel path to interpret our modern view
of the Earth’s structure.
Section 21.1
Seismic Wave Travel
Through the Earth’s Interior

S waves do not travel
through the liquid
outer core.

P waves are refracted
at density boundaries.
Section 21.1
Structure of the Earth’s Interior

Scientists think that the Earth is composed of four
concentric layers or zones:
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Inner core
Outer core
Mantle
Crust
Different compositions and/or physical properties
characterize each of these layers.
Section 21.1
Interior of the Earth – Four Layers

Evidence suggests that the inner core is solid and is
composed of iron (85%) and nickel.

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The outer core is interpreted to have the same
composition as the inner core, but is liquid.

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
Radius of approximately 1230 km
Thickness of approximately 2240 km
The mantle’s composition is distinctly different from the
outer core.
The thin solid outer layer where we live is called the
crust.
Section 21.1
Interior Structure of the Earth
Section 21.1
The Earth’s Crust

The Earth’s crust ranges in thickness from 5 to 11
kilometers for oceanic crust and 19 to 40 kilometers for
continental crust.
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Oceanic crust is mainly composed of basalt.
Continental crust is granitic in composition.
A sharp compositional boundary exists between the base
of the crust and the upper mantle.
This boundary is called the Mohorovicic discontinuity, or
simply Moho.
Section 21.1
Crust and Upper
Mantle – Physical Properties

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The crust and upper mantle can be divided differently if
we take into account its physical properties and behavior.
Lithosphere - outermost rigid, brittle layer
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Composed of the entire crust and uppermost mantle
Most faults and earthquakes occur in the lithosphere.
The asthenosphere lies beneath the crust, extending
down to approximately 70 km.

Due to its high temperature this layer is plastic, mobile, and
is essential for tectonic plate motion.
Section 21.1
Lithosphere and Asthenosphere

The lithosphere is solid and slowly moves over the plastic
asthenosphere.
Section 21.1
Continental Drift
Formulation of the Theory
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As one looks at a current world map, it is apparent that
the coastlines of eastern South America and western
Africa fit together fairly well.
Is this a coincidence or were these two and the other
continents once attached?
Over the past several hundred years scientists have
speculated as to the meaning of this observation. Have
the continents drifted?
Section 21.2
Continental Drift – Alfred Wegener
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Alfred Wegener (1880 –1930) was a German
meteorologist and geophysicist.
In the early 1900’s he revived/proposed the hypothesis of
continental drift.
Wegener proposed that about 200 million years ago all
the continents were together in a supercontinent he
called Pangea.
During the past 200 million years Pangea broke apart and
the newly formed continents slowly drifted apart.
Section 21.2
Starting 200 Million Years Ago
Pangea Started to Break Apart
Section 21.2
Breakup of Pangea
100 Million Years Ago
Section 21.2
Breakup of Pangea
Present Day Continental Configuration
Section 21.2
Continental Drift – Scientific Evidence
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Wegener brought together various pieces of geologic
evidence that supported his theory.
There were three prominent lines of evidence that
Wegener highlighted:
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Biological evidence
Continuity of geologic features
Glacial evidence
Section 21.2
Continental Drift
Biologic & Paleontologic Evidence
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Present-day biological species on widely separated
continents have similarities that suggest that these land
masses were once together.
Identical fossil plants and animals have been found on a
number of continents, once again strongly suggesting that
these continents were together when these organisms
were alive.
Section 21.2
Continental Drift
Continuity of Geologic Features
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If the continents of North America, Europe, South
America, and Africa were all put back together the
continuity of several geologic features would become
evident.
In the northern hemisphere the Caledonian, Hebrides,
Labrador, and Canadian Appalachians match up.
In the southern hemisphere the Cape and Sierra
mountains of South Africa and Brazil would line up nicely.
Section 21.2
Continuity of Continental
Features Illustrated

If the continents had
once been together
and drifted apart, we
would expect the
continuity of geologic
features when put back
together
Section 21.2
Continuity of Geologic
Features
Section 21.2
Continental Drift
Glacial Evidence
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Geologic evidence suggests that the southern areas of
South America, Africa, India, and Australia were covered
with glaciers 300 million years ago (mya).
There is no evidence for glaciation at this time (300 mya)
in Europe and North America.
This indicates that the glaciated areas were once located
at very high latitudes, while North America and Europe
were at low latitudes.
Section 21.2
Glacial Evidence of Continental Drift
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Only when the
continents are put
back into their
“Pangea” positions
does this glacial
episode make sense.
All of these glaciated
areas were located
close to the south
polar region 300
million years ago.
Section 21.2
Continental Drift
Not Generally Accepted
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Although a number of lines of evidence supported
Wegener’s theory it was not widely accepted at the time
(early 1900’s).
At the time, Wegener’s theory still had one critical flaw.
He nor anyone else could devise a mechanism that could
explain how continental crust could “move” through the
oceanic crust.
Section 21.2
Seafloor Spreading
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In 1960, the American geologist Harry Hess suggested a
viable mechanism that could explain continental drift.
At the time the mid-ocean ridge system and the deep sea
trenches had been mapped in fair detail throughout the
world’s oceans.
The mid-ocean ridge system was known to stretch
throughout the world.
The trenches were known to be very deep and very long
and narrow.
Section 21.2
Seafloor Spreading
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Hess proposed the theory of seafloor spreading.
In this theory the seafloor slowly spreads by moving
sideways away from the mid-ocean ridges.
New magma wells up and cools as each side of the midocean ridge slowly splits apart.
The entire ocean floor can be viewed as a giant conveyor
belt where the new seafloor moves away from the ridges,
and eventually descends back into the mantle at the
trenches.
Section 21.2
Paleomagnetics Supports
Seafloor Spreading
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As new magma wells up and cools along the mid-ocean
ridge system one of the component minerals of this new
rock is magnetite. (Fe3O4)
When this mineral crystallizes (at cooling) it becomes
magnetized in the direction of the Earth’s prevailing
magnetic field, a phenomenon called remanent magnetism.
We know that the Earth’s magnetic field has abruptly and
frequently reversed itself during geologic time.
Section 21.2
Seafloor Spreading – Evidence
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Remanent magnetism of the ocean crust reveals long,
narrow, symmetric bands of magnetic anomalies on either
side of the Mid-Atlantic Ridge.
These magnetic anomalies indicate that the Mid-Atlantic
Ridge has been continuously spreading and that the
Earth’s magnetic field has reversed itself many times.

The mid-ocean ridge spreading rates are in the range of a
few centimeters per year.
Section 21.2
Magnetic Anomalies
Showing Reversals in
the Earth’s Magnetic
Fields
From Chernicoff, Stanley and Donna Whitney, Geology, T
hird Edition © 2002 by Houghton Mifflin Company. Used with permission
Seafloor Spreading
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Wegener’s original evidence (biologic, paleontologic,
geologic, and glacial) supports the theory of continental
drift.
Hess’s theory and evidence (remanent magnetism)
supports the idea of seafloor spreading.
These two ideas have now been merged into the modern
theory of plate tectonics.
We now know that both the oceanic and continental
crusts are carried as part of a thicker layer called the
lithosphere.
Section 21.2
Plate Tectonics
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We now visualize ocean basins to be in a constant cycle with
new crust being created at the mid-ocean ridges and old crust
descending along the ocean trenches.
We also know that the lithosphere is composed of a series of
solid segments called plates.
These plates are constantly moving and interacting with other
plates.
The theory of plate tectonics encompasses all these processes.
The lithosphere is divided into approximately 20 plates.
Section 21.3
From From Chernicoff, Stanley and Donna Whitney, Geology,Third
Edition © 2002 by Houghton Mifflin Company. Used with permission
Plate Boundaries
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The most active areas of the Earth’s crust are along the
plate boundaries.
There are three types of plate boundaries:
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Divergent – located along mid-ocean ridges where the two
plates are moving apart
Convergent – zones along which two plates are driven
together, one plate is consumed
Transform – boundaries along which two plates slide
horizontally past one another
Section 21.3
The Asthenosphere
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Underlying the Earth’s solid lithosphere is a higher
temperature layer - the asthenosphere.
This layer, although basically solid, is so close to its
melting temperature that it is relatively plastic and easily
deforms.
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The asthenosphere is much more easily deformed than the
lithosphere.
The lithosphere may be viewed as actually “floating” on
top of the asthenosphere.
Section 21.3
Asthenosphere and Isostasy
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Isostasy – the concept that the solid lithosphere floats in
gravitational equilibrium (buoyancy) on the plastic
asthenosphere
Continental plates float higher because they are less
dense than oceanic plates.
At any given time, all of the plates are in isostatic
equilibrium.
Mountain ranges simply represent thicker masses of
continental material and therefore float higher.
Section 21.3
Isostasy
Similar to less dense ice that floats in water, the less dense
continental
crust floats on the more dense asthenosphere
Section 21.3
From Chernicoff, Stanley and Donna Whitney, Geology, T
hird Edition © 2002 by Houghton Mifflin Company. Used with permission
Plate Movement
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The movement of the lithospheric is due to forces within
the asthenosphere.
Most geologists think that movement within the
asthenosphere is caused by convection cells.
Unequal temperature distribution within the
asthenosphere and upper mantle results in the hot, less
dense material rising, and the cooler, more dense material
sinking.
Section 21.3
Convection Cells in the Asthenosphere
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Drag from the more active asthenosphere drives the
outermost solid lithosphere.
Section 21.3
Divergent Boundary
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The mid-ocean ridge system represents a zone where
two plates are moving apart – a divergent boundary.
The initially molten magma is shouldered to each side of
the rift and causes the lithospheric plates to slowly
separate.
Drag from the underlying asthenosphere keeps the plates
in motion.
Section 21.3
Spreading at the Mid-Ocean Ridge
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As the two plates move apart, new magma wells up and
cools along the rift zone creating new crust.
Section 21.3
Divergent Boundaries
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As a portion of the plate moves away from the hot,
spreading center it cools, contracts, and becomes more
dense.
Due to the increase in density going away from the
spreading center (rift), the plate gradually subsides
(isostatic equilibrium) and the oceans grow progressively
deeper.
Section 21.3
Convergent Boundary
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The result of two plates converging depends on the type
of plates that are interacting.
Three combinations are possible:
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Oceanic-oceanic convergent boundary
Oceanic-continental convergent boundary
Continental-continental convergent boundary
In two of these converging boundary-types one of the
plates descends beneath the other plate, a process called
subduction.

A ‘subduction zone’ is where this happens.
Section 21.3
Oceanic-Oceanic Convergence
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Two oceanic plates will have essentially the same density,
about 3.0 g/cm3.
When two oceanic plates collide one is eventually
subducted beneath the other.
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Long narrow deep see trenches mark the zones where the
plate is subducted.
The plate subducted begins to melt as it comes in contact
with the asthenosphere.
Molten material begins to rise, forming a volcanic island
arc on the overriding plate.
Section 21.3
Oceanic-Oceanic Convergence

The deep sea
trench and the
volcanic island arc
are parallel and
close to each other.
Section 21.3
Oceanic-Continental Convergence
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Since continental crust is less dense (2.7 g/cm3), it is the
oceanic crust that is always subducted.
A trench will develop along the zone where the oceanic
crust is subducted.
As the oceanic crust descends toward the asthenosphere
it begins to melt.
Magma rises up through the overriding continental plate
forming volcanic mountain ranges at the surface.

The Andes and Cascades are volcanic mountains.
Section 21.3
Oceanic-Continental Convergence

The ocean
trench and the
volcanic
mountains are
parallel and
close to each
other.
Section 21.3
The Andes Mountains were formed by oceaniccontinental convergence.
Copyright © Bobby H. Bammel. All rights reserved
Section 21.3
Cascade Mountains were formed by
oceanic-continental convergence.
Copyright © Bobby H. Bammel. All rights reserved
Section 21.3
Continental-Continental Convergence
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Continental plates have a relatively low density. (2.7
g/cm3)
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Subduction of continental crust is minimal due to its low
density.
During convergence the plate edges are intensely
deformed to construct fold-mountain ranges.
Continents can increase in size during this process by
suturing themselves together along fold-mountain
systems.
Section 21.3
Continental-Continental Convergence
The Himalayas, Alps,
and Appalachians are examples.
Section 21.3
Transform Boundary
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Linear zones where adjacent plates slide past each other
in opposite directions.
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This is a zone of shearing, or transform motion.
Crust is not destroyed or created along a transform
boundary since neither subduction nor magma upwelling
occur.
Periodic movements along these faults result in sudden
energy release and repeated earthquakes.

These zones are said to be seismically active.
Section 21.3
Plate Motion and Volcanoes
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The term ‘volcano’ can refer to either a vent from which
hot molten material escapes or a mountain created by
solidified volcanic rock.
Although the specific occurrence of volcanic activity is
usually unpredictable, the locations of eruptions and
potential eruptions are known.
Section 21.4
Active Volcanoes of the World
The vast majority of active
volcanoes lies along plate boundaries
Section 21.4
Most Volcanoes occur
along Convergent Boundaries
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When two plates collide, one plate is usually subducted
beneath the other
During the subduction process, rock just above the
subducting plate margin melts, resulting in the molten
rock rising to the surface to form volcanic islands or
mountains.
Section 21.4
Subduction of Oceanic Lithosphere
Forms a Volcanic Island Arc – such as Japan
Section 21.4
Earthquakes
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Earthquake – sudden release of energy due to a sudden
movement in the Earth’s crust or mantle, resulting from
stresses
Seismology – the study of earthquakes
Earthquakes cause the Earth’s surface to vibrate and
sometimes result in violent movements, depending on the
amount of energy released
Section 21.5
Earthquakes
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Earthquakes occur when rocks grind past each other
along plate boundaries.
During this process vibrations radiate out in all directions
from the disturbance.
The major danger from earthquakes is not the vibrations
but rather the human-made structures that collapse.
Section 21.5
Causes of Earthquakes
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Most earthquakes are caused by movements of the
lithospheric plates.
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They can also result from explosive volcanic eruptions or by
human-caused explosions.
Movements of lithospheric plates generally cause faults in
the crustal material.
Fault – a fracture in rock along which there has been
visible movement of the two sides relative to each other
Section 21.5
Causes of Earthquakes
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Earthquakes are most likely to occur along plate
boundaries.
Stresses are exerted on the rock formations in adjacent
plates, as movement occurs.
Since rock possess elastic properties, energy is stored
until the stresses can overcome the friction between the
two plates.
At the moment of energy release, the rocks along the
fault suddenly move, the energy is released, and an
earthquake occurs.
Section 21.5
World Map of Recorded
Earthquake Locations
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Earthquake occurrence closely follows the volcanic ring of fire.
Section 21.5
Causes of Earthquakes

Generally aftershocks will occur after a major
earthquake.
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These are caused by the rocks continuing to adjust to their
new positions.
Transform plate boundaries are the locations of many of
the world’s longest continuous faults (transform faults.)
The San Andrea Fault in California is a transform fault
that lies between the Pacific and North American plates.
Section 21.5
San Andreas Fault of California

The San Andreas Fault is the master fault of an intricate
fault zone that runs along the coastal area of south and
central California.
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Many earthquakes have occurred along this fault.
In 1906 a major earthquake occurred in the San Francisco
area, resulting in hundreds of lives lost and millions of
dollars of damage.
In 1989 a major earthquake in the area caused severe
bridge, building, and highways damage.
Little can be done to control this fault line.
Section 21.5
San Andreas Fault

In about 10 million years
Los Angeles will move far
enough north to be
adjacent with San
Francisco.
Section 21.5
Anatomy of an Earthquake
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The point of the initial movement, or energy release,
along the fault is called the focus.
The focus is generally located underground.

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From a few miles to perhaps several hundred miles in depth
The point on the Earth’s surface directly above the focus
is designated the epicenter.

This is the surface position that receives the greatest impact
from the earthquake.
Section 21.5
Focus and Epicenter
Photo Source: From Dolgoff, Anatole Physical Geology
Copyright © 1996 by Houghton Mifflin Company. Used with permission
Section 21.5
Earthquake – Energy Release
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When an earthquake occurs the energy released from
the focus propagates outward in all directions as seismic
waves.
A seismograph monitors and measures the seismic waves.
The greater the energy released in the quake, the greater
the amplitude (height) of the traces (lines) on the
recorded seismogram.
Section 21.5
Seismograph

During a quake, the spool vibrates and the light beam is
relatively still.
Section 21.5
Earthquake Severity

The severity and strength of earthquakes are measured
on two common scales:
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
The Richter Scale and the modified Mercalli Scale.
The Richter scale measures the amount of absolute
energy released during a quake by calculating the seismic
wave energy at a standard distance.
The modified Mercalli scale describes the results of the
earthquake in terms of felt and observed effects.
Section 21.5
Modified Mercalli Scale
Section 21.5
Richter Scale

This scale was developed in 1935 by Charles Richter of
Cal Tech.

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
This is the most common earthquake measurement.
This scale correlates the largest seismogram peak during
a given quake to the amount of energy released by the
quake.
The Richter scale gives the earthquake’s magnitude,
expressed as numbers, usually between 3 and 9.
Section 21.5
Richter Scale
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The Richter scale is logarithmic.
Each whole number increment represents a 10-fold
increase in amplitude tracings.
Each whole number increment represents a 31-fold
increase in energy release.
Therefore, an earthquake of magnitude 5 releases 31
times the energy of a magnitude 4.
An earthquake of magnitude 6 releases more than 900
times the energy of a magnitude 4.
Section 21.5
Richter Scale


One significant drawback of the Richter scale is that the
magnitude of the earthquake gives no indication of the
damage it may cause.
Earthquake damage depends on many factors including
focus location, geologic rock types, population density, and
construction types.

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Relatively moderate quakes in areas with high populations
and/or poor construction techniques can cause considerable
property damage and loss of life.
More severe quakes in sparsely populated areas may cause
very little damage.
Section 21.5
Richter Scale – Earthquake Severity
Section 21.5
Related Earthquake Damage


Damage from earthquakes may be directly related to the
vibrational tremors or it may result from a number of
secondary effects.
Landslides are commonly triggered by quakes

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
Much of the damage in the 1964 Alaskan quake was due to a
series of small landslides.
Fires that are initiated by the initial tremors are difficult
to fight due to broken water lines.
The U.S. government (FEMA) has literature available on
earthquake preparedness.
Section 21.5
Submarine Earthquakes

When a submarine earthquake occurs, some of the
energy may be release into the water to form huge waves
called tsunamis.
Section 21.5
Crustal Deformation
and Mountain Building



Along plate boundaries tremendous forces may be
exerted that result in the buckling, fracturing, or shifting
of rock units.
These forces can rupture the plate edges into huge
displace blocks and may eventually result in the formation
of mountain ranges.
Two basic types of structural deformation are common:

Folding and Faulting.
Section 21.6
Crustal Deformation - Folding




Folding – buckling of the rock layers into anticlines
(arches) and synclines (toughs)
Folding occurs when slow compressive forces apply
extreme pressures on the rock layers.
The forces that cause folding may be exerted either
horizontally or vertically.
In general, folding occurs mainly during the early stages of
mountain building.
Section 21.6
Crustal Folding

In an extensively folded area, it is a particular rock layer’s
resistance to erosion that determines what type of
topographic feature is formed.

Note that anticlines do not always form high ridges.
Section 21.6
Crustal Deformation - Faulting


Fault – a fracture in rock along which there has been
visible movement of the two sides relative to each other
Stresses that form faults may be compressional.


If the compression is vertical uplifts are produced. If the
compression is horizontal the crust will be shortened.
Tensional (pull-apart) stresses can also form faults.

Tension causes the crust to lengthen.
Section 21.6
Fault Terminology




Fault plane – an approximately planar surface along which
the actual movement takes place
Hanging wall – this is the fault block that is on the
uppermost side of an inclined fault plane
Footwall – this is the fault block that is on the lowermost
side of an inclined fault plane
The fault block that has moved up relative to the other
side is termed the upthrown side.
Section 21.6
Fault Types

There are three basic types of faults:


Normal, Reverse, and Transform.
Normal fault – the hanging wall (uppermost side) moves
down with respect to the footwall


Reverse fault – the footwall (lowermost side) moves down
with respect to the hanging wall


Tensional forces (pull-apart) cause normal faults.
Compressive forces cause reverse faults.
Strike-slip (transform) fault – stresses are parallel to the fault
plane (horizontal motion)
Section 21.6
Fault Terminology Illustrated
Photo Source: From Dolgoff, Anatole Physical Geology Copyright © 1996 by Houghton Mifflin Company. Used with permission
Section 21.6
Mountain Building


The mountain building process occurs most often along
and because of converging plate boundaries.
Mountains can be classified into three broad categories,
based on their characteristic features:
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Volcanic
Fault-block
Fold
Section 21.6
Volcanic Mountains
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These mountains are primarily formed through a series of
volcanic eruptions.
Most volcanic mountains are located along convergent
boundaries, since that is where most volcanoes occur.
Along an oceanic-oceanic convergent boundary, chains of
volcanic islands will form on the plate overlying the
subduction zone.
The Aleutian Islands, Japan, and the Lesser Antilles are all
examples of volcanic mountains.
Section 21.6
Volcanic Mountains
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Volcanic mountains also form along a continental-oceanic
convergent boundary.
The continental plate always overlies the subduction zone
and this is where volcanic mountains will form.
The Andes Mountains of South America were formed as
the oceanic Nazca plate is subducted beneath the
continental South American plate.
The Cascade Mountains are also volcanic mountains.
Section 21.6
Fault-Block Mountains
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Normal faulting can produce tilting and uplift of large
crustal blocks.
This will result in dramatic fault-block mountains rising
abruptly above the surrounding lowlands.
The Grand Tetons of Wyoming, the Sierra Nevada
Mountains of California, and the Wasatch range of Utah
are all examples of fault-block mountains in the U.S.
Section 21.6
Grand Teton Mountains, Wyoming
Copyright © Bobby H. Bammel. All rights reserved.
Section 21.6
Fold Mountains
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Fold mountains are characterized by prolific folding of the
rock strata.
The Alps, Himalayas, and Appalachians are all examples of
fold mountains.
These mountains are also characterized by thick packages
of marine sedimentary strata.
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This sedimentary strata was originally deposited below sea
level and then uplifted and incorporated into the fold
mountains.
Marine fossils are regularly found high in a fold mountain
range.
Section 21.6
Formation of the
Himalayas – Fold Mtns.
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During the breakup of Pangea (200 m.y.a.) the
subcontinent of India broke away from Africa.
As the Indian plate moved north toward Asia, oceanic
lithosphere was continually subducted beneath Eurasia.
During the time before continental collision, sediments
were deposited in the marine waters between India and
Eurasia.
Section 21.6
After breaking away
from Africa, India
moved north and
eventually collided
with Eurasia.
Section 21.6
Formation of the
Himalayas – Fold Mtns.
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The sedimentary strata that was deposited between
Eurasia and India was eventually uplifted and folded into
the mountains.
When the continental crust portions finally collided,
subduction was significantly slowed.
The edge of the Eurasian plate was uplifted as the
continental Indian plate wedged under it.
The Indian plate continues to move north today, resulting
in continued uplift of the Himalayas.
Section 21.6
Formation of the Himalayas
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
The oceanic crust was
subducted beneath
Asia until the
continental crusts
collided.
The collision of the
Eurasian and Indian
continental plates
resulted in the lofty
Himalayas.
Section 21.6