Chapter 22
Structural Geology
Sections 22.1-22.4
Introduction
• Recall in Chapter 21 we learned that the
locations of most of the volcanoes and
igneous activity are due to the
movement of lithospheric plates in the
Earth’s outer shell.
• In this chapter we will also learn that
earthquakes, crustal deformation, and
mountain building are also largely due
to plate tectonics.
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Intro
22 | 2
Introduction - Earthquakes
• 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.
• Some of these vibrations travel through the
interior of the Earth and their study provides
scientists with significant information.
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Intro
22 | 3
Continental Drift
Formulation of the Theory
• 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?
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Section 22.1
22 | 4
Continental Drift – Alfred Wegener
• 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.
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Section 22.1
22 | 5
Starting 200 Million Years Ago Pangea
Started to Break Apart
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Section 22.1
22 | 6
Breakup of Pangea
100 Million Years Ago
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Section 22.1
22 | 7
Breakup of Pangea
Present Day Continental Configuration
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Section 22.1
22 | 8
Continental Drift – Scientific Evidence
• Wegener brought together various
pieces of geologic evidence that
supported his theory.
• There were three prominent lines of
evidence that Wegener highlighted:
– Biological evidence
– Continuity of geologic features
– Glacial evidence
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Section 22.1
22 | 9
Continental Drift
Biologic & Paleontologic Evidence
• 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.
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Section 22.1
22 | 10
Continental Drift
Continuity of Geologic Features
• 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.
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Section 22.1
22 | 11
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
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Section 22.1
22 | 12
Continuity of
Geologic Features
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Section 22.1
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Continental Drift
Glacial Evidence
• Geologic evidence suggests that the southern
areas of South America, Africa, India, and
Australia were covered with glaciers 300
million years ago.
• There is no evidence for glaciation at this
time in Europe and North America.
• This indicates that the glaciated areas were
located at very high latitudes, while North
America and Europe were at low latitudes.
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Section 22.1
22 | 14
Glacial Evidence of Continental Drift
• 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.
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Section 22.1
22 | 15
Continental Drift
Not Generally Accepted
• Although a number of lines of evidence
supported Wegener’s theory it was not
widely accepted.
• 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.
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Section 22.1
22 | 16
Seafloor Spreading
• 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.
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Section 22.1
22 | 17
Seafloor Spreading
• 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 mid-ocean 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.
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Section 22.1
22 | 18
Paleomagnetics Supports Seafloor
Spreading
• 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.
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Section 22.1
22 | 19
Seafloor Spreading – Evidence
• Remanent magnetism of the ocean crust
displays 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.
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Section 22.1
22 | 20
Magnetic Anomalies
Showing Reversals in
the Earth’s Magnetic
Fields
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Section 22.1
22 | 21
Seafloor Spreading
• 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.
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Section 22.1
22 | 22
Plate Tectonics
• 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.
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Section 22.2
22 | 23
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Section 22.2
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Plate Boundaries
• The most active areas of the Earth’s crust are
along the plate boundaries. (Chapter 21)
• There are three types of plate boundaries:
– 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
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Section 22.2
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The Asthenosphere
• 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.
– The asthenosphere is much more easily deformed
than the lithosphere.
• The lithosphere may be viewed as actually
“floating” on top of the asthenosphere.
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Section 22.2
22 | 26
Asthenosphere and Isostasy
• 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.
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Section 22.2
22 | 27
Plate Movement
• 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.
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Section 22.2
22 | 28
Convection Cells in the Asthenosphere
• Drag from the more active asthenosphere
drives the outermost solid lithosphere.
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Section 22.2
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Divergent Boundary
• 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.
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Section 22.2
22 | 30
Spreading at the Mid-Ocean Ridge
• As the two plates move apart, new magma wells up
and cools along the rift zone creating new crust.
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Section 22.2
22 | 31
Divergent Boundary
• 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.
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Section 22.2
22 | 32
Convergent Boundary
• The result of two plates converging depends
on the type of plates that are interacting.
• Three combinations are possible:
– 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.
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Section 22.2
22 | 33
Oceanic-Oceanic Convergence
• Both oceanic plates have essentially the
same density, about 3.0 g/cm3.
• When two oceanic plates collide one is
eventually subducted beneath the other.
– 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.
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Section 22.2
22 | 34
Oceanic-Oceanic Convergence
• The deep sea
trench and the
volcanic island
arc are parallel
and close to
each other.
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Section 22.2
22 | 35
Oceanic-Continental Convergence
• 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.
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Section 22.2
22 | 36
Oceanic-Continental Convergence
• The ocean
trench and
the volcanic
mountains
are parallel
and close to
each other.
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Section 22.2
22 | 37
The Andes Mountains were formed by
oceanic-continental convergence.
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Section 22.2
22 | 38
Cascade Mountains were formed by
oceanic-continental convergence.
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Section 22.2
22 | 39
Continental-Continental Convergence
• Continental plates have a relatively low
density. (2.7 g/cm3)
– Subduction of continental crust is minimal
due to its low density.
• During convergence the plate edges are
intensely deformed to construct foldmountain ranges.
• Continents can increase in size during
this process by suturing themselves
together along fold-mountain systems.
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Section 22.2
22 | 40
Continental-Continental Convergence
The Himalayas, Alps, and Appalachians are
examples.
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Section 22.2
22 | 41
Transform Boundary
• Linear zones where adjacent plates slide past
each other in opposite directions.
– 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.
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Section 22.2
22 | 42
Earthquakes
• 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
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Section 22.3
22 | 43
Causes of Earthquakes
• Most earthquakes are caused by movements
of the lithospheric plates.
– 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
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Section 22.3
22 | 44
Causes of Earthquakes
• 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.
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Section 22.3
22 | 45
Recorded Earthquake Locations
• Earthquake occurrence closely follows the volcanic
ring of fire.
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Section 22.3
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Causes of Earthquakes
• Generally aftershocks will occur after a
major earthquake.
– 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.
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Section 22.3
22 | 47
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.
– 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.
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Section 22.3
22 | 48
San Andreas Fault
• In about 10 million
years Los Angeles
will move far
enough north to be
adjacent with San
Francisco.
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Section 22.3
22 | 49
Anatomy of an Earthquake
• The point of the initial movement, or energy
release, along the fault is called the focus.
• The focus is generally located underground.
– 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.
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Section 22.3
22 | 50
Anatomy of an Earthquake
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Section 22.3
22 | 51
Earthquake – Energy Release
• 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.
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Section 22.3
22 | 52
Seismograph
• During a quake, the spool vibrates and the light beam
is relatively still.
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Section 22.3
22 | 53
Earthquake Severity
• The severity and strength of earthquakes are
measured on two common scales:
– 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.
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Section 22.3
22 | 54
Modified Mercalli Scale
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Section 22.3
22 | 55
Richter Scale
• This scale was developed in 1935 by Charles
Richter of Cal Tech.
– 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.
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Section 22.3
22 | 56
Richter Scale
• 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.
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Section 22.3
22 | 57
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.
– 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.
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Section 22.3
22 | 58
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
– 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.
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Section 22.3
22 | 59
Submarine Earthquakes
• When a submarine earthquake occurs, some
of the energy may be release into the water
to form huge waves called tsunamis.
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Section 22.3
22 | 60
Earthquake Waves Help Reveal the
Earth’s Interior
• Despite their destructive character, seismic
waves are used by scientists to gain a better
understanding of the Earth’s interior.
– As these wave propagate through the Earth’s
interior their speed and direction are a function of
the types of material that they penetrate.
• Scientists use seismic waves, the
composition of meteorites, and high
pressure/temperature laboratory experiments
to decipher the composition of the Earth’s
interior.
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Section 22.3
22 | 61
Seismic Waves
• 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.
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Section 22.3
22 | 62
Body Waves
• Two types of body waves can be
distinguished by their type of motion and
speed:
– P (primary) waves are compressional in
nature.
• 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.
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Section 22.3
22 | 63
P and S Waves
• 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.
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Section 22.3
22 | 64
Body Waves - Refraction
• 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.
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Section 22.3
22 | 65
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.
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Section 22.3
22 | 66
Structure of the Earth’s Interior
• Scientists think that the Earth is
composed of four concentric layers or
zones:
– Inner core
– Outer core
– Mantle
– Crust
• Different compositions and/or physical
properties characterize each of these
layers.
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22 | 67
Interior of the Earth – Four Layers
• Evidence suggests that the inner core is solid
and is composed of iron (85%) and nickel.
– Radius of approximately 1230 km
• The outer core is interpreted to have the same
composition as the inner core, but is liquid.
– 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.
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Section 22.3
22 | 68
Interior Structure of the Earth
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Section 22.3
22 | 69
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.
– 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, simply Moho.
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Section 22.3
22 | 70
Crust and Upper Mantle –
Physical Properties
• The crust and upper mantle can be divided
differently if we take into account its physical
properties and behavior.
• Lithosphere - outermost rigid, brittle layer
– 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.
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Section 22.3
22 | 71
Lithosphere and Asthenosphere
• The lithosphere is solid and slowly
moves over the plastic asthenosphere.
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Section 22.3
22 | 72
Crustal Deformation
• 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.
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Section 22.4
22 | 73
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.
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22 | 74
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.
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Section 22.4
22 | 75
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.
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Section 22.4
22 | 76
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.
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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
• Tensional forces (pull-apart) cause normal faults.
– Reverse fault – the footwall (lowermost side)
moves down with respect to the hanging wall
• Compressive forces cause reverse faults.
– Strike-slip (transform) fault – stresses are parallel
to the fault plane (horizontal motion)
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Section 22.4
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Fault Terminology Illustrated
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Section 22.4
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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:
– Volcanic
– Fault-block
– Fold
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Volcanic Mountains
• 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.
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Volcanic Mountains
• 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.
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Fault-Block Mountains
• 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.
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Section 22.4
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Grand Teton Mountains, Wyoming
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Fold Mountains
• 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.
– 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.
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Formation of the Himalayas – Fold Mtns.
• 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.
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After breaking away
from Africa, India
moved north and
eventually collided
with Eurasia.
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Formation of the Himalayas – Fold Mtns.
• 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.
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Formation of the Himalayas
• 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.
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