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Chapter 22 Lecture
Conceptual
Integrated Science
Second Edition
Plate Tectonics
© 2013 Pearson Education, Inc.
This lecture will help you understand:
• The layered structure of the Earth
• How seismic waves reveal Earth's internal
structure
• The concept of continental drift and how it led to
the theory of plate tectonics
• How the motion of tectonic plates affects Earth's
surface
• Evidence for the theory of plate tectonics
• Differences among the three kinds of plate
boundaries
• How the process of subduction relates to
volcanism and earthquakes
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Earth Science Is an Integrated Science
• A system is a collection of components that constantly
affect one another.
• Systems cannot be understood simply in terms of their
individual parts. The way the parts work together and the
emergent properties that arise from that interaction are
also attributes of the system.
• Earth is a system with living and nonliving parts that
interact in tremendously complex ways. This is why
Earth science is an integrated science. No one science
discipline is sufficient to describe all of Earth's parts,
processes, and properties. Physics, chemistry, biology,
and astronomy all come into play.
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•
•
•
•
Earth is a rocky sphere
¾ of it is covered by water
A thin atmosphere (layer of gases) surrounds it
Geology is the broadest Earth science discipline
– Meteorology – study of weather
– Petrology – study of rocks
– Mineralogy – study of minerals within rocks
– Paleontology – study of Earth’s early history
– Oceanography – study of the oceans
– Volcanology – study of volcanoes
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Earth Has Distinct Compositional Layers
• Earth has three basic layers: crust, mantle, and
core. These layers differ in their chemical
composition.
• The crust is composed mainly of rocks that have
minerals rich in silicon and oxygen.
– Most common element in crust is oxygen.
• Earth's crust is thin and cool overall but divided
into basaltic oceanic crust and granitic
continental crust.
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Earth Has Distinct Compositional Layers
• Continental crust
– Makes up landmasses with lighter, less dense
granitic rock
– Made up of mainly of silicon and oxygen
– 40 km thick
• Oceanic crust
– Makes up the ocean floor
– Made of mostly basalt
• With higher amounts of iron and magnesium than
granitic rock
– 7 km thick
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Details of the Crust
The Earth's crust has two distinct regions:
• Oceanic crust is compact and averages about
10 kilometers in thickness.
– It is composed of dense basaltic rocks.
• Continental crust varies between 20 and 60
kilometers in thickness.
– It is composed of less dense granitic rocks.
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• Mountains are like tips of icebergs
• Supported by deep “roots”
• They are continuously replenished from below
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• The crust “floats” on
the mantle
• The mountain rises
higher on the mantle
when it “unloads”
mass by erosion.
• This vertical
positioning of the
crust on the mantle is
called isostasy.
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Isostasy Relates to the Vertical Position of
the Crust
• The word isostasy is derived from the Greek
roots iso meaning "equal" and stasis meaning
"standing"—equal standing.
• Isostasy is the vertical positioning of the crust so
that gravitational and buoyant forces balance
one another.
• Low-density crust floats on the denser
underlying mantle.
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Why Are Continents High And Oceans Low
In Elevation?
• Isostasy! Variations in surface elevations result
from variations in the thickness and the density
of the crust.
• Areas of continental crust stand higher than
areas of oceanic crust because continental crust
is thicker and less dense than oceanic crust.
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• Oceanic crust is denser so it sinks lower on the
mantle than continental crust.
• Earth’s crust doesn’t just move up and down
• It can move in many more ways
• Ocean crust and continental crust do intermix
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Isostasy
CHECK YOUR NEIGHBOR
The Earth's crust is thicker beneath a mountain
because
A. the roots of the mountain are heavier than the
mountain at the surface.
B. mountains sink until the upward buoyant force
balances the downward gravitational force.
C. mantle rock is weak beneath the mountain.
D. oceanic crust is thin.
Explain your answer to your neighbor
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Earth Has Distinct Compositional Layers
• The mantle is thick and consists of hot rock rich
in silicon and oxygen–like the crust, except the
mantle contains more magnesium, iron, and
calcium.
– From molten rock in volcanic vents
• We cannot drill into the mantle because the rock
is so hard that drilling equipment breaks before
reaching the bottom of the crust.
– 82% of the mass and 65% of the volume of
the Earth is the Mantle.
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Earth Has Distinct Compositional Layers
• The mantle is 2900 km thick
• It is denser than the crust
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Earth Has Distinct Compositional Layers
• The core is composed of scorching hot metal,
mostly iron with some nickel.
– More dense than the crust and mantle
– Has a radius of about 3500 km
• Approximately the distance between San
Francisco to New York
– We don’t know a lot about the core
– Most of what we know comes from
seismology – the study of earthquake waves
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Differentiation
• Earth was molten when it was a young planet,
due to collisions with space debris.
• Denser elements were able to sink toward
Earth's center when the planet was fluid. This is
the process of differentiation.
– The process by which gravity separates fluids
according to density.
• As a result of differentiation, Earth is layered
according to density and therefore also
according to composition.
– So Earth’s elements are not evenly distributed
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Details of the Mantle
• The mantle makes up 82% of Earth's volume
and 65% of Earth's mass.
• The mantle is Earth's thickest layer—2900 km
from top to bottom.
• Mantle rock is rich in silicon and
oxygen.
– It also contains heavier
elements, such as iron,
magnesium, and calcium.
• The mantle is divided into two
regions—upper mantle and lower mantle.
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It's Also Useful to View Earth in Terms of
Structural Layers
• Earth consists of layers that differ by properties
rather than chemical composition.
• The properties that determine Earth's layers
include temperature, pressure, strength, and
ability to flow.
• Earth's structural layers are: lithosphere,
asthenosphere (including crust and upper
mantle), lower mantle, outer core, and inner
core.
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• Lithosphere
– “rock sphere” is a shell of cool, rigid rock.
– Includes the crust and some upper portions of
the Earth’s mantle.
– On average it is about 100 km thick.
• It is thickest below the continents and thinnest
below the oceans.
– It is not continuous like the other layers
• it is broken into interlocking pieces called tectonic
plates
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• Asthenosphere
– Is under the lithosphere
– Made of mantle rock
• The rock is soft and flows slowly
• It is called “plastic”
– A solid that can flow slowly.
– An hour hand moves about 10,000 times faster than the
flowing asthenosphere.
– So it would look solid rock if you could see it.
• Pieces of the lithosphere are carried by
asthenosphere.
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The Lower Mantle
• The lower mantle extends from a depth of 660
kilometers to the outer core.
• Radioactive decay produces heat throughout the
mantle.
• The lower mantle is under great pressure,
making it less plastic than the upper mantle.
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• The lower mantle
– Sits between the asthenosphere and outer
core.
– Made of strong, rigid mantle rock
– Despite its extreme heat, it is not nearly as
“plastic” as the asthenosphere.
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Details of the Core
• The core is composed predominantly of metallic
iron.
• The core has two layers—a solid inner core and a
liquid outer core.
• The inner core is solid due to great pressure.
• The outer core is under less pressure and flows
in a liquid phase.
• Flow in the outer core produces Earth's magnetic
field.
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• The outer core
– Hot, liquid metal – mostly iron with some
nickel.
– Found beneath the mantle.
– Since it is a liquid, it spins as the Earth
rotates.
– It also swishes and flows, because its
enormous heat stirs up convection currents.
– Creates Earth’s magnetic field.
• This geomagnetic field shields Earth from solar
winds.
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• The inner core
– Solid sphere of hot metal.
– Mostly iron like the outer core.
– Its temperature is estimated to be over 7000
degrees Celsius.
• About as hot as the surface of the sun!
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Question
• So how does the inner core stay solid at that
temperature?
• Discuss with your neighbor
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Seismology
• Seismology is the study of earthquakes and
seismic (earthquake) waves.
– Earthquakes are the shaking or trembling of
the ground that occurs when big sections of
rock underground shift or break
• Earthquakes release stored elastic energy.
– Energy radiates outward in all directions.
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Seismology
• Energy travels in the form of seismic waves,
which cause the ground to shake and vibrate.
• Analysis of seismic waves provides a detailed
view of Earth's layered interior.
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Seismic Waves
• Two main types of seismic waves:
– Body waves travel through
Earth's interior.
• Primary waves (P-waves)
– Longitudinal waves
– Compress and expand the material
through which they move.
– Fastest seismic waves
» 1.5 – 8 km/s
» In solid rock, magma, water, and
air
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Seismic Waves
• Two main types of seismic waves:
– Body waves travel through
Earth's interior.
• Secondary waves (S-waves)
– Transverse waves
– Vibrate the particles up and down
and side to side
– Slower than P waves
– Can only travel through solid
materials, not liquids
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Seismic Waves
• Two main types of seismic waves:
– Surface waves travel on
Earth's surface.
• Rayleigh waves
– Roll over and over in a tumbling
motion like ocean waves
• Love waves
– Move more snakelike
» Side-to-side transverse motion
– Particularly damaging to tall
buildings
– Not loved by people that live/work in
skyscrapers!
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Body Waves: Primary Waves (P-waves)
• Primary waves are longitudinal.
– They compress and expand the material
through which they move.
– Compression and expansion occur parallel to
the wave's direction of travel.
• Primary waves travel through any type
ofmaterial—solid rock, magma, water, or air.
• Primary waves are the fastest of all seismic
waves—the first to register on a seismograph.
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Body Waves: Secondary Waves (S-waves)
• Secondary waves are transverse.
– They vibrate rock in Earth in an up-and-down
or side-to-side motion.
– Transverse motion occurs perpendicular to a
wave's direction of travel.
• Secondary waves travel through solids; they are
unable to move through liquids.
• Secondary waves are slower than P-waves—the
second to register on a seismograph.
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Surface Waves
• Surface waves are the slowest seismic waves
and the last to register on a seismograph.
• Surface waves are the most destructive type of
seismic waves.
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Surface Waves
• Rayleigh waves have a rolling-type of motion.
– They roll over and over in a tumbling motion,
similar to ocean wave movement.
– The tumbling motion occurs backward
compared to the wave's direction of travel.
– The ground moves up and down.
• Love waves have motion similar to S-waves.
– Horizontal surface motion is side to side.
– Whiplike, side-to-side motion occurs
perpendicular to the wave's direction of travel.
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Seismic Waves
A situation to ponder…
Surface waves are the most destructive type of
seismic waves. Why?
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Seismic Waves
CHECK YOUR NEIGHBOR
The most destructive earthquakes are caused by
the passage of surface waves because
A. they travel faster than other seismic waves.
B. they occur in the crust, the densest layer of the
Earth.
C. they occur at the surface, where the ground
shakes up and down and from side to side.
D. they travel deep into the Earth's interior.
Explain your answer to your neighbor
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Wave Properties Reveal Earth's Interior
The velocity of a seismic wave as well as how it
reflects or refracts and whether or not it is
transmitted all depend on density and other
properties of the medium the wave is traveling
through. Thus wave properties can reveal density
differences, compositional layers, physical state,
and other characteristics of Earth's interior.
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Discovery of the Core–Mantle Boundary
• In 1906, Richard Oldham observed that
P-waves and S-waves travel together for a
distance, then encounter a boundary where the
S-waves stop and the P-waves refract.
• He had discovered the core–mantle boundary.
– A place where the solid Earth became liquid.
– Because S-waves can’t travel through liquids
and P-waves can.
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Discovery of the Crust–Mantle Boundary
ˇ ´ observed a sharp
• In 1909, Andrija Mohorovicic
increase in seismic velocity at a shallow layer
within Earth.
ˇ ´ had discovered the crust–mantle
• Mohorovicic
boundary.
– The increase in speed was because the wave
was passing form a low-density solid to a
high-density solid.
• Earth is composed of a thin outer crust that sits
upon a layer of denser material, the mantle.
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Determining the Depth of the Core–Mantle
Boundary
• In 1913, Beno Guttenberg
refined Oldham's work by
locating the depth of the
core–mantle boundary
(2900 km).
• When P-waves reach this
depth, they refract so strongly
that the boundary casts a
P-wave shadow (where no
waves are detected) over part
of the Earth.
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Seismic Waves Revealed Inner Core and
Outer Core
• In 1936, Ingre Lehman observed that P-waves
also refract at a certain depth within the core.
• At this depth, P-waves show an increase in
velocity, indicating higher–density material.
• Lehman discovered the inner core.
– The core has two parts: a liquid outer core
and a solid inner core.
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Continental Drift: An Idea Before Its Time
Alfred Wegener (1880–1930)
• Continental drift hypothesis:
– The world's continents are in motion and have
been drifting apart into different configurations
over geologic time.
• Proposed that the continents were at one time
joined together to form the supercontinent of
Pangaea—"universal land"
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Evidence for Continental Drift
Wegener used evidence from many
disciplines to support his hypothesis:
• Jigsaw fit of the continents
• Fossil evidence
• Matching rock types
• Structural similarities in
mountain chains on different
continents
• Paleoclimatic evidence
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Continental Drift: Lacking a Mechanism
• Despite evidence to support continental drift,
• Wegener could not explain how the continents
moved.
• Without a suitable explanation, Wegener's ideas
were dismissed.
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A Mechanism for Continental Drift
• Detailed mapping of the seafloor revealed:
– Huge mountain ranges in the middle of ocean
basins
– Deep trenches alongside some continental
margins
• So, the deepest parts of the ocean are near the
continents, and out in the middle of the ocean,
the water is relatively shallow.
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Seafloor Spreading
Harry Hess's hypothesis of seafloor spreading
provided the mechanism for continental drift.
• The seafloor is not permanent; it is constantly being
renewed.
• Mid-ocean ridges are sites of new lithosphere formation.
• Oceanic trenches are sites of lithosphere destruction
(subduction).
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• The Earth is not getting bigger.
• This means that the lithosphere is being
destroyed as fast as it is created.
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Seafloor Spreading Is Supported by
Magnetic Studies of the Ocean Floor
• Lava erupted at the mid-ocean ridges is rich in
iron.
• Magnetite crystals in lava align themselves to
Earth's magnetic field.
• Earth's magnetic poles flip—the north and south
poles exchange positions. This is known as
magnetic reversal.
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Seafloor Spreading Is Supported by
Magnetic Studies of the Ocean Floor
• The seafloor holds a record of
Earth's magnetic field at the
time the rocks of the seafloor
cooled.
• The magnetic record appears
as parallel, zebralike stripes on
both sides of mid-ocean ridges.
• The age of the ocean floor and
the rate of seafloor spreading
could be determined
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Plate Tectonics: A Modern Version of an Old
Idea
Plate tectonics is the unifying theory that explains
the dramatic, changing surface features of the
Earth in terms of the shifting of tectonic plates.
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Features of Tectonic Plates
• Plates are sections of Earth's strong, rigid outer
layer—the lithosphere.
– They are up to 100 km thick.
• Because plates are composed of lithosphere,
they consist of the uppermost mantle and
oceanic or continental crust.
• Plates overlie the weaker asthenosphere. They
ride along with the asthenosphere as it flows.
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Plate Tectonics
• Earth is divided into a dozen or so major
lithospheric plates as well as a few smaller ones.
• Plates are in motion and continually changing in
shape and size.
• The largest plate is the Pacific Plate.
• Several plates include an entire continent plus a
large area of seafloor.
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Speed of Tectonic Plates
• Earth's plates move in different directions and at
different speeds.
• Continental plates tend to move slowly.
• Oceanic plates tend to move faster.
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Plate Tectonics
CHECK YOUR NEIGHBOR
Continental plates tend to move slower than
oceanic plates because
A.
B.
C.
D.
their roots extend deep into the mantle.
they are heavier.
they are convergent.
of gravity.
Explain your answer to your neighbor
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Plate Boundaries
• Interactions between plates occur along plate
boundaries.
• The creation and destruction of lithosphere
occur along plate boundaries.
• Earthquakes, volcanoes, and mountains occur
along plate boundaries—and sometimes along
former plate boundaries.
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• Forces that pull the seafloor apart
– Force of gravity
• Creates slab pull
– The cool, more dense edge of the plate sinks into the
asthenosphere.
– Escape of Earth’s internal heat by convection
• 3 sources of this heat
– Radioactivity
» From mantle and core
– Heat is stored from the “great bombardment”
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Plate Tectonics:
Three Types of Plate Boundaries
• Divergent plate boundaries
– Magma generation and lithosphere formation
• Convergent plate boundaries
– Magma generation and lithosphere
destruction
• Transform–fault boundaries
– Plates slide past each other.
– No magma generation, no formation or
destruction of lithosphere
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Divergent Boundary Features
• Plates move away from one another.
• As plates move apart, the asthenosphere rises
and partially melts to form lava.
– New crust is formed as lava fills in the gaps
between plates.
• In the ocean, there is seafloor spreading.
– Mid-ocean ridge
– The Mid-Atlantic Ridge rate of spreading
ranges between 1 cm and 6 cm per year.
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Divergent Boundary Features
• On land, continents tear apart.
– Rift valley
• Shallow earthquakes occur.
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Convergent Boundary Features
• Plates move slowly toward each other.
• Oceanic crust is destroyed when plates converge
at a subduction zone.
– Subduction is when one plate descends below
another.
• Continental crust is deformed when plates collide.
• Deep earthquakes occur at subduction zones.
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Types of Convergent Boundaries
Oceanic–oceanic convergence
– When two oceanic plates converge, the older, cooler
and denser plate descends beneath the other plate.
– As the denser plate descends, partial melting of
mantle rock generates magma and volcanoes.
– This creates a deep ocean trench.
• They can be 8 – 12 km deep and 100 km wide!
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Types of Convergent Boundaries
Oceanic–oceanic convergence
– If the volcanoes emerge as islands, a volcanic
island arc is formed (examples: Japan,
Aleutian islands, Tonga islands).
• These islands parallel the trench
• Earthquakes are common
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Types of Convergent Boundaries
Oceanic–continental convergence
– The denser oceanic slab sinks into the asthenosphere.
– As the denser plate descends, partial melting of mantle
rock generates magma.
– The mountains produced by volcanic activity from
subduction of oceanic lithosphere are called continental
volcanic arcs (Andes and Cascades) or island arcs
(Aleutian Islands).
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Types of Convergent Boundaries
Oceanic–continental convergence
– Why does oceanic plate partially melt?
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Types of Convergent Boundaries
Continental–continental convergence
– Continued subduction can bring two continents together.
– The blocks of rock have the same density, and
buoyancy, so neither sinks below the other when they
collide (no subduction).
– The result is a collision between two continental blocks.
– The process produces mountains (Himalayas, Alps,
Appalachians).
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Types of Convergent Boundaries
Continental–continental convergence
– The result is a collision between two continental blocks,
that push one another upward.
– The process produces mountains (Himalayas, Alps,
Appalachians).
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Continental–Continental Convergence
The continent-to-continent collision of India with
Asia produced—and is still producing (1 cm per
year)—the Himalayas.
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Transform-Fault Boundaries
• Plates slide past one
another and no new
lithosphere is created
or destroyed.
• Most transform faults
join two segments of
a mid-ocean ridge.
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Transform-Fault Boundaries
• Transform faults are
oriented
perpendicular to midocean ridges.
– Permits plates to move
from offset ridge
segments
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Transform-Fault Boundaries
• The plates slide past
each other very
slowly.
• In some locations the
friction is so great that
entire sections of rock
become “stuck.”
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Transform-Fault Boundaries
• When the
compressional and
tensional forces
pulling and pushing
exceed friction
• The stressed rock
suddenly breaks
loose and slips.
• Releasing its stored
energy.
• Shallow but strong
earthquakes occur.
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Transform-Fault Boundaries
• Most transform-fault boundaries are located
within ocean basins.
• A few transform-fault boundaries, such as the
infamous San Andreas Fault, cut through
continental crust.
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• The San Andreas Fault is the thickest fault
between the North American Plate and the
Pacific Plate in California.
• The slice of CA to the west of the fault is slowly
moving NW, while the rest of CA is moving SE.
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• LA will not split off and fall into the ocean!
– It is actually slowly! Moving NW towards San
Francisco
– And San Francisco is slowly moving SE.
– In ~ 16 million years the two cities should be
side by side.
• In the meantime, CA residents can expect plenty
of earthquakes.
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The Real San Andreas Fault
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Life in the Trenches
• The Mariana Trench is the deepest trench.
– 11,000 m (7 miles) below sea level.
– If you sank Mt. Everest to the bottom of the
trench there would still be a mile of ocean
above it.
• Conditions are extreme.
– Water is nearly frozen.
– Pressure reaches more than 1000 atm
(~16,000 lb per square inch)
– Sunlight is totally absent.
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Life in the Trenches
• Yet life thrives in the trenches.
• Extremophiles are organisms adapted to the
extreme conditions found at great depth in
ocean trenches.
– Species include worms, shrimp, fish, crabs,
and microbes.
– We don’t understand how these animals can
survive in this type of environment.
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Life in the Trenches
• Some ocean trenches contain hydrothermal
vents, which are cracks in the crust often
associated with tectonic activity.
• Heat and chemicals such as hydrogen sulfide
are released.
• Pressure-adapted bacteria consume these
materials and convert the chemicals into energy
they need for life.
– They produce oxygen as a waste product
through a process called chemosynthesis.
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Life in the Trenches
• A food chain can develop around a hydrothermal vent
where chemosynthesis, rather than photosynthesis,
provides food for the lowest-level organisms in the chain.
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