Transcript Chapter 13 - The Theory of Plate Tectonics

Plate Tectonics
Major Earthquake
and Volcano Zone
Ridges and Trenches
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Earth’s Internal Layers
Cross-Section
of the Earth
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Earth’s Internal Layers
 The Earth’s interior consists of multiple layers:
the inner core, the outer core, the mantle, and the crust
 Based on 2008 seismic data, the inner core is thought to be solid and
composed mainly of iron crystals
 The entire inner core is 2,400 kilometers (1,491 miles) in diameter
 inner core has an inner core 1,180 km (733 miles) in diameter
 inner core has a temperature of 5,000˚C (9,032˚F) – nearly the same
as the surface of the sun
 inner core is theorized to be solid due to intense pressure
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Outer Core
 Outer core consists of the same elements at the same
temperature as inner core
 with less pressure, it is liquid
 thickness is 2,270 km (1,411 miles)
 outer core has thermal plumes - localized areas of high
heat release possibly related to volcanic activity
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Mantle
 The mantle is the layer above the core and is
approximately 2,900 kilometers (1,802 miles) in
thickness
 mantle contains mostly silicon and oxygen with some
iron and magnesium.
 Seismic wave studies give shape to the mantle’s structure
 lower mantel is made of hot, dense magma (molten
rock)
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Upper Mantle
 The upper mantle is composed of the asthenosphere
and a portion of the lithosphere
 asthenosphere is solid magma that flows slowly over
time
 lithosphere is the uppermost, rigid part of the upper
mantle and the crust
 Lithosphere = cool, solid rock portion of the outer Earth
that rests on the warmer asthenosphere
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Crust
 outermost layer of the Earth
 composed mainly of oxygen, silicon, magnesium, and
iron.
 varies in thickness
 is the outer layer of the lithosphere
 Low density magma from the asthenosphere rises and
can eventually flow from a volcano or other opening in
the crust
 Lava - when molten rock exits the opening
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The Rock Cycle
 The three rock types found in the crust are:
 Igneous
 Sedimentary
 Metamorphic.
 Rocks form or change over long periods due to the
processes of the rock cycle
 One type is converted into another type
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The Rock Cycle
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Igneous Rock
 Igneous rocks form when magma or lava cools and hardens.
 Granite and basalt are common examples
 Igneous and other rock types break apart into rock particles due to
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weathering and water flow
Water flow carries away the rock particles, along with loose soil and
particles of organic material. This process is called erosion
Wind, glaciers, and gravity cause erosion
Chemical processes break down and change rock through reactions
When the water flow slows or stops, the particles may be deposited
as sediment
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Sedimentary rock
 sediments are slowly compressed and cemented
together to form sedimentary rock
 sediments contain organic matter
 form rock in which fossils, coal, petroleum, and other
fossil fuels are found
 layers build up on top of each other
 upper layers subject lower layers to increasing pressure
and heat
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Metamorphic rock
 Metamorphic rock forms when pressure and heat
become great enough to change the rock chemically
 Rock may return to the mantle as part of plate tectonic
processes
 Subjected to the heat of the Earth’s interior
 rocks remelt and become magma, returning to the crust
as igneous rock
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Floating crust
 currently thought that the crust doesn’t sit on anything
rigid, but literally floats on the mantle
 continental crust (crust under the continents) consists
primarily of granite
 ocean crust (crust under ocean basins) consists
primarily of basaltic rock
 With the crust floating on the mantle, there must
be a balance between the weight of the crust and
the upward force of buoyancy
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Archimedes Principle of buoyancy
 Archimedes’ Principle of buoyancy states:
 an object immersed in a fluid (gas or liquid) is
buoyed up by a force equal to the weight of the
fluid displaced.
 An object that weighs less than the fluid it
displaces will float.
 If the weight of the crust changes, according to
Archimedes’ Principle the landmass must rise or
subside (sink) to compensate.
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Isostatic Equilibrium
 Isostatic equilibrium: balance between the weight
of the crust and the buoyancy provided by the
mantle
 balance is disrupted as material adds to the oceanic
crust (from deposition by sedimentation and
volcanoes) or leaves the continental crust (from
erosion)
 additional weight causes the crust to deflect
downwards, while the removal of material causes the
crust to deflect upwards
 called isostatic rebound
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Isostatic Equilibrium
 This is one theorized cause of earthquakes. To restore
equilibrium, landmasses will sink or rise slightly along
a weak area called a fault.
 During an earthquake, the landmasses (continental
or ocean basin) on either side of the involved fault
do not move together.
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Alfred Wegener
 a German
 meteorologist and
polar explorer
 proposed what
was in 1912 a
startling idea
 Not accepted
while he was alive
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Alfred Wegener and Pangaea
 He proposed:
 that in the distant past all the Earth’s continents had
been a single giant continent called Pangaea
 Surrounding Pangaea was a single
large ocean he called Panthalassa
sepetjian.wordpress.com
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Alfred Wegener and Pangaea
 Wegener was the first to formally propose a process that explained the
fit and present placement of the continents.
 Wegener theorized that because the less dense continents floated
on the molten rock of the mantle, Pangaea broke by floating apart
into separate pieces.
 The separate continents reached their present locations by drifting
apart for more than 200 million years.
 The theory that the continents were once a single landmass
that drifted apart (and are still doing so) is called the theory
of continental drift
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Evidence for Continental Drift
 In the 1600s, the first accurate world maps became
available
 continents apparently fit together like jigsaw-puzzle
pieces
 Continents match best on the edge of the continental
shelf beneath the ocean water
 First evidence of continental drift
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Evidence for Continental Drift
 In 1855, the German scientist Edward Suess found
fossils of the Glossopteris fern in South America,
Africa, Australia, India, and Antarctica.
 Seeds of this fern are too heavy to travel by wind and
too fragile to survive significant sea crossings
 Landmasses must have once been much closer
together for the fern to be so widespread
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Evidence for Continental Drift
 Continental drift proponents also studied the distribution of animals
and fossils.
 Two examples were the extinct aquatic reptile Mesosaurus and an
extinct bear-like animal called Lytrosaurus.
 Based on the distribution of fossils for these animals, it was
hypothesized that Pangaea had split into two continents about 200
million years ago.
 Laurasia, name of hypothetical northern continent
 It included today’s North America, Greenland, and Eurasia
 Gondwanaland, name of the southern continent which included
the remaining continents
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Evidence for Continental Drift
Map of Pangaea
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Evidence for Continental Drift
 Wegener also saw the distribution of coal as evidence
 In 1908, the famed polar explorer Ernest Shackleton discovered coal
in the Antarctic
 Scientists theorize that coal originates when geological processes
bury vegetation in warm, swampy climates faster than it can
decompose
 Pressure and heat alter the vegetation, eventually turning it into
coal
 Wegener reasoned Antarctica must have been in a different place
with a different climate in the past
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New Technology and Seafloor Knowledge
 The invention of Sonar made it
possible to “see” long distances
under water
 Sonar detects objects under water
by transmitting a sound and
receiving an echo
 Based on the echo’s angle, how long
it took to return, and changes in
frequency
 can determine where an object is,
its distance, and whether it’s
moving
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Sonar
 an acronym for Sound Navigating And Ranging
 Sonar made it possible to detect an otherwise invisible
iceberg 5 kilometers (3.1 miles) away
 Scientists began using an echo sounder –
 essentially modified sonar specifically for mapping
bottom terrain
 ability to map the seafloor in greater detail revealed
important new features
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Seafloor Discoveries
 A 70,000-kilometer (43,497-mile) mountain range
that extends through the Atlantic, Indian, and Pacific
Oceans.
 trenches, which are deep ravines in the seafloor
 rift valleys, which are deep valleys running through
the center of the Mid-Atlantic Ridge and other midocean ridges
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Seafloor Creation and Destruction
a theory of seafloor spreading
 seafloor is in a constant state of creation and
destruction
 New crust emerges from the rift valley in a mid-ocean
ridge
 Magma pushes up through the rift and solidifies into
new crust
 New seafloor near the ridge continuously pushes old
seafloor away from the ridge
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Seafloor Creation and Destruction
 Old seafloor subsides (sinks) at the trenches
 Old seafloor is drawn downward by gravity and inertia,
eventually reaching the asthenosphere and melting
into magma again
 new seafloor forms at the rift valleys and mid-ocean
ridges
 spreads away from the ridges until it returns to magma
at subduction zones (trenches)
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Seafloor Creation and Destruction
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Evidence of Seafloor Spreading
 There are several forms of evidence that support
the theory of seafloor spreading.
 Radiometric dating.
 Ocean-bottom sediment samples.
 Evidence is supplied by rheology.
 Magnetometer data.
 Since about 1990, geodesists
have been measuring plate
movements directly.
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Evidence of Seafloor Spreading
Radiometric Dating of the Seafloor
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Geomagnetic reversals
 basalt—the iron-rich, volcanic rock making up the
ocean floor—contains a strongly magnetic mineral
(magnetite) and can locally distort compass readings
 When these magnetic patterns were mapped over a
wide region, the ocean floor showed a zebra-like
pattern
 one stripe with normal polarity and the adjoining
stripe with reversed polarity
 stripes are symmetrical around the crests of the midocean ridges
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Alternating Polarity of mid-ocean ridge
evidence of seafloor spreading
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Birth of Plate Tectonicsa unifying theory
 In 1965, Canadian geophysicist John Tuzo Wilson
introduced a new theory that united the theories of
continental drift and seafloor spreading
 Earthquakes, volcanic activity, mountain-building,
and oceanic trench formation occur along these
plate boundaries
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Tectonic Plates
 The lithosphere is broken up into tectonic plates
 The plates are rigid and float on the asthenosphere.
 there are seven or eight plates, depending on how they are
defined and many minor plates.
 As plates move, they collide in some places, separate or
slide side-by-side in others
 Where plates meet their relative motion determines the
type of boundary: convergent, divergent, or transform.
 The lateral relative movement of the plates typically varies
from zero to 100 mm annually
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Tectonic Plates
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Seafloor Spreading and
Continental Drift Combine
Earth’s Crustal Plates
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Ring
of
Fire
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Transform boundary or fault
 At a transform boundary or fault, two plates slide
past each other.
 In the United States, perhaps the most well-known of
these is the San Andreas Fault in California.
 The plate to the west of the fault is moving north, while
the plate to the east is moving south.
 Earthquakes result as rocks move when the plates slide
next to each other.
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The San Andreas Fault
Fault
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Seafloor Spreading and
Continental Drift Combine
 At a spreading or divergent boundary, two plates are moving
apart
 The crust pulls apart and forms valleys
 Mid-ocean ridges and rift valleys mark divergent boundaries
 Magma from the asthenosphere flows up through the rift valley
where the plates separate, creating new crust and widening the
seafloor
 Besides forming new seafloor, volcanic activity at a divergent
boundary may build mountains higher than sea level
 This builds islands such as Iceland
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Rift Valleys: Dead Sea, Red Sea, Loch Ness
Divergent Boundary: Because new crust forms there,
divergent boundaries are also called constructive boundaries
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Global plate motion
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Seafloor Spreading and
Continental Drift Combine
 At a colliding or convergent boundary, two plates
push together
 Convergent boundaries are also called destructive
boundaries because
movements along the
boundaries destroy
crust
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3 types of plate collision
 The first occurs between two oceanic plates
 In this collision, one plate is subducted under (sinks
beneath) the other
 This can result in a chain
of volcanic islands
or an island arc
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Subduction and Volcanoes
 The second type of plate collision occurs between
oceanic and continental plates
 The more dense oceanic plate is subducted under the
less dense continental plate
 This subduction occurs in trenches
 A range of volcanic
mountains may form
at the edge of the
continental plate as
molten rock from the
oceanic plate rises
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Mountains fold and thrust up
 The third type of collision occurs between two
continental plates
 when two continental plates collide, mountains form
as the crust folds
 The Himalayas are thought to still be forming as the
Indo-Australian Plate pushes into the Eurasian Plate.
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Hot Spots
 What makes the hot spot theory significant is the
concept that hot spots do not move with tectonic
plates.
 Explains the nature of features forming away from
plate boundaries
 Hot spots originate in the mantle, so the volcanic areas
change on the plate as it moves over the hot spot.
 This results in a line or row of volcanic formations.
 Volcanic island chains result from the plate moving over
a hot spot
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Hot Spots
 Example- hot spot under the Pacific Plate that created
the Emperor Seamounts
and Hawaiian Islands
 formed from a hot
spot as the plate moved
northwest over the last
35 million years
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Hawaii hotspot
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Plate Movement
 Convection is a primary force driving seafloor spreading and
continental drift.
 Convection is a vertical circulation pattern in a gas or liquid caused
by hot material rising and cold material sinking
 occurs in the mantle when warm molten rock rises and cool magma
sinks
 This creates a current that moves the plates away from each other at
the divergent boundaries
 toward each other at the convergent boundaries
 past each other at the transform boundaries
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Convection
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Volcanoes are located near plate boundaries
eofdreams.com volcano
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Volcanic Mont Serrat
Lesser Antilles island arc
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Epicenters concentrate at plate boundaries
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Plate Movement
 What will happen over the next several million years?
 the Atlantic Ocean and Indian Ocean will expand
the Pacific Ocean will shrink
 Australia will continue to drift toward Eurasia.
 Southern California will pass San Francisco as it moves
to the northwest
 a new ocean will form in the East African rift valley.
 The Mediterranean Sea will close as Africa moves
northward.
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