Transcript Chapter 13 - The Theory of Plate Tectonics

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Chapter 13 Page 13-4 to 13-9
The Earth Inside and Out
The Earth Inside and Out
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Chapter 13 Pages 13-4 to 13-6
The Earth Inside and Out
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 inner core has an inner core 1,180 kilometers (733
miles) in diameter. The entire inner core is thought to be
2,400 kilometers (1,491 miles) in diameter.
 The inner course is thought to have a temperature of
5,000˚C (9,032˚F) – nearly the same temperature as the
surface of the sun.
 The inner core is theorized to be solid due to intense
pressure.
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Chapter 13 Pages 13-4 to 13-6
The Earth Inside and Out
Earth’s Internal Layers
 The outer core consists of the same elements at the
same temperature.
 Scientists theorize that with less pressure, it is
liquid.
 The thickness is 2,270 kilometers (1,411 miles).
 In some places the outer core has thermal
plumes, which are localized areas of high heat
release that might possibly be related to volcanic
activity.
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Chapter 13 Pages 13-4 to 13-6
The Earth Inside and Out
Earth’s Internal Layers
 The mantle is the layer above the core and is
approximately 2,900 kilometers (1,802 miles) in
thickness.
 The mantle is thought to contain mostly silicon and
oxygen with some iron and magnesium. Seismic
wave studies have given scientists a picture of the
mantle’s structure – it consists of the upper mantle
and the lower mantle. The lower mantel is made of
hot, dense magma (molten rock).
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Chapter 13 Pages 13-4 to 13-6
The Earth Inside and Out
Earth’s Internal Layers
 The upper mantle is composed of the asthenosphere
and a portion of the lithosphere.
 The asthenosphere is solid magma that flows
slowly over time.
 The lithosphere is the uppermost, rigid part of
the upper mantle and the crust. It is the cool,
solid rock portion of the outer Earth that rests
on the warmer astehnosphere.
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Chapter 13 Pages 13-4 to 13-6
The Earth Inside and Out
Earth’s Internal Layers
 The outermost layer of the Earth is the crust.
 The crust is composed mainly of oxygen, silicon,
magnesium, and iron.
 It varies in thickness and 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. When
molten rock exist the opening, it is called lava.
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Earth’s Internal Layers
Chapter 13 Pages 13-4 to 13-6
The Earth Inside and Out
Cross-Section
of the Earth
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Chapter 13 Pages 13-4 to 13-6
The Earth Inside and Out
Earth’s Internal Layers
 Scientists separate the uppermost mantle from the
crust because they think the mantle’s elemental
composition changes little.
 The crust, however, consists of different rock types
thought to undergo change over long periods.
 Besides chemical properties, conditions such as
temperature and pressure differentiate the crust
from the mantle.
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Chapter 13 Pages 13-6 to 13-7
The Earth Inside and Out
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.
 You can think of this as the Earth’s recycling
machine, endlessly converting rock from one type to
another.
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Chapter 13 Pages 13-6 to 13-7
The Earth Inside and Out
The Rock Cycle
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Chapter 13 Pages 13-6 to 13-7
The Earth Inside and Out
The Rock Cycle
 Igneous rocks form when magma or lava cools and hardens.
Granite is a common example of igneous rock.
 Igneous and other rock types break apart into rock particles
due to 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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Chapter 13 Pages 13-6 to 13-7
The Earth Inside and Out
The Rock Cycle
 Scientists have concluded that, over time, sediments
are compressed and cemented together to form
sedimentary rock.
 Because sediments contain organic matter, this is
the only type of rock in which scientists find fossils,
coal, petroleum, and other fossil fuels.
 As layers build up on top of each other, the upper
layers subject lower layers to increasing pressure
and heat.
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The Rock Cycle
Chapter 13 Pages 13-6 to 13-7
The Earth Inside and Out
 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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Chapter 13 Pages 13-7 to 13-9
The Earth Inside and Out
Isostatic Equilibrium
 It is currently thought that the crust doesn’t sit on
anything rigid, but literally floats on the mantle.
 The continental crust (the crust under the
continents) consists primarily of granite, whereas
the ocean crust (the crust under the 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. This is the
application of Archimedes’ Principle of buoyancy.
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Chapter 13 Pages 13-7 to 13-9
The Earth Inside and Out
Isostatic Equilibrium
 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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 The balance between the weight of the crust and
the buoyancy provided by the mantle is called
isostatic equilibrium.
 As material adds to the oceanic crust (from
deposition by sedimentation, glaciers, and volcanic
activity) or leaves the continental crust (from erosion),
this balance becomes disrupted.
Chapter 13 Pages 13-7 to 13-9
The Earth Inside and Out
Isostatic Equilibrium
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Isostatic Equilibrium
Chapter 13 Pages 13-7 to 13-9
The Earth Inside and Out
 The additional weight will cause the crust to deflect
downwards, while the removal of material causes the
crust to deflect upwards.
 This is called isostatic rebound.
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Isostatic Equilibrium
Chapter 13 Pages 13-7 to 13-9
The Earth Inside and Out
 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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 Until the last half of the 20th century, the prevailing
view in science was that earthquakes and continental
movement only involved vertical rises and fall.
 This changed with the acceptance of the theory of
plate tectonics, which suggests that the continents
move in horizontal directions and that earthquakes
also result from that movement.
Chapter 13 Pages 13-7 to 13-9
The Earth Inside and Out
Isostatic Equilibrium
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Chapter 13 Page 13-10 to 13-13
The Theory of Continental Drift
The Theory of Continental Drift
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 In 1912, a German meteorologist and polar explorer
named Alfred Wegener proposed what was at the
time a startling idea.
 He proposed that in the distant past all the Earth’s
continents had been a single giant continent.
Wegener called this continent Pangaea.
 Surrounding Pangaea, he said, was a single large
ocean he called Panthalassa.
Chapter 13 Page 13-10
The Theory of Continental Drift
Alfred Wegener and Pangaea
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Alfred Wegener and Pangaea
Chapter 13 Page 13-10
The Theory of Continental Drift
 Over the years, since the first accurate maps, others
(including Leonardo da Vinci and Sir Francis Bacon)
had observed how well the continents could be
pieced together like a puzzle.
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 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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The Theory of Continental Drift
Alfred Wegener and Pangaea
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 In the 1600s, the first accurate world maps became
available.
 People noticed that the continents apparently fit
together like jigsaw-puzzle pieces.
 This was the first evidence of continental drift. It
was hundreds of years later that Wegener began
to see further evidence from other sources.
Chapter 13 Page 13-11 to 13-13
The Theory of Continental Drift
Evidence for Continental Drift
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Chapter 13 Page 13-11 to 13-13
The Theory of Continental Drift
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.
 The seeds of this fern are too heavy to travel by wind
and too fragile to survive significant sea crossings.
 To Wegener and other early advocates of
continental drift, this suggested that these
landmasses must have once been much closer
together for the fern to have spread so widely.
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Chapter 13 Page 13-11 to 13-13
The Theory of Continental Drift
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,
Wegener and others who supported the theory of continental
drift hypothesized that Pangaea had split into two continents
about 200 million years ago.
 They called the hypothetical northern continent Laurasia. It
included today’s North America, Greenland, and Eurasia.
 They named the southern continent Gondwanaland which
included the remaining continents.
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Evidence for Continental Drift
Chapter 13 Page 13-11 to 13-13
The Theory of Continental Drift
Map of Pangaea
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Chapter 13 Page 13-11 to 13-13
The Theory of Continental Drift
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.
 Since Antarctica doesn’t presently have the appropriate
climate for this kind of vegetation, Wegener reasoned, it
must have in the past been in a different place with a
different climate.
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Evidence for Continental Drift
Chapter 13 Page 13-11 to 13-13
The Theory of Continental Drift
 Despite these examples and other evidence, the
theory of continental drift wasn’t widely accepted
while Wegener was still alive.
 This may have partly been because Wegener was a
meteorologist, not a geologist.
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Chapter 13 Pages 13-14 to 13-20
The Theory of Seafloor Spreading
The Theory of Seafloor Spreading
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Chapter 13 Pages 13-14 to 13-15
The Theory of Seafloor Spreading
New Technology and Seafloor Knowledge
 The invention of Sonar (an
acronym for Sound Navigating
And Ranging) made it possible to
“see” through 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,
sonar operators can determine
where an object is, its distance,
and whether it’s moving.
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 Sonar made it possible to detect an otherwise
invisible iceberg 5 kilometers (3.1 miles) away.
 Sonar underwent improvements and played a pivotal
role in World War II.
 Scientists began using an echo sounder essentially modified sonar specifically for mapping
bottom terrain.
Chapter 13 Pages 13-14 to 13-15
The Theory of Seafloor Spreading
New Technology and Seafloor Knowledge
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Chapter 13 Pages 13-14 to 13-15
The Theory of Seafloor Spreading
New Technology and Seafloor Knowledge
 The ability to map the seafloor in greater detail
revealed important new features to scientists.
 One of the most important discoveries was a 70,000kilometer (43,497-mile) mountain range that extends
through the Atlantic, Indian, and Pacific Oceans.
 Scientists also discovered trenches, which are
deep ravines in the seafloor, and rift valleys,
which are deep valleys running through the
center of the Mid-Atlantic Ridge and other midocean ridges.
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Chapter 13 Pages 13-14 to 13-15
The Theory of Seafloor Spreading
New Technology and Seafloor Knowledge
Major Earthquake
and Volcano Zone
Patterns of Ridges
and Trenches
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Chapter 13 Page 13-16
The Theory of Seafloor Spreading
The Creation and Destruction of Seafloor
 In 1960, geologists Harry Hess and Robert S. Dietz
proposed an explanation of seafloor features. They
hypothesized that the seafloor is in a constant state
of creation and destruction through a process called
seafloor spreading.
 In the theory of seafloor spreading, new crust
emerges from the rift valley in a mid-ocean ridge.
 Magma from the asthenosphere pushes up through
the rift and solidifies into new crust.
 Through this process, new seafloor near the ridge
continuously pushes old seafloor away from the
ridge.
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 Other scientists proposed that old seafloor subsides
(sinks) at the trenches. The old seafloor is drawn
downward by gravity and inertia, eventually reaching
the asthenosphere and melting into magma again.
 Therefore, the theory of seafloor spreading says that
new seafloor forms at the rift valleys and mid-ocean
ridges, spreading away from the ridges until it returns
as part of the rock cycle at subduction zones
(trenches).
Chapter 13 Page 13-16
The Theory of Seafloor Spreading
The Creation and Destruction of Seafloor
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Chapter 13 Page 13-16
The Theory of Seafloor Spreading
The Creation and Destruction of Seafloor
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Chapter 13 Pages 13-17 to 13-20
The Theory of Seafloor Spreading
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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Chapter 13 Pages 13-17 to 13-20
The Theory of Seafloor Spreading
Evidence of Seafloor Spreading
Radiometric Dating of the Seafloor
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Chapter 13 Pages 13-17 to 13-20
The Theory of Seafloor Spreading
Evidence of Seafloor Spreading
Alternating Polarity of the Mid-Ocean Ridge
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Chapter 13 Pages 13-22 to 13-29
The Unifying Theory: Plate Tectonics
The Unifying Theory: Plate Tectonics
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 In 1965, Canadian geophysicist John Tuzo Wilson
introduced a new theory that united the theories of
continental drift and seafloor spreading.
 This was the birth of today’s theory of plate
tectonics. This theory combined ideas from both
theories, along with some of the original isostatic
equilibrium concepts.
Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
Seafloor Spreading and
Continental Drift Combine
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 According to the theory of plate tectonics, the
Earth’s lithosphere consists of more than a dozen
separate plates.
 Some plates have entirely oceanic crust, some
have continental crust, and some have both types.
 The plates are rigid and float on the
asthenosphere.
Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
Seafloor Spreading and
Continental Drift Combine
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Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
Seafloor Spreading and
Continental Drift Combine
Earth’s Crustal Plates
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Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
Seafloor Spreading and
Continental Drift Combine
 Scientists can measure plate movement. Studies
show that the plates move a few centimeters a year.
 As plates move, they collide in some places,
separate in others, and slide side-by-side in yet
others.
 Where plates meet at plate boundaries, there are
three possible motions relative to each other;
spreading apart, pushing together, or passing
side-by-side.
 The interaction where plates meet each other
explains how mid-ocean ridges, rift valleys and
trenches relate to plate tectonic concepts.
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Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
Seafloor Spreading and
Continental Drift Combine
 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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Seafloor Spreading and
Continental Drift Combine
Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
The San Andreas
Fault
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Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
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.
 Because new crust forms there, divergent boundaries
are also called constructive boundaries.
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Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
Seafloor Spreading and
Continental Drift Combine
Divergent Boundary
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 At a colliding or convergent boundary, two plates
push together.
 Convergent boundaries are also called destructive
boundaries because movements along these
boundaries destroy crust.
Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
Seafloor Spreading and
Continental Drift Combine
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 There are three 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.
Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
Seafloor Spreading and
Continental Drift Combine
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Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
Seafloor Spreading and
Continental Drift Combine
 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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Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
Seafloor Spreading and
Continental Drift Combine
Island Arcs - Trench Development
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 The third type of collision occurs between two
continental plates.
 According to the theory, 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.
Chapter 13 Pages 13-22 to 13-25
The Unifying Theory: Plate Tectonics
Seafloor Spreading and
Continental Drift Combine
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Chapter 13 Page 13-26
The Unifying Theory: Plate Tectonics
Hot Spots
 What makes the hot spot theory significant is the
concept that hot spots do not move with tectonic
plates.
 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, for example, result from
the plate moving over a hot spot.
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Chapter 13 Page 13-26
The Unifying Theory: Plate Tectonics
Hot Spots
 Hot spot theory is important to oceanographers
because it explains the nature of features forming
away from plate boundaries.
 Perhaps the most prominent of these is a hot spot
under the Pacific Plate that scientists think created
the Emperor Seamounts
and the Hawaiian Islands.
 They formed from a hot
spot as the plate moved
northwest over the last
35 million years.
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Chapter 13 Pages 13-27 to 13-29
The Unifying Theory: Plate Tectonics
Plate Movement
 The theory of continental drift wasn’t accepted early in its
conception because no one could explain how continents could
move. The theory of plate tectonics provided the explanation.
 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.
 This 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, and past each other at the transform
boundaries.
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Chapter 13 Pages 13-27 to 13-29
The Unifying Theory: Plate Tectonics
Plate Movement
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Chapter 13 Pages 13-27 to 13-29
The Unifying Theory: Plate Tectonics
Plate Movement
 Based on present theorized plate movements, what
will happen over the next several million years? Will
the plates ever rejoin as a super continent?
 According to current theory, the Atlantic Ocean
and Indian Ocean will expand while the Pacific
Ocean will shrink.
 Australia will continue to drift toward Eurasia.
 Southern California will pass San Francisco as it
moves to the northwest, and a new ocean will
form in the East African rift valley.
 The Mediterranean Sea will close as Africa moves
northward.
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