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Chapter 13 - The Theory of Plate Tectonics

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The Earth Inside and Out The Theory of Continental Drift The Theory of Seafloor Spreading The Unifying Theory: Plate Tectonics Chapter Topic Menu – PowerPoint PPT presentation

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Title: Chapter 13 - The Theory of Plate Tectonics


1
  • Choose to view chapter section with a click on
    the section heading.
  • The Earth Inside and Out
  • The Theory of Continental Drift
  • The Theory of Seafloor Spreading
  • The Unifying Theory Plate Tectonics

Chapter Topic Menu
2
The Earth Inside and Out
The Earth Inside and Out
Chapter 13 Page 13-4 to 13-9
3
Earths Internal Layers
  • The Earths 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 5,000C (9,032F) nearly the same
    temperature as the surface of the sun.
  • The inner core is theorized to be solid due to
    intense pressure.

The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6
4
Earths 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.

The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6
5
Earths 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 mantles structure it consists of the upper
    mantle and the lower mantle. The lower mantel is
    made of hot, dense magma (molten rock).

The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6
6
Earths 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.

The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6
7
Earths 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.

The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6
8
Earths Internal Layers
Cross-Section of the Earth
The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6
9
Earths Internal Layers
  • Scientists separate the uppermost mantle from the
    crust because they think the mantles 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.

The Earth Inside and Out
Chapter 13 Pages 13-4 to 13-6
10
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 Earths recycling
    machine, endlessly converting rock from one type
    to another.

The Earth Inside and Out
Chapter 13 Pages 13-6 to 13-7
11
The Rock Cycle
The Earth Inside and Out
Chapter 13 Pages 13-6 to 13-7
12
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.

The Earth Inside and Out
Chapter 13 Pages 13-6 to 13-7
13
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.

The Earth Inside and Out
Chapter 13 Pages 13-6 to 13-7
14
The Rock Cycle
  • 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
    Earths interior, rocks remelt and become magma,
    returning to the crust as igneous rock.

The Earth Inside and Out
Chapter 13 Pages 13-6 to 13-7
15
Isostatic Equilibrium
  • It is currently thought that the crust doesnt
    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.

The Earth Inside and Out
Chapter 13 Pages 13-7 to 13-9
16
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.

The Earth Inside and Out
Chapter 13 Pages 13-7 to 13-9
17
Isostatic Equilibrium
  • 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.

The Earth Inside and Out
Chapter 13 Pages 13-7 to 13-9
18
Isostatic Equilibrium
  • 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.

The Earth Inside and Out
Chapter 13 Pages 13-7 to 13-9
19
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.

The Earth Inside and Out
Chapter 13 Pages 13-7 to 13-9
20
Isostatic Equilibrium
  • 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.

The Earth Inside and Out
Chapter 13 Pages 13-7 to 13-9
21
The Theory of Continental Drift
The Theory of Continental Drift
Chapter 13 Page 13-10 to 13-13
22
Alfred Wegener and Pangaea
  • 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
    Earths continents had been a single giant
    continent. Wegener called this continent Pangaea.
  • Surrounding Pangaea, he said, was a single large
    ocean he called Panthalassa.

The Theory of Continental Drift
Chapter 13 Page 13-10
23
Alfred Wegener and Pangaea
  • 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.

The Theory of Continental Drift
Chapter 13 Page 13-10
24
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.

The Theory of Continental Drift
Chapter 13 Page 13-10
25
Evidence for Continental Drift
  • 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.

The Theory of Continental Drift
Chapter 13 Page 13-11 to 13-13
26
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.

The Theory of Continental Drift
Chapter 13 Page 13-11 to 13-13
27
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 todays North America,
    Greenland, and Eurasia.
  • They named the southern continent Gondwanaland
    which included the remaining continents.

The Theory of Continental Drift
Chapter 13 Page 13-11 to 13-13
28
Evidence for Continental Drift Map of Pangaea
The Theory of Continental Drift
Chapter 13 Page 13-11 to 13-13
29
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 doesnt 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.

The Theory of Continental Drift
Chapter 13 Page 13-11 to 13-13
30
Evidence for Continental Drift
  • Despite these examples and other evidence, the
    theory of continental drift wasnt widely
    accepted while Wegener was still alive.
  • This may have partly been because Wegener was a
    meteorologist, not a geologist.

The Theory of Continental Drift
Chapter 13 Page 13-11 to 13-13
31
The Theory of Seafloor Spreading
The Theory of Seafloor Spreading
Chapter 13 Pages 13-14 to 13-20
32
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
    echos angle, how long it took to return, and
    changes in frequency, sonar operators can
    determine where an object is, its distance, and
    whether its moving.

The Theory of Seafloor Spreading
Chapter 13 Pages 13-14 to 13-15
33
New Technology and Seafloor Knowledge
  • 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.

The Theory of Seafloor Spreading
Chapter 13 Pages 13-14 to 13-15
34
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
    70,000-kilometer (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 mid-ocean
    ridges.

The Theory of Seafloor Spreading
Chapter 13 Pages 13-14 to 13-15
35
New Technology and Seafloor Knowledge
The Theory of Seafloor Spreading
Chapter 13 Pages 13-14 to 13-15
Major Earthquake and Volcano Zone
Patterns of Ridges and Trenches
36
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.

The Theory of Seafloor Spreading
Chapter 13 Page 13-16
37
The Creation and Destruction of Seafloor
  • 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).

The Theory of Seafloor Spreading
Chapter 13 Page 13-16
38
The Creation and Destruction of Seafloor
The Theory of Seafloor Spreading
Chapter 13 Page 13-16
39
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.

The Theory of Seafloor Spreading
Chapter 13 Pages 13-17 to 13-20
40
Evidence of Seafloor Spreading
The Theory of Seafloor Spreading
Chapter 13 Pages 13-17 to 13-20
Radiometric Dating of the Seafloor
41
Evidence of Seafloor Spreading
The Theory of Seafloor Spreading
Chapter 13 Pages 13-17 to 13-20
Alternating Polarity of the Mid-Ocean Ridge
42
The Unifying Theory Plate Tectonics
The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-29
43
Seafloor Spreading and Continental Drift Combine
  • 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 todays theory of plate
    tectonics. This theory combined ideas from both
    theories, along with some of the original
    isostatic equilibrium concepts.

The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
44
Seafloor Spreading and Continental Drift Combine
  • According to the theory of plate tectonics, the
    Earths 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.

The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
45
Seafloor Spreading and Continental Drift Combine
The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
Earths Crustal Plates
46
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.

The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
47
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.

The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
48
Seafloor Spreading and Continental Drift Combine
The San Andreas Fault
The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
49
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.

The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
50
Seafloor Spreading and Continental Drift Combine
The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
Divergent Boundary
51
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 these
    boundaries destroy crust.

The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
52
Seafloor Spreading and Continental Drift Combine
  • 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.

The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
53
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.

The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
54
Seafloor Spreading and Continental Drift Combine
The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
Island Arcs - Trench Development
55
Seafloor Spreading and Continental Drift Combine
  • 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.

The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-22 to 13-25
56
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.

The Unifying Theory Plate Tectonics
Chapter 13 Page 13-26
57
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.

The Unifying Theory Plate Tectonics
Chapter 13 Page 13-26
58
Plate Movement
  • The theory of continental drift wasnt 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.

The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-27 to 13-29
59
Plate Movement
The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-27 to 13-29
60
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.

The Unifying Theory Plate Tectonics
Chapter 13 Pages 13-27 to 13-29
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