Plate Tectonics

1
Chapter 9

• Plate Tectonics

2
Section 9.2

• Plate Tectonics

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Plate tectonics

• By the 1960s, accumulated evidence supporting
  the hypothesis of continental drift and seafloor
  spreading led to the formation of a more
  far-reaching theory.
• This theory is called plate tectonics.
• The theory of plate tectonics not only describes
  continental movement but also proposes a possible
  explanation of why and how continents move.
• The term tectonics comes from the Greek word
  tekonikos, meaning construction.
• Tectonics is the study of the formation of
  features in the earths crust.

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Plate tectonics

• According to the plate tectonics theory, the
  uppermost mantle, along with the overlying crust,
  behaves as a strong, rigid layer.
• This layer is known as the lithosphere.
• This outer shell lies over a weaker region in the
  mantle known as the asthenosphere.
• The lithosphere floats upon the asthenosphere and
  permits plate motion.
• The lithosphere is divided into segments called
  plates, which move and continually change shape
  and size.
• There are 7 major plates and many smaller plates.
• Seven Major Plates Eurasian, African,
  Australian-Indian, Antarctic, Pacific, North
  American, and South American.

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Plate tectonics
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Plate tectonics

• The largest plate is the Pacific plate, which
  covers most of the Pacific Ocean.
• Several of the plates include an entire
  continents plus a large area of the seafloor.
• Continental and Oceanic lithosphere.
• This is a major departure from Wegeners
  hypothesis of continental drift, which proposed
  that the plates moved through the ocean floor,
  not with it.

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Plate tectonics

• The lithospheric plates move relative to each
  other at a very slow but continuous rate that
  averages about 5 centimeters per year.
• This movement is driven by the unequal
  distribution of heat within Earth (Earths
  convection cycle).
• Hot mantle rises, cooler mantle sinks, setting
  Earths outer shell into motion.
• This grinding movement generates earthquakes,
  creates volcanoes, and deforms large masses of
  rock into mountains.

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According to the theory of plate tectonics,

1. The asthenosphere is divided into plates.
2. The lithosphere is divided into plates.
3. The asthenosphere moves over the lithosphere.
4. The asthenosphere is strong and rigid.

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Which of the following statements correctly
describes the asthenosphere?

1. It is a thin, cold, and rigid layer.
2. It is the source of Earths heat.
3. It permits plate motion.
4. It occurs only near subduction zones.

10
In the plate tectonics theory, the lithosphere is
divided into

1. 100 major plates.
2. 7 major plates and many smaller plates.
3. Many small plates, but no large plates.
4. 50 major plates and many smaller plates.

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The lithospheric plates move an average of

1. 5 inches per year.
2. 50 inches per year.
3. 5 centimeters per year.
4. 50 centimeters per year.

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A tectonic plate consists of

1. The crust and uppermost mantle.
2. The oceanic and continental crust only.
3. The crust and entire mantle.
4. The asthenosphere only.

13
In the plate tectonic theory, a plate can be made
up of

1. Continental lithosphere only.
2. Oceanic lithosphere only.
3. Both continental and oceanic lithosphere.
4. Both continental and oceanic asthenosphere.

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Plate tectonics

• All major interaction among individual plates
  occur along their boundaries.
• There are three main types of boundaries.
• Convergent.
• Divergent.
• Transform.

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Plate tectonics

• Convergent
• Form where two plates move together.
• This results in oceanic lithosphere plunging
  beneath an overriding plate, and descending into
  the mantle.
• Scientists refer to the region along a plate
  boundary where one plate moves under another as a
  subduction zone.
• A deep ocean trench generally forms along a
  subduction zone.
• Ex The Andes in South America.

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Plate tectonics

• 2. Divergent
• Occur when two plates move apart.
• Also called spreading centers.
• Results in upwelling of material from the mantle
  to create new seafloor.
• This formation is called a rift valley.
• Ex The Mid-Ocean Ridge and the East African Rift
  Valley.

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Plate tectonics

• 3. Transform Fault
• Margins where two plates grind past each other
  without the production or destruction of
  lithosphere.
• Ex San Andreas Fault

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Plate tectonics

• Each plate contains a combination of these three
  types of boundaries.
• The total surface area of the Earth does not
  change, but the plates may shrink or grow.
• This shrinking/growing depends on the locations
  of convergent and divergent boundaries.
• Ex The Antarctic plate is getting larger while
  the Phillipine plate is becoming smaller.
• New plate boundaries can be created because of
  changes in the forces acting on these rigid slabs.

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What kind of plate boundary occurs where two
plates grind past each other without destroying
or producing lithosphere?

1. Divergent boundary.
2. Convergent boundary.
3. Transitional boundary.
4. Transform fault boundary.

20
A divergent boundary at two oceanic plates can
result in a

1. Rift valley.
2. Volcanic island arc.
3. Continental volcanic arc.
4. Subduction zone.

21
What type of boundary occurs where two plates
move together, causing one plate to descend into
the mantle beneath the other plate?

1. Transform fault boundary.
2. Divergent boundary.
3. Convergent boundary.
4. Transitional boundary.

22
Which of the following is a geographic example of
a transform fault boundary?

1. The East African Rift valley.
2. The San Andreas Fault.
3. The Mid-Atlantic Ridge.
4. The Andes Mountains.

23
New ocean crust is formed at

1. Divergent boundaries.
2. Convergent boundaries.
3. Continental volcanic arcs.
4. Transform fault boundaries.