Plate Tectonics

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Chapter 9

• Plate Tectonics

2
Continental drift

• As people have studied maps , they were impressed
  by the similarity of the continental shorelines
  on either side of the Atlantic Ocean.
• In 1915, Alfred Wegener, a German scientist,
  proposed his hypothesis of continental drift.
• His hypothesis stated that the continents had
  once been joined to form a single continent.
• He called this super continent Pangaea, meaning
  all land.
• Surrounding Pangaea was a huge ocean,
  Panthalassa, meaning all seas.
• Wegener also hypothesized that about 200 million
  years ago Pangaea began breaking up into smaller
  continents.
• These continents then drifted to their present
  positions.

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Continental drift
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Wegeners continental drift hypothesis stated
that all the continents once joined together to
form

• Two major supercontinents.
• Two major supercontinents and three smaller
  continents.
• One major supercontinent.
• Three major supercontinents.

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The supercontinent in the continental drift
hypothesis was called

• Panthalassa.
• Pangaea.
• Mesosaurus.
• Africa.

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What hypothesis states that the continents were
once joined to form a single supercontinent?

• Plate tectonics
• Continental drift
• Seafloor spreading
• Paleomagnetism

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Continental drift

• Evidence to support continental drift
• The Continental Puzzle
• When looking at maps of the time Wegener thought
  that the continents might have been joined when
  he noticed the similarities between coastlines on
  opposite sides of the South Atlantic Ocean.
• His opponents correctly argued that erosion
  continually changes shorelines over time.

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CONTINENTAL DRIFT
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Continental drift

• 2. Matching Fossils
• If the continents had once been joined, Wegener
  reasoned, research should uncover fossils of the
  same plants and animals in areas that had been
  adjoining parts of Pangaea.
• Wegener knew that identical fossil remains of
  Mesosauras, a small, extinct land reptile that
  lived 270 million years ago, had already been
  found in both eastern South America and western
  Africa.
• Wegener knew that it was impossible for these
  reptiles to have swum across the Atlantic.
• His opponents thought that there were land
  bridges that might have connected the continents
  at some earlier time.
• Ex Bering Straight between Asia and North
  America
• There was no evidence that suggested that South
  America and Africa were connected by a land
  bridge.
• Wegener thus concluded that South America and
  Africa must have been joined at one time.

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Continental drift
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Continental drift

• 3. Rock Types and Structures
• The clear picture in the continental drift puzzle
  is one of matching rock types and mountain belts.
• Rock evidence for continental drift exists in the
  form of several mountain belts that end at one
  coastline, only to appear on a landmass across
  the ocean.
• Ex Appalachian Mountain Belt runs northeastward
  through the eastern U.S., ending off the coast of
  Newfoundland. Mountains of the same age with
  similar rocks and structures are found in the
  British Isles and Scandinavia.
• When these landmasses are fit together the
  mountain chains form a nearly continuous belt.

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Continental drift
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Continental drift

• 4. Ancient Climates
• Wegener was a meteorologist, so he was interested
  in obtaining data about ancient climates to
  support continental drift.
• He found glacial deposits showing that between
  220 million and 300 million years ago, ice sheets
  covered large areas of the Southern Hemisphere.
• Layers of glacial till were found in southern
  Africa and South America, as well as in India and
  Australia.
• The land area that shows evidence of this
  glaciation now lies near the equator in a
  subtropical or tropical climate.
• Large tropical swamps existed during the same
  time in the Northern Hemisphere with lush
  vegetation which eventually became the coal
  fields of the eastern U.S., Europe, and Siberia.

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Continental drift
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One kind of evidence that supports Wegeners
hypothesis is that

• The same magnetic directions exist on different
  continents.
• Major rivers on different continents match.
• Land bridges still exist that connect major
  continents.
• Fossils of the same organism have been found on
  different continents.

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Evidence about ancient climates indicates that

• Glacial ice once covered much of what is now
  India and Australia.
• Continents in the Northern hemisphere today were
  once centered over the South Pole.
• Continents in the Southern Hemisphere today were
  once centered onver the North Pole.
• No continents occupied the Southern Hemisphere.

17
The geographic distribution of the swimming
reptile Mesosaurus provides evidence that

• Europe was covered by a shallow sea when
  Mesosaurus lived.
• A land bridge existed between Australia and
  India.
• South America and Africa were once joined.
• The Atlantic Ocean was wider when Mesosaurus
  lived than it is now.

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Which of the following was not used in support of
the continental drift hypothesis?

• Fossil evidence
• Paleomagnetism
• Ancient climate
• Fit of South America and Africa

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How did opponents of continental drift account
for the existence of similar fossils on widely
separated continents?

• Parallel evolution
• Oceanic currents
• Large ocean rafts
• Migration across land bridges

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Continental drift

• Wegeners drift hypothesis faced a great deal of
  criticism from other scientists.
• One objection was that Wegener could not describe
  a mechanism that was capable of moving the
  continents across the globe.
• Wegener proposed that the tidal influence of the
  Moon was strong enough to give the continents a
  westward motion.
• Physicists responded that tidal friction of the
  size needed to move the continent would stop
  Earths rotation.

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Continental drift

• Wegener also proposed that the larger and smaller
  continents broke through the oceanic crust, much
  like ice breakers cut through ice.
• There was no evidence to suggest that the ocean
  floor was weak enough to permit passage of the
  continents without the ocean floor being broken
  and deformed in the process.
• Most scientists rejected Wegeners hypothesis,
  but a few geologists continued to search for
  additional evidence.

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Continental drift

• With major strides in technology, scientists were
  able to map the ocean floor.
• Extensive data on earthquake activity and Earths
  magnetic field also became available.
• By 1968, these findings led to a new theory,
  known as plate tectonics.
• This theory provides the framework for
  understanding most geologic processes.

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What was the main reason Wegeners continental
drift hypothesis was rejected?

• He was not well liked by other scientists.
• He could not provide a mechanism for the movement
  of the continents.
• He could provide only illogical explanations for
  the movement of the continents.
• His evidence was incorrect.

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Wegeners idea that tidal forces might cause
continental drift was shown to be impossible when
it was

• Determined that Earths magnetic field was too
  strong.
• Shown that the tidal forces needed to move
  continents would stop Earths rotation.
• Determined that Earths density was too low.
• Shown that no tides occurred 200 million years
  ago.

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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 in 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.

30
According to the theory of plate tectonics,

• The asthenosphere is divided into plates.
• The lithosphere is divided into plates.
• The asthenosphere moves over the lithosphere.
• The asthenosphere is strong and rigid.

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Which of the following statements correctly
describes the asthenosphere?

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

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In the plate tectonics theory, the lithosphere is
divided into

• 100 major plates.
• 7 major plates and many smaller plates.
• Many small plates, but no large plates.
• 50 major plates and many smaller plates.

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The lithospheric plates move an average of

• 5 inches per year.
• 50 inches per year.
• 5 centimeters per year.
• 50 centimeters per year.

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A tectonic plate consists of

• The crust and uppermost mantle.
• The oceanic and continental crust only.
• The crust and entire mantle.
• The asthenosphere only.

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

• Continental lithosphere only.
• Oceanic lithosphere only.
• Both continental and oceanic lithosphere.
• 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.

41
What kind of plate boundary occurs where two
plates grind past each other without destroying
or producing lithosphere?

• Divergent boundary.
• Convergent boundary.
• Transitional boundary.
• Transform fault boundary.

42
A divergent boundary at two oceanic plates can
result in a

• Rift valley.
• Volcanic island arc.
• Continental volcanic arc.
• Subduction zone.

43
What type of boundary occurs where two plates
move together, causing one plate to descend into
the mantle beneath the other plate?

• Transform fault boundary.
• Divergent boundary.
• Convergent boundary.
• Transitional boundary.

44
Which of the following is a geographic example of
a transform fault boundary?

• The East African Rift valley.
• The San Andreas Fault.
• The Mid-Atlantic Ridge.
• The Andes Mountains.

45
New ocean crust is formed at

• Divergent boundaries.
• Convergent boundaries.
• Continental volcanic arcs.
• Transform fault boundaries.

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Actions at plate boundaries

• Divergent Boundaries
• Most divergent boundaries are located along the
  crests of oceanic ridges.
• Can be thought of as constructive plate margins
  because this is where new oceanic lithosphere is
  generated.
• As plates move apart, fractures are created,
  which are in turn filled with molten rock, that
  eventually cools to produce new seafloor.

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Actions at plate boundaries

• Along well-developed divergent boundaries, the
  seafloor is elevated, forming the oceanic ridge.
• The system of ocean ridges is the longest
  physical feature on Earths surface, stretching
  more than 70,000 kilometers in length and between
  1,000 to 4,000 kilometers wide.
• Deep faulted structures called rift valleys are
  found along the axes of some segments.
• Rift valleys and spreading centers can also
  develop on land.
• Seafloor spreading is the process by which plate
  tectonics produces new lithosphere.
• None of the oceans floors that has been dated is
  older than 180 million years old.

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Actions at plate boundaries

• Continental rifts occur when the spreading
  centers are within a continent, splitting the
  landmass into two or more smaller segments.
• Ex East African Rift Valley and the Rhine Valley
  in Northwest Europe.
• The most widely accepted model for continental
  breakup suggests that forces stretching the
  lithosphere must be acting on the plate.
• These stretching forces are not large enough to
  tear the lithosphere, but the rupture of the
  lithosphere is thought to begin in those areas
  where plumes of hot rock rise up from the mantle.
• This weakens the lithosphere and creates domes in
  the crust directly above the hot rising plume.
• Uplifting stretches and thins the crust.
• Faulting and volcanism can also form a rift
  valley.

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Actions at plate boundaries

• If the stretching continues, the rift valley will
  lengthen and deepen until the continent splits in
  two.
• At this point the rift becomes a narrow sea with
  an outlet to the ocean.
• If this continues even farther, an ocean basin
  and ridge system is created.
• Ex Red Sea Formed about 20 million years ago
  when the Arabian Peninsula rifted from Africa.
  Scientists believe that this may have been how
  the Atlantic Ocean was first formed.

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Actions at plate boundaries
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Actions at plate boundaries
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Actions at plate boundaries
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Which of the following results when divergence
occurs between two oceanic plates?

• Seafloor spreading.
• A subduction zone.
• An ocean trench.
• A volcanic island arc.

54
The Red Sea is theorized to be the site of a
recently formed

• Convergent Boundary.
• Transfrom Fault Boundary.
• Divergent Boundary.
• Hot Spot.

55
An example of a divergent plate boundary on
continental lithosphere would be

• The Mid-Atlantic Ridge.
• East Pacific Rise.
• The Eastern African Rift Valley.
• The Andes Mountains.

56
Which list places the locations in the correct
sequence, with the initial seafloor spreading
stage first, followed by more advanced stages in
the seafloor spreading process?

• Red Sea, African Rift Valley, Atlantic Ocean
• Atlantic Ocean, African Rift Valley, Red Sea
• African Rift Valley, Atlantic Ocean, Red Sea
• African Rift Valley, Red Sea, Atlantic Ocean

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Actions at plate boundaries

• 2. Convergent Boundaries
• To accommodate the newly created lithosphere,
  older portions of oceanic plates return to the
  mantle along convergent boundaries.
• They are also called destructive plate boundaries
  because they destroy old lithosphere.
• As two plates converge, the leading edge of one
  is bent downward, allowing it to slide beneath
  the other.
• Destructive plate margins where oceanic crust is
  being pushed down into the mantle are called
  subduction zones.
• The surface feature produced by the descending
  plate is an ocean trench.

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Actions at plate boundaries

• Convergent boundaries are controlled by the type
  of crust involved and the forces acting on the
  plate.
• There are three different types of crust
  collisions
• Oceanic-Continental
• Oceanic-Oceanic
• Continental-Continental

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Actions at plate boundaries

• Oceanic-Continental
• When these two types of crust converge, the less
  dense continental plate remains floating while
  the denser oceanic plate sinks into the
  asthenosphere.
• When it reaches a depth of about 100 to 150
  kilometers, some of the asthenosphere above the
  descending plate melts.
• The newly formed magma rises and may reach the
  surface and cause volcanic eruptions.
• These are called continental volcanic arcs.
• Ex The Andes in South America.

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Actions at plate boundaries
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Actions at plate boundaries

• B. Oceanic-Oceanic
• When two oceanic crust collide, one descends
  beneath the other causing volcanic activity
  similar to continental volcanic arcs.
• Instead the volcanoes form on the ocean floor and
  will eventually build a chain of volcanic
  structures that become islands.
• These are called volcanic island arcs.
• Ex The Aleutian Islands off the shore of Alaska.

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Actions at plate boundaries
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Actions at plate boundaries

• C. Continental-Continental
• Since continental lithosphere is buoyant, it
  prevents it from being subducted to any great
  depth, which results in the formation of complex
  mountains.
• Ex Himalayas in South Asia formed from the
  collision of the subcontinent of India with Asia.
• Other examples include the Alps, Appalachians,
  and Urals.

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Actions at plate boundaries
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Actions at plate boundaries
66
What forms when one oceanic plate is forced
beneath another plate?

• An ocean basin.
• An ocean ridge.
• A subduction zone.
• A rift valley.

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Deep ocean trenches are associated with

• Ocean ridge systems.
• Subduction zones.
• Transform fault boundaries.
• Rift zones.

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Which of the following does not occur at a
subduction zone?

• The leading edges of both plates are bent upward.
• Oceanic crust is pushed down into the mantle.
• A continental plate moves over an oceanic plate.
• One oceanic plate moves beneath another oceanic
  plate.

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Oceanic lithosphere is destroyed at

• Transform fault boundaries.
• Divergent boundaries.
• Ocean ridges.
• Convergent boundaries.

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The Himalayas in South Asia are an example of
what type of plate boundary?

• Convergent oceanic-continental boundary.
• Convergent continental-continental boundary.
• Divergent boundary.
• Transform fault boundary.

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Actions at plate boundaries

• 3. Transform Fault Boundaries
• Most transform faults join two segments of a
  mid-ocean ridge.
• They are present about every 100-kilometers along
  the ridge axis.
• Active transform faults lie between the two
  offset ridge segments.
• The seafloor produced at one ridge axis moves in
  the opposite direction as seafloor is produced at
  an opposing ridge segment.
• Most transform faults are located within the
  ocean basins, a few cut through the continental
  crust.
• Ex San Andreas Fault of California The Pacific
  plate is moving toward the NW, and if this
  movement continues, that part of California west
  of the fault one will become an island off the
  coast of the U.S. and Canada, and could
  eventually reach Alaska.

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Actions at plate boundaries
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Actions at plate boundaries
74
At a transform fault boundary,

• New oceanic crust is destroyed.
• Oceanic lithosphere is destroyed.
• Lithosphere is neither destroyed nor produced.
• New continental crust is formed.

75
Where are most transform faults found?

• In California.
• Joining two segments of a mid-ocean ridge.
• Joining two segments of a subduction zone.
• In areas where two continents have collided.

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Actions at plate boundaries
77
Testing Plate tectonics

• Evidence supporting plate tectonics
• Paleomagnetism
• The natural remnant magnetism in rock bodies the
  permanent magnetization acquired by rock that can
  be used to determine the location of the magnetic
  poles at the time it became magnetized.
• When certain rocks containing iron-rich minerals
  (magnetite) are heated above a certain
  temperature, they lose their magnetic properties
  (convergent boundary).
• When they are again cooled, they become
  magnetized in the direction parallel to the
  existing magnetic field (divergent boundary).
• Rocks formed millions of years ago show the
  location of the magnetic poles at the time of
  their formation.

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Testing Plate tectonics

• Geophysicists have learned that Earths magnetic
  field periodically reverses polarity.
• The north magnetic pole becomes the south and
  vice-versa.
• When rocks show the same magnetism as the present
  magnetic field, they are described as having
  normal polarity.
• Rocks that show the opposite magnetism are said
  to have reverse polarity.
• A relationship was discovered between the
  magnetic reversals and the seafloor spreading
  hypothesis.
• Ships towed instrument called magnetometers
  across segments of the ocean floor and discovered
  alternating strips of high and low-intensity
  magnetism that ran parallel to the ridges.
• The strips of high-intensity magnetism are
  regions where paleomagnetism is of the normal
  type. The low-intensity strips represent regions
  where the ocean crust is polarized in the reverse
  direction, and therefore weaken the magnetic
  field.
• The discovery of strips of alternating polarity,
  which lie as mirror images across the ocean
  ridges, is amongst the strongest evidence of
  seafloor evidence.

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Testing Plate tectonics
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Testing Plate tectonics
81
According to the property of paleomagnetism,

• Iron-rich rocks show the location of the magnetic
  poles at the time of their formation.
• All rocks, regardless of when they are formed,
  have the same polarity.
• All rocks have a reversed polarity.
• Rocks do not possess magnetic properties.

82
Magnetic reversals,

• Cause the movements of tectonic plates.
• Confirmed the existence of subduction zones.
• Provide strong evidence for seafloor spreading.
• Have never occurred during geologic time.

83
Strips of alternating magnetic polarities found
in rocks in the ocean basins

• Conflict with the theory of plate tectonics.
• Provide evidence that Earths magnetic field has
  never reversed polarity.
• Indicate changes in Earths gravitation field.
• Provide evidence for seafloor spreading.

84
What do the strips of low-intensity magnetism
represent on the ocean floor?

• Areas where there is no magnetism.
• Areas where the rocks have a normal polarity.
• Areas where the rocks have a reversed polarity.
• Areas of different types of rock.

85
Testing Plate tectonics

• 2. Earthquake Patterns
• Scientists found a close link between deep-focus
  earthquakes and ocean trenches.
• Also, the absence of deep-focus earthquakes along
  the oceanic ridge system was shown to be
  consistent with the new theory.
• When the depth of earthquake foci and their
  locations within the trench system are plotted, a
  pattern emerges.

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Testing Plate tectonics
87
Testing Plate tectonics

• In the plate tectonics model, deep-ocean trenches
  are produced where cool, dense slabs of oceanic
  lithosphere plunge into the mantle.
• Shallow-focus earthquakes are produced as the
  descending plate interacts with the lithosphere
  above.
• As the slab descends farther, deeper-focus
  earthquakes are produced.
• No earthquakes have been recorded below
  700-kilometers.
• At this depth, the slab is heated enough to
  soften.

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Testing Plate tectonics
89
Testing Plate tectonics

• 3. Ocean Drilling
• The Deep Sea Drilling Project from 1968 to 1983
  used the drilling ship Glomar Challenger to drill
  hundreds of meters into the sediments and
  underlying crust.
• When the oldest sediment from each drill site was
  plotted against its distance from the ridge
  crest, its was revealed that the age of the
  sediment increased with increasing distance from
  the ridge.
• The data on the ages of seafloor sediment
  confirmed what the seafloor-spreading hypothesis
  predicted.
• The youngest oceanic crust is at the ridge crest
  and the oldest oceanic crust is at the
  continental margins.
• No sediment older than 180 million years old was
  found.
• Which means that the oceanic crust is relatively
  young as compared to some continental crust which
  has been dated at 4.0 billion years old.

90
The age of the rocks in the ocaen basins was
determined by

• Ocean drilling.
• The fit of continents across ocean basins.
• The depth of earthquake foci.
• The amount of magnetism in the rocks.

91
How does the age of seafloor sediments change
with increasing distance from the ocean ridge?

• Age decreases.
• Age stays the same.
• Age increases.
• Age varies without a pattern.

92
Testing Plate tectonics

• 4. Hot Spots
• Mapping of the Pacific seafloor revealed a chain
  of volcanic structures that extended from the
  Hawaiian Islands to Midway Island, and then north
  to the Aleutian Trench.
• Dates of volcanoes in this chain showed that the
  volcanoes increase in age with increasing
  distance from Hawaii.
• Suiko Seamount is 65 million years old, Midway
  Island is 27 million years old, and the island of
  Hawaii formed less than a million years ago and
  is still forming.

93
Testing Plate tectonics

• A rising plume of mantle material is located
  below the island of Hawaii.
• Melting of this hot rock as it nears the surface
  creates a volcanic area, or hot spot.
• As the Pacific plate moves over the hot spot,
  successive volcanic mountains have been created.
• The age of each volcano indicates the time when
  it was situated over the hot spot.
• Kauai is the oldest of the large islands in the
  Hawaiian chain.
• Hot spot evidence supports the idea that the
  plates move over Earths surface.

94
Testing Plate tectonics
95
The Hawaiian Islands were formed when the Pacific
Plate moved over

• A subduction zone.
• An ocean ridge.
• The Aleutain Plate.
• A hot spot.

96
The formation of the Hawaiin Islands is
associated with

• A divergent plate boundary.
• A convergent plate boundary.
• A transform fault boundary.
• No plate boundary of any kind.

97
Mechanisms of Plate Motion

• Scientists generally agree that convection
  occurring in the mantle is the basic driving
  force for plate movement.
• Warm, less dense material rises, and cooler,
  denser material sinks.
• This motion is called convective flow.
• These movements are driven by the unequal heat
  distribution of Earths heat.
• The heat is generated by the radioactive decay of
  elements in the Earths mantle and crust.
• Example Uranium

98
Mechanisms of Plate Motion

• One mechanism of plate motion is called the
  slab-pull.
• This occurs because old oceanic crust sinks into
  the asthenosphere and pulls the trailing
  lithosphere along.
• Slab-pull is though to be the primary downward
  arm of convective flow into the mantle.

99
Mechanisms of Plate Motion

• Another mechanism of plate movement is ridge-push
  which results from the elevated position of the
  oceanic ridge system.
• Ridge-push causes oceanic lithosphere to slide
  down the sides of the oceanic ridges.
• The downward slide is the result of gravity
  acting on the oceanic lithosphere.
• Ridge-push, although active in some spreading
  centers, is probably less important that
  slab-pull.

100
Mechanisms of Plate Motion
101
The main source of downward convection flow in
the mantle is called

• Ridge-pull.
• Slab-pull.
• Slab-push.
• Ridge-push.

102
The downward sliding characteristic of ridge-push
is the result of

• Gravity.
• Uneven heat distribution.
• Paleomagnetism.
• Continental rifting.

103
The thermal convection that drives plate motion
if caused by

• Seafloor spreading.
• An unequal distribution of heat.
• Gravity.
• Subduction.

104
Mechanisms of Plate Motion

• Another mechanism for plate movement is mantle
  convection.
• Most models suggest that hot plumes of rock are
  the upward flowing arms in mantle convection.
• These rising mantle plumes sometimes shoe
  themselves on Earths surface as hot spots and
  volcanoes.
• There are two types of mantle convection
• Whole-Mantle Convection
• Deep-Layer Model

105
Mechanisms of Plate Motion

• Whole-Mantle Convection
• In this model, slabs of cold oceanic lithosphere
  descend into the lower mantle.
• This process provides the downward arm of
  convective flow.
• At the same time, hot mantle plumes originating
  near the mantle-core boundary move heat toward
  the surface.

106
Mechanisms of Plate Motion
107
Mechanisms of Plate Motion

• Deep-Layer Model
• In this model, heat from Earths interior causes
  the two layers (upper mantle and lower mantle) to
  slowly swell and shrink in complex patterns
  without much mixing.
• A small amount of material from the lower layer
  flows upward as mantle plumes, creating hot-spot
  volcanism at the surface.

108
Mechanisms of Plate Motion
109
Which one of the following has not been proposed
as a mechanism of plate motion?

• Slab-pull.
• Ridge-push.
• Mantle convection.
• Crust-core convection.

110
According to whole-mantle convection,

• Small amounts of material from the lower mantle
  move upward to the surface.
• Slabs of cold oceanic lithosphere move down and
  into the lower mantle.
• Large chunks of continental crust are pulled down
  into the lower mantle.
• Material from the inner core rises into the
  mantle to form super hot plumes.

111
Mechanisms of Plate Motion