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An Idea Before Its Time
Continental Drift
Wegener’s continental drift hypothesis
stated that the continents had once been
joined to form a single supercontinent.
• Wegener proposed that the supercontinent,
Pangaea, began to break apart 200 million years
ago and form the present landmasses.
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An Idea Before Its Time
Continental Drift
Evidence• The Continental Puzzle
• Matching Fossils
- Fossil evidence for continental drift includes
several fossil organisms found on different
landmasses.
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An Idea Before Its Time
Continental Drift
Evidence
• Ancient Climates
• Rock Types and Structures
- Rock evidence for continental exists in theform of several mountain belts that end at
one coastline, only to reappear on a
landmass across the ocean.
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Matching Mountain Ranges
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Glacier Evidence
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Rejecting the Hypothesis
Continental Drift
A New Theory Emerges
• Wegener could not provide an explanation of
exactly what made the continents move. Newstechnology lead to findings which then lead to
a new theory called plate tectonics.
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Earth’s Major Roles
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.
A plate is one of numerous rigid sections of thelithosphere that move as a unit over the material of
the asthenosphere.
•
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Types of Plate Boundaries
Plate Tectonics
o Divergent boundaries (also called spreading
centers) are the place where two plates move
apart.
o Convergent boundaries form where two
plates move together.
o Transform fault boundaries are marginswhere two plates grind past each other without
the production or destruction of the
lithosphere.
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Three Types of
Plate Boundaries
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Divergent Boundaries
Actions at Plate Boundaries
Oceanic Ridges and Seafloor Spreading
o Oceanic ridges are continuous elevated zones on the floor of
all major ocean basins. The rifts at the crest of ridgesrepresent divergent plate boundaries.
o Rift valleys are deep faulted structures found along the axes
of divergent plate boundaries. They can develop on the
seafloor or on land.
oSeafloor spreading produces new oceanic lithosphere.
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Spreading Center
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Divergent Boundaries
Actions at Plate Boundaries
Continental Rifts
• When spreading centers develop within a
continent, the landmass may split into twoor more smaller segments, forming a rift.
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East African Rift Valley
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Convergent Boundaries
Actions at Plate Boundaries
A subduction zone occurs when one
oceanic plate is forced down into the
mantle beneath a second plate.
• Denser oceanic slab sinks into the asthenosphere.
Oceanic-Continental
• Pockets of magma develop and rise.
• Continental volcanic arcs form in part by volcanicactivity caused by the subduction of oceaniclithosphere beneath a continent.
• Examples include the Andes, Cascades, and
the Sierra Nevadas.
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Oceanic-Continental
Convergent Boundary
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Convergent Boundaries
Actions at Plate Boundaries
• Two oceanic slabs converge and one descends
beneath the other.
Oceanic-Oceanic
• This kind of boundary often forms volcanoes onthe ocean floor.
• Volcanic island arcs form as volcanoes emerge
from the sea.• Examples include the Aleutian, Mariana, and
Tonga islands.
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Oceanic-Oceanic
Convergent Boundary
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Convergent Boundaries
Actions at Plate Boundaries
• When subducting plates contain continental
material, two continents collide.
Continental-Continental
• This kind of boundary can produce new
mountain ranges, such as the Himalayas.
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Continental-Continental
Convergent Boundary
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Collision of India and Asia
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Transform Fault Boundaries
Actions at Plate Boundaries
At a transform fault boundary, plates grind
past each other without destroying the
lithosphere.
Transform faults
• Most join two segments of a mid-ocean ridge.
• At the time of formation, they roughly parallel thedirection of plate movement.
• They aid the movement of oceanic crustalmaterial.
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Transform Fault Boundary
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Evidence for Plate Tectonics
Testing Plate Tectonics
Paleomagnetism is the natural remnant
magnetism in rock bodies; this permanent
magnetization acquired by rock can beused to determine the location of the
magnetic poles at the time the rock became
magnetized.
• Normal polarity—when rocks show the samemagnetism as the present magnetism field
• Reverse polarity—when rocks show theopposite magnetism as the present magnetism
field
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Paleomagnetism Preserved
in Lava Flows
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Evidence for Plate Tectonics
Testing Plate Tectonics
The discovery of strips of alternating
polarity, which lie as mirror images across
the ocean ridges, is among the strongestevidence of seafloor spreading.
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Polarity of the Ocean Crust
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Evidence for Plate Tectonics
Testing Plate Tectonics
Earthquake Patterns
• Scientists found a close link between deep-focus
earthquakes and ocean trenches.• The absence of deep-focus earthquakes along
the oceanic ridge system was shown to beconsistent with the new theory.
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Evidence for Plate Tectonics
Testing Plate Tectonics
Ocean Drilling• The data on the ages of seafloor sediment
confirmed what the seafloor spreadinghypothesis predicted.
• The youngest oceanic crust is at the ridge crest,and the oldest oceanic crust is at the continentalmargins.
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Evidence for Plate Tectonics
Testing Plate Tectonics
Hot Spots• A hot spot is a concentration of heat in the
mantle capable of producing magma, which risesto Earth’s surface; The Pacific plate moves over a hot spot, producing the Hawaiian Islands.
• Hot spot evidence supports that the plates moveover the Earth’s surface.
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Hot Spot
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Causes of Plate Motion
Mechanisms of Plate Motion
Scientists generally agree that convection
occurring in the mantle is the basic driving
force for plate movement.• Convective flow is the motion of matter resulting
from changes in temperature.
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Causes of Plate Motion
Mechanisms of Plate Motion
Slab-Pull and Ridge-Push
• Ridge-push causes oceanic lithosphere to slide
down the sides of the oceanic ridge under thepull of gravity. It may contribute to plate motion.
• Slab-pull is a mechanism that contributes to
plate motion in which cool, dense oceanic crustsinks into the mantle and “pulls” the trailinglithosphere along. It is thought to be the primarydownward arm of convective flow in the mantle.
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Causes of Plate Motion
Mechanisms of Plate Motion
Mantle Convection
• The unequal distribution of heat within Earthcauses the thermal convection in the mantle thatultimately drives plate motion.
• Mantle plumes are masses of hotter-than-
normal mantle material that ascend toward thesurface, where they may lead to igneous activity.
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Mantle Convection Models
Get handout with this Diagram!!!!!!!!!!!!