Plate Tectonics Chapter 19

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Plate TectonicsChapter 19
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Continental drift An idea before its time

• Alfred Wegener
• Proposed hypothesis in 1915
• Published The Origin of
• Continents and Oceans
• Continental drift hypothesis
• Supercontinent Pangaea began breaking apart about
  200 million years ago

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Pangaea approximately 200 million years ago
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The great debate

• Objections to drift hypothesis
• Inability to provide a mechanism capable of
  moving continents across globe
• Wegner suggested that continents broke through
  the ocean crust, much like ice breakers cut
  through ice

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Matching of mtn ranges on continents
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Paleoclimatic evidence for Continental Drift
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The great debate

• Continental drift and the scientific method
• Wegners hypothesis was correct in principle, but
  contained incorrect details
• For any scientific viewpoint to gain wide
  acceptance, supporting evidence required

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Continental drift and paleomagnetism

• Renewed interest in continental drift came from
  rock magnetism
• Magnetized minerals in rocks
• Show direction to Earths magnetic poles
• Provide a means of determining their original
  latitude

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Continental drift and paleomagnetism

• Polar wandering
• Apparent movement of magnetic poles in volcanic
  rocks indicates continents move
• Shows Europe was closer to equator when
  coal-producing swamps existed

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Apparent polar-wandering paths for Eurasia and
North America
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The scientific revolution begins

• During the 1950s and 1960s technological strides
  permitted extensive mapping of the ocean floor
• Seafloor spreading hypothesis was proposed by
  Harry Hess in the early 1960s

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The scientific revolution begins

• Geomagnetic reversals
• Earth's magnetic field periodically reverses
  polarity north magnetic pole becomes south
  magnetic pole, vice versa
• Dates when polarity of Earths magnetism changed
  were determined from lava flows

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Paleomagnetic reversals recorded by basalt at
mid-ocean ridges
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Age of Oceanic Crust
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• Geomagnetic reversal
• Paleomagnetism was the most convincing evidence
  to support concepts of continental drift and
  seafloor spreading

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Plate tectonics The new paradigm

• More encompassing theory than continental drift
• Mix of ideas that explained motion of Earths
  lithosphere by subduction and seafloor spreading

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Plate tectonics The new paradigm

• Earths major plates
• Associated with Earth's strong, rigid outer layer
• Known as the lithosphere
• Consists of uppermost mantle and overlying crust
• Overlies a weaker region in the mantle called the
  asthenosphere

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Basal tractions drive plate motions
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• Earths major plates
• Seven major lithospheric plates
• Plates are in motion and change in shape and size
• Largest plate is the Pacific plate
• Several plates include an entire continent plus a
  large area of seafloor

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• Earths major plates
• Plates move relative to each other at a very slow
  but continuous rate
• Average about 5 centimeters (2 inches) per year
• Cooler, denser slabs of oceanic lithosphere
  descend into the mantle
• Motion defined by rotation around a pole

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• Plate boundaries
• Interactions among individual plates occur along
  their boundaries
• Types of plate boundaries
• Divergent plate boundaries
• Convergent plate boundaries
• Transform fault boundaries

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Types of Plate Margins
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Divergent plate boundaries

• Most are located along the crests of oceanic
  ridges
• Oceanic ridges and seafloor spreading
• seafloor is elevated forming oceanic ridges

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• Oceanic ridges and seafloor spreading
• Seafloor spreading occurs along the oceanic ridge
  system
• Spreading rates and ridge topography
• Ridge systems exhibit topographic differences
• Topographic differences are controlled by
  spreading rates (see map of age of oceanic crust
  for width of ridges relative to their age)

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Divergent boundaries are located mainly along
oceanic ridges
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• Spreading rates and ridge topography
• Topographic differences are controlled by
  spreading rates
• Slow rates (1-5 cm/year), rift valley develops on
  ridge crest (30 to 50 km wide, 1500-3000 m deep)
• Intermediate spreading rates (5-9 cm/year), rift
  valleys are shallow with subdued topography
• At rates gt 9 cm/year no rift valley develops or
  are narrow and extensively faulted

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Divergent boundaries in Continents

• Continental rifts
• Splits landmasses into two or more smaller
  segments

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Divergent boundaries

• Continental rifts
• Example includes East African rifts
• Produced by extensional forces acting on the
  lithospheric plates
• Not all rift valleys develop into spreading
  centers
• Otherwise Nevada would be an ocean!

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The East African Rift
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Development of Continental Rift into Ocean Basin
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Convergent plate boundaries

• Old portions of oceanic plates are returned to
  the mantle
• Surface expression of descending plate is an
  ocean trench
• Called subduction zones
• Average angle at which oceanic lithosphere
  descends into the mantle is about 45?

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• All have same basic characteristics, but can have
  highly variable features
• Types of convergent boundaries
• Oceanic-continental convergence
• Denser oceanic slab sinks into the asthenosphere
• Bathymetry marked by trench
• As plate descends, partial melting of mantle rock
  makes basaltic or andesitic magmas
• Volcanic mountains associated with subduction of
  oceanic lithosphere are called continental
  volcanic arcs (Andes and Cascades)

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Types of Arcs
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• Types of convergent boundaries
• Oceanic-oceanic convergence
• When two oceanic slabs converge, one descends
  beneath the other
• Often forms volcanoes on the ocean floor
• If the volcanoes emerge as islands, a volcanic
  island arc is formed (Japan, Aleutian islands,
  Tonga islands)

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Swim through the Marianas Trench
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Types of Arcs
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• Types of convergent boundaries
• Continental-continental convergence
• Continued subduction brings continents together
• Less dense, buoyant continental lithosphere does
  not subduct
• Result is a collision between two continental
  blocks
• Process produces mountains (Himalayas, Alps,
  Appalachians)

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The collision of India and Asia produced the
Himalayas
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Transform fault boundaries

• Third type of plate boundary
• Plates slide past one another and no new
  lithosphere is created or destroyed
• Transform faults
• Most join two segments of a mid-ocean ridge as
  parts of linear breaks in the oceanic crust known
  as fracture zones
• Accommodate simultaneous movement of offset
  ridges

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Transform faults accommodate movement on offset
ridge segments
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Testing the plate tectonics model

• Plate tectonics and earthquakes
• Plate tectonics model accounts for the global
  distribution of earthquakes
• Absence of deep-focus earthquakes along the
  oceanic ridge is consistent with tectonic theory
• Deep-focus earthquakes associated with subduction
  zones
• The pattern of earthquakes along a trench
  provides method to track plate's descent

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Deep-focus earthquakes occur along convergent
boundaries
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Earthquakes near Japan trench
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• Evidence from ocean drilling
• Most convincing evidence confirming seafloor
  spreading comes from drilling directly into
  ocean-floor sediment
• Age of deepest sediments
• Thickness of ocean-floor sediments verifies
  seafloor spreading

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• Hot spots
• Caused by rising plumes of mantle material
• Volcanoes form over them (Hawaiian Island chain)
• Mantle plumes are long-lived structures and
  originate at great depth, perhaps at core-mantle
  boundary

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The Hawaiian Islands form over stationary hot spot
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• No one driving mechanism accounts for all major
  facets of plate tectonics
• Researchers agree that convective flow in 2,900
  km-thick mantle is main driving force of plate
  tectonics (by basal traction)
• Other mechanisms generate forces that contribute
  to plate motion
• Slab-pull
• Ridge-push

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Importance of plate tectonics

• Provides a unified explanation of Earths major
  surface processes, especially oceans
• Within framework of plate tectonics, we find
  explanations for the distribution of earthquakes,
  volcanoes, and mountains
• Plate tectonics provides explanations for
  distribution/evolution of plants and animals and
  climate record