Chapter 8: Plate Tectonics

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Chapter 8 Plate Tectonics

• 8.1 Earth has several layers
• 8.2 Continents change position over time
• 8.3 Plates move apart
• 8.4 Plates converge or scrape past each other

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8.4 Plates converge or scrape past each other

• Before, you learned
• Plates move apart at divergent boundaries
• In the oceans, divergent boundaries mark where
  the sea floor spreads apart
• On land, continents split apart at divergent
  boundaries
• Now, you will learn
• What happens when two continental plates converge
• What happens when an oceanic plate converges with
  another plate
• What happens when one plate scrapes past another
  plate

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Tectonic plates push together at convergent
boundaries

• Recall new crust forms at divergent boundaries
• At convergent boundaries, plates push together
  and crust is folded or destroyed
• A plate with older, denser oceanic crust will
  sink beneath another plate
• The crust melts in the athenosphere and is
  destroyed
• Subduction when one plate sinks beneath another
• Sub-, under and ducere, to lead led under

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Continental-Continental Collision

• Both crusts are the same density, so neither
  plate can sink beneath the other
• The edges crumple and fold
• Push together two blocks of clay one or both
  will buckle
• Folded crust may push up high enough to form a
  mountain
• Ex European Alps where the African and European
  Plates are colliding
• Ex Himalaya Indian Plate colliding with the
  European
• These mountains keep rising higher plates are
  still moving

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Oceanic-Oceanic Subduction

• Occurs where an older plate with oceanic crust
  sinks (subducts) under another with newer oceanic
  crust
• Older plate is colder and more dense than the
  younger plate subducts into the asthenosphere

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Oceanic-Oceanic Subduction

• Deep-Ocean Trenches like deep canyons that form
  in the ocean floor as a plate sinks
• Most are found in the Pacific Ocean
• Ex Marina Trench Pacific Plate sinking under
  the Philippine Plate
• Deepest place in the worlds ocean 11,000m
  (36,000ft) into the sea floor

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Oceanic-Oceanic Subduction

• Island Arcs chains of volcanic islands that form
  on the top plate, parallel to a deep-ocean trench
• Ocean crust of the sinking plate melts and magma
  rises through the top plate
• Overtime, flows can build up a series of islands
• Ex Philippine Islands, Aleutian Islands
  (Alaska), islands of Japan

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Oceanic-Continental Subductions

• Occurs when ocean crust sinks under continental
  crust ocean crust is colder and denser than
  continental crust
• Deep-Ocean Trenches
• In the Pacific Ocean, the Pacific Plate is
  sinking under the North American Plate can cause
  underwater earthquakes

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Oceanic-Continental Subductions

• Coastal Mountains as oceanic crust sinks under a
  continent, the continental crust buckles to form
  a range of mountains
• Similar to island arcs, parallel a deep-ocean
  trench
• Some of theses mountains are volcanoes (when
  melted oceanic crust rises through the top plate)
• Ex Cascade Mountains in Oregon and Wash.
• As the Juan de Fuca Plate began sinking under the
  North American Plate
• Mount St. Helens active volcanoes

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Tectonic Plate Boundaries

• Divergent Boundaries new crust formed
• Convergent Boundaries crust folds/destroyed
• Transform Boundaries
• Plates move past each other in opposite
  directions
• Edges scrape and grind against each other
• Occur mostly on the sea floor near mid-ocean
  ridges
• Can occur on land San Andreas Fault
• Runs from the Gulf of California to San Francisco
• Pacific Plate and North American Plate are moving
  opposite directionsLos Angeles will be a suburb
  of San Francisco in about 10 million years!!

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The theory of plate tectonics helps geologists
today

• Changed scientists view of Earth the lithosphere
  has been in motion for millions of yearsand can
  help predict what may happen in the future
• Eastern U.S the deformed and folded rocks in the
  Appalachian Mountains are evidence of an ancient
  convergent boundary
• These rocks are the same as those in northwest
  Africa!
• Indicate these mountains formed when North
  America collided with Africa and Eurasia as part
  of Pangaea
• As they pulled apart formed rift valleys (U.S.
  east coast)
• Predictions
• more earthquakes are likely where plates slide
  past each other
• volcanic activity where plates are sinking
  beneath other plates
• mountains will continue to rise where plates push
  together