Inside Earth: Chapter 1 Plate Tectonics

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Inside Earth Chapter 1- Plate Tectonics

• Section 5 The Theory of Plate Tectonics

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Guide For Reading

• What is the theory of plate tectonics?
• What are the three types of plate boundaries?

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The Canadian scientist, _______ , observed that
there are cracks in the _______ similar to those
on the ocean floor.

• J. Tuzo Wilson
• continental crust

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Plates

• A section of the lithosphere that slowly moves,
  carrying pieces of continental and oceanic crust
• The plates fit closely together along cracks in
  the crust
• Plates carry the continents or parts of the ocean
  floor, or both

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A Theory of Plate Motion
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Wilsons theory of plate tectonics combines what
geologists knew about _____, _____, and _____.

• sea-floor spreading
• Earths plates
• continental drift

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Scientific Theory

• A well tested concept that explains a wide
  range of observations

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Plate Tectonics

• A geological theory that states that pieces of
  Earths crust are in constant, slow motion,
  driven by convection currents in the mantle
• Convection Currents The movement of a fluid,
  caused by differences in temperature, that
  transfers heat from one part of the fluid to
  another

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How can Earths plates move?

• The plates of the lithosphere float on the top of
  the asthenosphere
• Convection currents rise in the asthenosphere and
  spread out beneath the lithosphere
• Geologists believe this causes plate movement

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As plates move, they ____, ____, or ____ past
each other, producing spectacular changes in the
earths surface.

• collide
• pull apart
• grind

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These changes include ____, ____, ____, and ____.

• volcanoes
• mountain ranges
• deep-sea trenches
• earthquakes

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Figure 26 Interpreting MapsWhich plates include
only ocean floor? Which plates include both
continents and ocean floor?

• Plates that include only ocean floor are the
  Pacific Plate and the Nazca Plate
• All other labeled plates in the figure include
  both continents and ocean floor

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Guide For Reading What is the theory of plate
tectonics?

• The theory of plate tectonics explains the
  formation, movement, and subduction of Earths
  crust
• Convection currents in the asthenosphere cause
  the plate to move
• As plates move they collide, pull apart or grind
  past each other
• Volcanoes
• Mountain ranges
• Deep sea trenches
• Earthquakes

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Sharpen Your Skills

• Predicting

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Study the map of Earths plates in Figure 26.
Notice the arrows that show the direction of
plate movement. Now find the Nazca plate on the
map. What direction is it moving?

• The Nazca plate is moving east

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Find the South American plate and describe its
movement.

• The South American plate is moving west

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What do you think will happen as these plates
continue to move?

• If the plates continue to move, they will collide
  (convergent boundary)
• Since the Nazca plate carries oceanic crust and
  the South American plate carries continental
  crust, the Nazca will plunge beneath the South
  American plate

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Plate Boundaries
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Faults

• A break in the Earths crust where slabs of rock
  slip past each other

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Guide For Reading What are the three types of
plate boundaries?

• Transform Boundaries
• Divergent Boundaries
• Convergent Boundaries
• Each type of plate boundary, there is a different
  type of plate movement

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Transform Boundaries
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Transform Boundaries

• A plate boundary where two plates move past each
  other in opposite directions
• Crust is neither created nor destroyed

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What Earth changing event occurs on a
transforming boundary?

• Earthquakes frequently occur at transform
  boundaries

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Divergent Boundaries
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Divergent boundaries

• A plate boundary where two plates move away from
  each other
• Most divergent boundaries occur at the mid-ocean
  ridge
• Divergent boundaries that occur on land are
  called rift valleys
• Example Great Rift Valley (South Africa)
• A 3,000 km crack in the earth

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Most divergent boundaries occur at the ____.

• mid-ocean ridge

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What could eventually happen at the Great Rift
Valley in Africa?

• The rift may someday split the eastern part of
  Africa away from the rest of the continent

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Checkpoint (Page 45) What is a rift valley? How
are rift valleys formed?

• A rift valley is a deep valley that forms along a
  divergent boundary on land
• Rift valleys form as two slabs of Earths crust
  slide apart

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Convergent Boundaries
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Convergent Boundaries

• A plate boundary where two plates move toward
  each other
• Collisions can occur between
• oceanic crust and oceanic crust
• oceanic crust and continental crust
• continental crust and continental crust

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When two plates collide, the ____ of the plates
determines which one comes out on top.

• density

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What occurs when plates carrying oceanic crust
collide?

• The more dense plate dives back into the mantle
  (subduction)
• The collision between oceanic plates causes
  deep-ocean trenches

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What occurs when plates carrying oceanic and
continental crust collide?

• The more dense oceanic crust sinks and plunges
  (subducts) beneath the less dense continental
  crust
• The collision between oceanic plates and
  continental plates can cause a volcanic eruption

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What occurs when plates carrying continental
crust collide?

• Plates crash head on causing mighty mountain
  ranges

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Checkpoint (Page 46) What types of plate movement
occur at plate boundaries?

• Plates slip past each other at transform
  boundaries
• Move apart at divergent boundaries
• Come together at convergent boundaries

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The Continents Slow Dance
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Plates move at a rate of between ____ and ____
centimeters per year.

• one
• ten

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The North American and Eurasian plates float away
from each other at a rate of fingernail growth.
This is a rate of ____ per year.

• 2.5 centimeters

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Figure 29 Posing QuestionsWhat questions would
you need to answer in order to predict where the
continents will be in 50 million years?

• You would need answers to how fast and in what
  direction each plate is moving