4.5 The Theory of Plate Tectonics

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4.5 The Theory of Plate Tectonics
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Objectives

• Explain the theory of plate tectonics
• Describe the three types of plate boundaries.

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Engage/Explore

• What is a plate?
• Think of other contexts in which the word is
  used.
• Metal plates that cover machinery
• Home plate in baseball,
• a reptiles plates or scales
• plate of photographs in a textbook

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Introduction

• J. Tuzo Wilson - Canadian scientist.
• He discovered that there are cracks in the
  continents similar to those on the ocean floor.

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• Wilson proposed that the lithosphere is broken
  into separate sections called plates.
• Plates can carry both continents or parts of the
  ocean floor.

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Great Rift Valley in East Africa
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A Theory of Plate Motion

• Wilson combined the thoughts of sea-floor
  spreading, Earths plates, and continental drift
  into a theory.
• A scientific theory is a well-tested concept that
  explains a wide range of observations.

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• Plate tectonics is the geological theory that
  state that pieces of Earths lithosphere are in
  constant, slow motion, driven by convection
  currents in the mantle. It explains the
  formation, movement, and subduction of Earths
  plates.

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How can Earths plates move?

• The plates of the lithosphere float on top of the
  asthenosphere. Convection currents rise in the
  asthenosphere and spread out beneath the
  lithosphere.

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Earths Lithospheric Plates
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Affects of the plates

• Collide
• Pull apart
• Grind past each other

• Volcanoes
• Mountain ranges
• Deep-sea trenches

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Earths Lithospheric Plates

• Which plates include only ocean floor?
• Which plates include both continents and ocean
  floor?

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Plate Boundaries

• Demonstration
• Materials - 2 woooden blocks
• Three types of blocks
• 1. Transform boundary - slide past
• 2. Divergent boundary - pull away
• 3. Convergent boundary - push two blocks
  together.
• Draw the blocks with labels and arrows showing
  the direction of each blocks movement.

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Block demonstration

• Demonstrate the boundaries using blocks.
• Why do you think earthquakes occur frequently at
  transform boundaries?
• Answer The plates cannot move smoothly past one
  another because of the irregular nature of
  faults.
• How fast do you think Earths plates are moving?
• Answer Only a few centimenters per year.

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Plate Boundaries

• Faults - breaks the Earths crust where rocks
  have slipped past each other which form along
  these boundaries.
• Three kinds of boundaries
• 1. Transform boundary - slide past
• 2. Divergent boundary - pull away
• 3. Convergent boundary - push two blocks
  together.

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Transform Boundary

• A place where two plates slip past each other,
  moving in opposite directions.
• Earthquakes occur along these boundaries.
• San Andreas Fault is an example. The Pacific
  plate is sliding past the North American plate.
• Crust is neither created nor destroyed.

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San Andreas Fault
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San Andreas Fault
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Divergent Boundary

• The place where two plates move apart.
• Most occur at the mid-ocean ridge and on land.
• The boundary forms rift valleys.
• Example - Great Rift Valley has a 3000 km crack.

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Rio Grande Rift

• Extends
• from central Colorado to El Paso, Texas.

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Exploring Exercise - Plate Tectonics

• Plate Tectonics
• What is magma?
• What is the magma coming from that is shown
  erupting through the mid-ocean ridge and the rift
  valley?
• Answer asthenosphere
• What new process are shown?

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Convergent Boundaries

• Two plates come together. This is called a
  collision
• When two continental plates collide, why isnt
  one subducted beneath the
• other?
• Appalachian Mountains formed when two
  continental plates collided.
• -The density of plates determines which plate
  comes out on top.

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Appalachian Mountains
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Appalachian Mountains
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Convergent Boundaries
Oceanic/oceanic Subduction occurs
Oceanic/continental Oceanic plate sinks
Continental/continental Mountain ranges form
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The Continents Slow Dance

• The continents move at slow rates one to ten
  centimeters per year.
• Pangea began to drift apart about 225 million
  years ago.