Plate Tectonics

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Plate Tectonics
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Objectives

• To be able to describe the history and supporting
  evidence for plate tectonic theory
• To be able to explain the basic principles of
  plate tectonic theory
• To be able to describe draw the 3 types of
  plate boundaries
• To be able to describe features on Earth
  explained by plate tectonic theory

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Definition

• from the Greek t??t?? tekton, meaning builder
  or mason
• The main features of plate tectonics are
• The Earth's surface is covered by a series of
  crustal plates.
• The ocean floors are continually moving,
  spreading from the center, sinking at the edges,
  and being regenerated.
• Convection currents beneath the plates move the
  crustal plates in different directions.
• The source of heat driving the convection
  currents is radioactivity deep in the Earths
  mantle.

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History

• Alfred Wegener first proposed concept of
  continental drift (1915)
• Wegener said all continents were joined together
  in one landmass called Pangaea
• He used circumstantial evidence (puzzle-like fit
  of continents, fossil data, similar rock types
  and ages, glacial striations) to support his
  theory
• He proposed a pole-fleeing force to explain
  Pangaeas drift away from the South Pole toward
  the equator and gravitational forces of the sun
  and moon to explain the westward drift of North
  America
• His theory was rejected he died in Greenland in
  1930. His body is presumed to be buried under
  gt300 feet of ice.

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Evidence Cited by Wegener
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History (cont.)

• Post World War II technological advances led to
  detailed mapping of the seafloor ridges and
  trenches discovered
• Age dating of oceanic crust showed seafloor rocks
  are youngest at the ridges and oldest toward the
  trenches
• Paleomagnetic data showed a record of reversals
  preserved in the volcanic rocks of the seafloor
• Seafloor spreading proposed in 1961-62 by Howard
  Hess and R. Dietz
• Convection model proposed as mechanism explaining
  crustal motion HEAT drives the system
• Plate tectonic theory accepted by the late 1960s

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Basic Principles

• The lithosphere exists as separate and distinct
  plates (lt10 major plates up to a few dozen
  including microplates)
• Plates ride on the fluid-like asthenosphere
• Plate motions range from 1-4 cm/yr (as fast as
  fingernails grow) to 16 cm/yr (as fast as hair
  grows)

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Compositional vs. Mechanical Layering

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The Lithosphere and Earths Interior

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Plate Tectonic Map
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Tectonic Plate Motions
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California Seismicity
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Types of Plate Boundaries

• Divergent (AKA constructive e.g. Salton Sea in
  Southern California)
• Transform (e.g. San Andreas Fault)
• Convergent (AKA destructive)
• ocean-ocean (e.g. Mariana Islands)
• ocean-continent (e.g. Pacific Northwest ?
  Cascadia Subduction Zone)
• continent-continent (e.g. India-Asia)

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Divergent
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Transform
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Convergent
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Phenomena on Earth Explained by Plate Tectonic
Theory

• Mountains
• Trenches
• Midocean Ridges
• Earthquakes
• Volcanoes
• Mineral/Ore Distribution
• Resource Distribution (i.e. geothermal, etc.)
• Island chains and arcs
• Faulting and Folding of Rocks
• Et Cetera

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Summary

• Alfred Wegener ? continental drift led to plate
  tectonics
• Supporting evidence includes puzzle-like fit of
  continents fossil, rock, glacial, and
  paleomagnetic data seafloor spreading
• Driven by convection
• Explains most geologic phenomena on Earth