Plate Tectonics: More Interesting than it Sounds

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Plate Tectonics More Interesting than it Sounds

• By Jess Friedman

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Before we get going, lets review

• Plate
• Lithosphere
• Asthenosphere
• Radioactive Decay
• Convection

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• The story of Plate Tectonics is a fascinating
  story of continents drifting majestically from
  place to place breaking apart, colliding, and
  grinding against each other of terrestrial
  mountain ranges rising up like rumples in rugs
  being pushed together of oceans opening and
  closing and undersea mountain chains girdling the
  planet like seams on a baseball of violent
  earthquakes and fiery volcanoes. Plate Tectonics
  describes the intricate design of a complex,
  living planet in a state of dynamic flux.

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A Combination of Two Ideas

• Continental Drift
• Sea-Floor Spreading

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The Basics

• The Earth's surface is covered by a series of
  hard crustal plates
• The ocean floors are continually, moving,
  spreading from the center, sinking at the edges,
  and being regenerated

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The Basics, Part II

• Convection currents beneath the plates move the
  crustal plates in different directions its
  like a conveyor belt, but a little bit slower
• The source of heat driving the convection
  currents is radioactive decay deep in the Earths
  mantle

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Plate Tectonic Environments

• Extensional
• Convergent (Compressional)
• Divergent (Transform)

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What about Hawaii?

• Hawaii those islands wouldnt be here without
  the Pacific Plate
• Pacific Plate, which is mostly oceanic
  lithosphere, moves northwest at a variable rate,
  about 8 cm/year on average

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• But the islands are more than 3,200 km from the
  nearest plate boundary!

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Sources

• http//hypertextbook.com/facts/ZhenHuang.shtml
• http//pubs.usgs.gov/publications/text/hotspots.ht
  ml
• http//volcano.und.nodak.edu/vwdocs/vwlessons/plat
  e_tectonics/part12.html
• http//volcano.und.nodak.edu/vwdocs/vwlessons/atg.
  html
• http//www.ucmp.berkeley.edu/geology/tectonics.htm
  l
• http//www.seismo.unr.edu/ftp/pub/louie/class/100/
  plate-tectonics.html
• http//volcano.und.nodak.edu/vwdocs/vwlessons/plat
  e_tectonics/introduction.html

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Hot Spots spots that are hot
As interpreted by Jeff Marlow
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Early Studies

• Early Hawaiians recognized relative ages
• James Dana 1840
• Two strands of volcanoes along parallel paths

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Mantle Plumes

• Tuzo Wilson 1963
• Lithosphere moving across stationary hot spot in
  the mantle
• small, long-lasting, very hot regions exist below
  the plates These hotspots provide the localized
  source of heat energy to sustain volcanism

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• Led to Mantle Plume Theory
• mantle plume  a buoyant mass of hot mantle
  material that rises to the base of the
  lithosphere. 
• Mantle plumes commonly produce volcanic activity
  and structural deformation in the central part of
  lithospheric plates

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Mantle Plume Origin

• Deep mantle origin
• Mantle-core boundary?
• Constant position
• Higher convection cells very transient, erratic

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Plumes on Mars?
- (Multiple) Plumes? - Heatflow issues -
75 of topography can be explained by
crustal thickness and surface volcanic
construction - Tectonic Uplift?
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Problems with Plume Theory

• Bend in Emperor / Hawaiian Island Chain
• Change in direction of Pacific plate 50 mya?
• No such change occurred

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Problems with Plume Theory

• Temperature Anomalies
• Hawaii mantle temp elevated 200 C
• Core-mantle boundary plumes require 600 C
• Melt comes from shallow asthenosphere
• Lavas indicate certain stability field
• 80-120 km deep

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Problems with Plume Theory

• Seismology has not detected a plume
• Expect low speed earthquake waves when it crosses
  hot mantle
• Low wave anomalies detected.but in wrong place

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In Support of Mantle Plume

• Fixed hot spot
• High rate of magma output
• 3He/4He isotope ratios
• ? Points to Deep Mantle Plume