Plate Tectonics

<p>Jun 20, 2014 10:19 AM &nbsp;(the history channel show on the Marianas Trench prompted

this entry: that show is wonderful and is essential knowledge (since it is intimately

related to hx of life on earth, evolution of rocks, etc... not discussed in that show).</p>

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motion of Earth's lithosphere;</p>

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contin. drift developed during 1st few decades of 20th century.</p>

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accepted after seafloor spreading was confirmed in late 50s and early 60s.</p>

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lithosphere = crust and upper mantle; asthenosphere is below it.</p>

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(lithosphere is cooler, denser, more rigid; &nbsp;asthenosphere is hot, molten, more visco-elastic);

ocean lithosphere is us about 100 km thick;</p>

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litho is thinness at mid-ocean ridges but as it spreads out it gets cooler and thicker

as subjacent mantle is added to it;</p>

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7 or 8 major plates &nbsp;(and many minor ones)</p>

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earthquakes, volcanic activ, mtn building and cean trench formation.</p>

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mvmt varies from 0 to 100 mm per year &nbsp;(pacif plate moves at abt 2 inches = 50 mm / yr)

like growth of fingernails.</p>

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at convergent boundaries: &nbsp;subduction carries plates into the mantle;</p>

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lost material = material formed by new oceanic crust at mid-ocean ridges...

so total global surface is the same.</p>

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= conveyor belt principle.</p>

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plate mvmt along seafloor from spreading ridge is due to crust density, topography,

gravit forces,downward suction at subduction zone &nbsp;(sinking crust pulls floor crust)

also lunar tidal force... etc. (debated)</p>