Lecture20_EAS1600_Fall08

Lecture20_EAS1600_Fall08 - EAS 1600 Introduction to...

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EAS 1600 Introduction to Environmental Sciences ____________________________ Lecture 20 - Plate Tectonics: Part 3 In our last lecture we finish up Plate tectonics
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ENERGETICS OF PLATE TECTONICS Plates move an average of about 4 cm/yr. (Not very fast, you say, but how far would that amount to after 1 million years? Compare that distance to the circumference of the earth.) What is the energy source that drives this motion? < It comes from the earth’s interior - geothermal energy < Amounts to about 0.1 W/m 2 (How does this compare to solar input?) < The source of this energy is: 1. Radioactive decay; and 2. Residual heat from Earth’s formation 1. Radioactive decay - arises from the spontaneous decomposition of unstable isotopes of potassium, uranium, and thorium contained within the solid earth > For example, 40 K decays into 40 Ar with a half-life of 1.25 billion years When these elements decay they give off energy (like an atomic fission reaction). > All radioactive elements in earth’s interior are decreasing in abundance as they decay. Radioactive heat production has decreased by 1/5 over earth’s 4.6 billion year history > This is not a permanent energy source . .. 2. Residual Heat - related to heat produced in earth’s interior when earth first accreted. ( Pull of gravity on planetesmals and their collision) Want to know more about this? Some of it is covered in EAS 1601.
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The Heat within the interior can not build up indefinitely. ¾ 2 nd Law of Thermodynamics dictates that there will be a tendency for this heat to dissipate by traveling to the surface. ¾ This occurs by way of convection cells traveling through the mantle ¾ much like this our simple glass of water experiment ¾ also like the atmosphere’s general circulation ¾ These cells give rise to plate motion by way of mantle drag ¾ Like wind gives rise to surface ocean circulation . Once in motion other forces (see figure 6-21 in textbook) such as slab-pull keep plates in motion.
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Thus far we have been learning about what happens when plates collide. Now let’s we take a look at some of the major geological features that are produced as a result of these collisions: < Islands and Mountains Along Plate Boundaries; < Islands Chains, Seamounts, and Hotspots; < Cratons, Terranes, and Continents
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I. Volcanoes ¾ volcanic eruption brings magma to the surface to form “extrusive” igneous rock ¾ (non-volcanic magma forms intrusive or “plutonic igneous rock.) ¾ Two types of classification schemes ¾ eruptive history ¾ active – erupted in historical times ¾ dormant – not active, but show no signs of erosion ¾ extinct - highly eroded ¾ topographic form ¾ shield volcanos ¾ stratovolcano ¾ cinder cone ¾ basalt plateau ¾ For our purposes we are only interested in shield and stratovolcanoes
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Shield Volcano: Rounded, low profile ¾ Formed from successive lava flows ¾ Typically of basaltic composition ¾ basalts tend to be quite fluid when melted ¾ not steep ¾ usually NOT explosive Example: Hawaii
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Lecture20_EAS1600_Fall08 - EAS 1600 Introduction to...

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