MIT12_009S11_lec2_5

MIT12_009S11_lec2_5 - 2 Plate tectonics: volcanic source...

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Unformatted text preview: 2 Plate tectonics: volcanic source Earths biological carbon cycle derives its energy from the sun, which fuels photosynthesis. A small portion of the organic carbon that is fixed is buried as rock. Likewise, some inorganic carbon is buried as carbonate. If this burial were to continue without replacement, all the carbon in the atmosphere and oceans would be gone within about 10 6 yr. There must therefore be a resupply of CO 2 from geologic sources. This resupply of CO 2 comes via volcanism. The relative magnitude of its ux is small [3]: source volcanism other degassing combustion respiration ux (Gt C/yr) 0.1 0.1 7 100 Volcanism is nevertheless hugely significant at evolutionary time scales: with- out it, the carbon cycle would have long ago grinded to a halt, and life as we know it would not exist! In this way the biological carbon cycle is inextricably tied to the geological carbon cycle, or rock cycle. The rock cycle is fueled by the heat ux that comes out of the Earth. We proceed to describe an important manifestation of this heat ux: plate tec- tonics . We then use mathematical and physical reasoning to quantitatively evaluate predictions of plate tectonics. 13 The 2.1 Thermal convection and plate tectonics There are two internal sources of heat: Heat accumulated during the process of planetary accretion, i.e., the gravitational energy dissipated by the formation of the Earth 4.5 Ga. Heat generated by the radioactive decay of uranium, thorium, and potas- sium. To first approximation, then, we can think of the Earth as being heated from within and cooled at its surface. We seek an understanding of how the resulting heat ux generates volcanism and crustal motion at the surface. 2.2 Conductive vs. convective heat ow Measurements of temperature within the Earths crust show that heat ows upward out of the Earth. In general, there are two types of heat ux: conductive and convective. Conductive heat ux: only heat is transported, but not the material being heated. Convective heat ux: material is transported along with heat. Which one characterizes the geophysical heat ux? To investigate this problem, we consider a simpler problem in which a uid confined between two parallel heat-conducting plates is heated from below. 14 T=T + T T=T + T d g T=T ( cold) fluid temperature pure conduction T (hot) In the absence of convectionthe transport of hot uid up and cold uid down the temperature gradient is constant. Recall, however, that hot uid rises in a thermally expansive uid. There are thus two cases of interest: T small: no convective motion, due to stabilizing effects of viscous friction....
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This note was uploaded on 01/13/2012 for the course GEOLOGY 12.001 taught by Professor Staff during the Fall '11 term at MIT.

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MIT12_009S11_lec2_5 - 2 Plate tectonics: volcanic source...

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