Energetics of the Earth (1980) / Chapter Skim
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1 ENERGY SINKS
1 ENERGY SINKS
Pages 1-14
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Heat can never be totally converted to mechanical work; the efficiency of the conversion- that is, the ratio of work output to heat input-depends on, among other things, the temperature differences
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HEAT FLOW Surface heat flow, or heat flow for short, refers to the rate at which heat flows out across the interface between the solid earth and the atmosphere or oceans. The heat flow q is determined by measuring the temperature gradient VT in near-surface rocks and their thermal conductivity k; by Fourier's law of heat conduction q =-k VT, (1.1)
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This regional variation may be related to the uneven vertical distribution in the crust of radioactive heat sources that will be discussed in Chapter 2. Heat flow on oceanic plates also varies with local age, defined (through magnetic lineations)
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Thus it would seem that the rate of heat lost to the atmosphere and oceans by surface volcanic activity, on land and on the seafloor, may be about 8 x 10~ W less than the uncertainty affecting the global heat flow, which, as we have just seen, is variously estimated as 3 x 10~3 or 4 x 10~3 W
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It is also difficult to define a "normal" expected temperature since continental geotherms vary according to the surface heat flow and to the vertical distribution of radioactive heat sources. The impression nevertheless remains that some types of high-grade metamorphic rocks, particularly granulites or rocks associated with migmatites and granitic melts, form only when there is an abnormally high rate of heat influx rising into the continental crust from below.
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From page 6... ...
If the crust contains neither sources nor sinks of heat, the steady-state temperature distribution is linear with a slope (gradient) dT/dz = qO/k, where k is the thermal conductivity.
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But since the fraction of the earth's surface undergoing orogeny and regional metamorphism at any time is small, metamorphic heat as defined here is probably but a small fraction of the global heat flow, of the same order perhaps as volcanic heat, and will not be further considered. STRAIN ENERGY; EARTHQUAKES In broad terms an earthquake is believed to occur when sudden yielding or fracturing releases strain energy that has slowly accumulated in the neighborhood of the focus.
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From page 8... ...
This is negligibly small in comparison with the global heat flux of 3_4 x 10~3 W PLATE TECTONICS Since earthquakes, faulting, and deformation in general are now generally considered to be part of the broader phenomenon of plate motion, it may be appropriate to disregard for the moment the strain energy and look instead at the energy required to drive plates.
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From page 9... ...
Clearly, without all these factors we would have no rain on land. Plates are moved as much by the positive buoyancy of hot material rising at a ridge as by the negative buoyancy of the downgoing subducted slab, both of which are elements of a single convectional process.
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From page 10... ...
It is important to remember that the pressure gradients cannot be calculated by looking at the buoyancy at only one point. The horizontal pressure gradient that drives horizontal flow in the upper level of a conventional Benard convection cell is the result of both the negative buoyancy in the cold descending flow and the positive buoyancy of the ascending flow.
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From page 11... ...
The kinetic energy change corresponding to a change dI is then dEK =-i/2 co2 dI. Thus, if gravitational separation occurs in the earth, with denser matter moving toward the center, I will decrease and the kinetic energy will increase at the expense of part of the gravitational energy released by the condensation.
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From page 12... ...
Since the acceleration of Com is presumably caused by an electromagnetic internal torque exerted by the core on the mantle, the corresponding change in kinetic energy must come from the core. Conversely, when the mantle decelerates, energy flows back into the core.
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From page 13... ...
remains locked in those metamorphic rocks that form by endothermic reactions such as dehydration. It is estimated that in regions undergoing such metamorphism the heat flux rising into the crust from below may be at least twice normal; but since the area undergoing metamorphism at any one time is probably very small compared to the earth's surface area, metamorphic heat, like volcanic heat, is at most only a few percent of the global heat flow.
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From page 14... ...
On the other hand, for convection to occur certain conditions must be met, and in particular, the temperature gradient must exceed a certain critical value; this again places constraints on the location and distribution of heat sources. Finally, no heat source can account for the expenditure of strain energy, or for the motion of plates, without a suitable mechanism for converting heat into other forms of mechanical or potential energy.
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