Energetics of the Earth (1980) / Chapter Skim
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2 ENERGY SOURCES
2 ENERGY SOURCES
Pages 15-28
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From page 15... ...
Radioactivity This classification is somewhat arbitrary, in the sense that, as we shall presently see, much of the original heat is probably gravitational energy released at the time of formation of the earth. ORIGINAL H EAT By original heat we mean the heat content of the earth very soon after formation.
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From page 16... ...
This gravitational energy is of the order of 1032 J for the whole earth, quite enough to raise the temperature by tens of thousands of degrees and cause the earth to evaporate back into space as fast as it forms. The manner in which the earth manages to get rid of most of this energy, and in particular the fraction of it that is retained, determines the temperature and heat content of the growing planetary body.
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From page 17... ...
Considerable heating could also be generated, and higher internal temperatures reached, if the earth accumulated partly by the continuing capture of a planetesimal swarm of meteoritic bodies (Safronov, 1969; Wetherill, 1976, 19774. These bodies, hitting the earth at orbital velocities considerably greater than the velocity of free-fall, would cause much melting (as seems to have occurred on the moon)
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From page 18... ...
Although their model cannot be accepted literally, if only because there is surely radioactivity below 64 km, it nevertheless shows that original heat might indeed still contribute significantly to the surface heat flow. GRAVITATIONAL ENERGY A source of heat comparable in magnitude to radioactive heating (see below)
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From page 19... ...
calculates t1 = 2.49 x 1032 J CORE FORMATION The change in density distribution most frequently considered corresponds to the separation of a dense core and lighter mantle in an earth formed by homogeneous accretion of material with uniform uncom
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From page 20... ...
, which may have had a dense core from the start; in that case the AT would be part of the 2.5 x 1032 J of the energy released at the time of formation, most of which was presumably lost by radiation from the accreting surface. More frequently, separation is assumed to have occurred at the time, of the order of half a billion years after formation, when radiogenic heating would have raised the temperature inside the undifferentiated earth to the melting point of iron, or of an Fe-FeS mixture, allowing liquid core material to trickle down much faster than solid material would.
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From page 21... ...
Finally, seismic wave velocities and densities in the lower mantle do not seem to allow the presence in it of much metallic iron, except perhaps in the lower 100 km, just above the core boundary. The density of the solid inner core being greater than that of the liquid outer core, progressive crystallization of a cooling core must release gravitational energy.
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From page 22... ...
roughly 10 percent of the present surface heat flow. There has been a long-lasting debate as to whether dissipation, and hence heat production, occurs mainly in the oceanic tides (particularly shallow-sea tides)
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From page 23... ...
sets a lower limit for uranium in the earth as equal to the amount of uranium known or believed to be present in the crust, and an upper limit based on geochemical considerations regarding the degree of enrichment that is likely to occur in formation of the crust. He thus sets the uranium content of the earth at 12 + 6 ppb.
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From page 24... ...
It is indeed fairly certain that the density and sound speed in the core do not agree with the values expected for liquid iron under core conditions of pressure and temperature, and it has long been surmised that the core must contain substantial amounts (1~20 percent?
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From page 25... ...
deny that such partitioning would occur to any appreciable degree, but Goettel (1976) insists that much of the terrestrial potassium could indeed be in the core, in which the potassium concentration would be of the order of 0.1 percent.
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From page 26... ...
CRUSTAL HEAT VERSUS MANTLE HEAT It seems that observed local variations in heat flow at the surface of continents provide a datum by which crustal and mantle contributions to the heat flow can be separated. The observation is (Roy et al., 1972)
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From page 27... ...
(2.8) There is no generally recognized mechanism by which radioactive elements become heavily concentrated in the uppermost continental crust (see also Heier, 19781.
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From page 28... ...
, possible release of gravitational energy (original heat, separation of core, separation of inner core) , tidal friction in the early days of the earthmoon system, and possible meteoritic impact in the early days of the earth itself, the total supply of energy may seem embarrassingly large.
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