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Define and discuss on
Geothermal Gradients
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The internal temperature of the earth increases with depth from the surface. Near the
surface, the average geothermal gradient is about 25 degrees centigrade (77 degrees
Fahrenheit) for every kilometer of depth. Some areas have much higher heat flows
because of deep fault zones, rifting, magmatic intrusions, or active tectonic forces. The
geothermal gradient can make conditions in deep mines quite uncomfortable and hot
enough to explode rocks or bend steel.
The geothermal gradient of 25 degrees centigrade/kilometer is thought to be restricted to
the upper part of the crust. If it continued at this rate uniformly from the surface, the
internal temperature of the earth would be greater than 2,000 degrees centigrade within
the lithosphere—a temperature that far exceeds the melting temperatures for all rocks at
that depth. Since the crust and upper mantle are solid and brittle, this gradient cannot
extend to these depths, where it is more likely about 1 degree centigrade/kilometer.
Recent laboratory studies have suggested the temperature is about 4,800 degrees
centigrade at the base of the lower mantle and about 7,000 degrees centigrade in the inner
core.
The heat flow is the amount of heat from the earth's interior that is lost at the surface.
The heat is probably generated by a still‐cooling core or by the radioactive decay of
elements such as uranium and thorium. Areas of higher heat flow are generally related to
magmatic activity or tectonic forces that bring wedges of hot mantle rock (mantle
plumes) into thin or faulted crustal areas.
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