Magmas come from deep within the Earth, but what conditions
lead to the generation of melts in the interior of the
Earth? The geothermal gradient is a measure of how temperature
increases with depth in the Earth, and it provides information
about the depths at which melting occurs and the
depths at which magmas form. The differences in the composition
of the oceanic and continental crusts lead to differing
abilities to conduct the heat from the interior of the Earth,
and thus different geothermal gradients. The geothermal gradients
show that temperatures within the Earth quickly
exceed 1,832°F (1,000°C) with increasing depth, so why are
these rocks not molten? The answer is that pressures are very
high, and pressure influences the ability of a rock to melt. As
the pressure rises, the temperature at which the rock melts
also rises. This effect of pressure on melting is modified greatly
by the presence of water, because wet minerals melt at
lower temperatures than dry minerals. As the pressure rises,
the amount of water that can be dissolved in a melt increases.
Therefore, increasing the pressure on a wet mineral has the
opposite effect to increasing the pressure on a dry mineral: it
decreases the melting temperature
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