Magma or liquid rock is less dense than the
solid that melts to form it. Consequently like a
balloon filled with hot air (relative to the
surrounding cooler air) the magma is
buoyant ...it tends to move upwards in the
gravitational field of the Earth.
As it moves upwards magma cools because heat
is transferred from the hot magma to the cooler
solid rock surroundings and also because the magma
expands as it decompresses. If it losses enough
heat it can actually freeze at depth. But if magma
moves quickly enough it will not lose sufficient
heat to crystallize (freeze), and exploiting or
creating cracks and fractures, it can ascend and
erupt upon the Earth's surface.
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