Thus, while the surface is glassy, the interior becomes
crystalline.
As to what are the causes of the great convulsions of nature known as
the volcano and the earthquake we know very little. Various theories
have been advanced, but nothing by any means sure has been discovered,
and considerable difference of opinion exists. In truth we know so
little concerning the conditions existing in the earth's interior
that any views concerning the forces at work there must necessarily be
largely conjectural.
Sir Robert S. Ball says, in this connection: "Let us take, for instance,
that primary question in terrestrial physics, as to whether the interior
of the earth is liquid or solid. If we were to judge merely from the
temperatures reasonably believed to exist at a depth of some twenty
miles, and if we might overlook the question of pressure, we should
certainly say that the earth's interior must be in a fluid state. It
seems at least certain that the temperatures to be found at depths of
two score miles, and still more at greater depths, must be so high that
the most refractory solids, whether metals or minerals, would at once
yield if we could subject them to such temperatures in our laboratories.
But none of our laboratory experiments can tell us whether, under the
pressure of thousands of tons on the square inch, the application of
any heat whatever would be adequate to transform solids into liquids.
It may, indeed, be reasonably doubted whether the terms solid and
liquid are applicable, in the sense in which we understand them, to the
materials forming the interior of the earth.
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