In many cases the irregularity of the volcano is due to subsequent
action of its forces, which may blow the mountain itself to pieces.
In the case of Krakatoa, in the East Indies, for instance, the whole
mountain was rent into fragments, which were flung as dust miles high
into the air. The main point we wish to indicate is that volcanoes are
never formed by ordinary elevating forces and that they differ in this
way from all other mountains. On the contrary, they have been piled up
like rubbish heaps, resembling the small mountains of coal dust near the
mouths of anthracite mines.
It is to the burning heat of the earth's crust and the influence of
pressure, and more largely to the influx of water to the molten rocks
which lie miles below the surface, that these convulsions of nature are
due. Water, on reaching these overheated strata, explodes into volumes
of steam, and if there is no free vent to the surface, it is apt to rend
the very mountain asunder in its efforts to escape. Such is supposed
to have been the case in the eruption of Krakatoa, and was probably the
case also in the recent case of Mt. Pelee.
GENERAL DESCRIPTION OF ERUPTIONS
If we should seek to give a general description of volcanic eruptions,
it would be in some such words as follows: An eruption is usually
preceded by earthquakes which affect the whole surrounding country,
and associated with which are underground explosions that seem like
the sound of distant artillery.
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