The solid matter thrown out by
volcanoes consists of lapilli, scoriae, dust and bombs.
Though on the first formation of the volcano, masses of non-volcanic
rock may be torn from the chimney or pipe of the mountain, only slightly
fused externally owing to the bad conducting power of most rocks,
and hurled to a distance; and though at the beginning of a subsequent
eruption the solid plug of rock which has cooled at the bottom of the
crater, or, in fact, any part of the volcano, may be similarly blown up,
the bulk of the solid particles of which the volcano itself is composed
is derived from the lake of lava or molten rock which seethes at the
orifice. Solid pieces rent from this fused mass and cast up by the
explosive force of the steam with which the lava is saturated are known
as lapilli. Cooling rapidly so as to be glassy in texture externally,
these often have time to become perfectly crystalline within.
Gases and steam escaping from other similar masses may leave them
hollow, when they are termed bombs, or may pit their surfaces with
irregular bubble-cavities, when they are called scoriae or scoriaceous.
Such masses whirling through the air in a plastic state often become
more or less oblately spheroidal in form; but, as often, the explosive
force of their contained vapors shatters them into fragments, producing
quantities of the finest volcanic dust or sand. This fine dust darkens
the clouds overhanging the mountain, mixes with the condensed steam to
fall as a black mud-rain, or lava di aqua (Italian, water lava), or
is carried up to enormous heights, and then slowly diffused by upper
currents of the atmosphere.
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