The mountain quivers with internal
convulsions, due to the efforts of its confined forces to find an
opening. The drying up of wells and disappearance of springs are apt to
take place, the water sinking downward through cracks newly made in the
rocks. Finally the fierce unchained energy rends an opening through the
crater and an eruption begins. It comes usually with a terrible burst
that shakes the mountain to its foundation; explosions following rapidly
and with increasing violence, while steam issues and mounts upward in
a lofty column. The steam and escaping gases in their fierce outbreaks
hurl up into the air great quantities of solid rock torn from the sides
of the opening. The huge blocks, meeting each other in their rise and
fall, are gradually broken and ground into minute fragments, forming
dust or so-called ashes, often of extreme fineness, and in such
quantities as frequently to blot out the light of the sun. There is
another way in which a great deal of volcanic dust is made; the lava is
full of steam, which in its expansion tears the molten rock into atoms,
often converting it into the finest dust.
The eruption of Mt. Skaptar, in Iceland, in 1783, sent up such volumes
of dust that the atmosphere was loaded with it for months, and it
was carried to the northern part of Scotland, 600 miles away, in such
quantities as to destroy the crops. During the eruption of Tomboro, in
the East Indies, in 1815, so great was the quantity of dust thrown up
that it caused darkness at midday in Java 300 miles away and covered the
ground to a depth of several inches.
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