Floating pumice formed a layer
on the ocean surface two and a half feet in thickness, through which
vessels had difficulty in forcing their way.
The steam which rises in large volumes into the air may become suddenly
condensed with the chill of the upper atmosphere and fall as rain,
torrents of which often follow an eruption. The rain, falling through
the clouds of volcanic dust, brings it to the earth as liquid mud, which
pours in thick streams down the sides of the mountain. The torrents of
flowing mud are sometimes on such a great scale that large towns, as in
the instance of the great city of Herculaneum, may be completely buried
beneath them. Over this city the mud accumulated to the depth of over 70
feet. In addition to these phenomena, molten lava often flows from the
lip of the crater, occasionally in vast quantities. In the Icelandic
eruption of 1783 the lava streams were so great in quantity as to fill
river gorges 600 ft. deep and 200 ft. wide, and to extend over an open
plain to a distance of 12 to 15 miles, forming lakes of lava 100 feet
deep. The volcanoes of Hawaii often send forth streams of lava which
cover an area of over 100 square miles to a great depth.
GREAT OUTFLOWS OF LAVA
In the course of ages lava outflows of this kind have built up in Hawaii
a volcanic mountain estimated to contain enough material to cover the
whole of the United States with a layer of rock 50 feet deep. These
great outflows of lava are not confined to mountains, but take place now
and then from openings in the ground, or from long cracks in the surface
rocks.
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