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.
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. Occasionally great eruptions have taken place beneath the ocean’s surface, throwing up material in sufficient quantity to form new islands.
The formation of mud is not confined to the method given, but great quantities of this plastic material flow at times from volcanic craters. In the year 1691 Imbaburu, one of the peaks of the Andes, sent out floods of mud which contained dead fish in such abundance that their decay caused a fever in the vicinity. The volcanoes of Java have often buried large tracts of fertile country under volcanic mud.
An observation of volcanoes shows us that they have three well marked phases of action. The first of these is the state of permanent eruption, as in case of the volcano of Stromboli in the Mediterranean. This state is not a dangerous one, since the steam, escaping continually, acts as a safety valve. The second stage is one of milder activity with an occasional somewhat violent eruption; this is apt to be dangerous, though not often very greatly so. The safety valve is partly out of order. The third phase is one in which long periods of repose, sometimes lasting for centuries, are followed by eruptions of intense energy. These are often of extreme violence and cause widespread destruction. In this case the safety valve has failed to work and the boiler bursts.