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.
OFTEN REST FOR LONG TERMS OF YEARS
Such are the general features of action in the vast powers which
dwell deep beneath the surface, harmless in most parts of the earth,
frightfully perilous in others.
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