The primary cause of volcanoes, as of geysers, earthquakes and other
similar phenomena of nature, is the intensely heated condition of the
earth’s interior. It is the same force that has produced the irregular
features of the earth’s surface--its mighty mountain chains, the sunken
basins of the oceans, and its hills, valleys and gorges. Quite
logically, volcanoes are most numerous and most intense along the deep
mountain fissures which establish a ready communication between the
interior and the surface of the earth. Consequently the significant
facts about them are: (1) Nearly all volcanoes are either along the
highest border of the continents, or in the great central zone of
fracture; (2) most of the volcanic groups exhibit a linear arrangement;
(3) the agent at work in these mighty engines is mainly vapor of water,
or steam power.