“When steam escapes from the summit of a volcanic conduit—which, in plain terms, is a tall vessel filled with intensely hot and more or less viscous liquid—masses of the liquid rock are blown into the air, and on falling build up a rim or crater about the place of discharge. Commonly the lava in the summit portion of a conduit becomes chilled and perhaps hardened, and when a steam explosion occurs this crust is shattered and the fragments hurled into the air and contributed to the building of the walls of the inclosing crater.
“The solid rock blown out by volcanoes consists usually of highly vesicular material which hardened on the surface of the column of lava within a conduit and was shattered by explosions beneath it. These fragments vary in size from dust particles up to masses several feet in diameter, and during violent eruptions are hurled miles high. The larger fragments commonly fall near their place of origin, and usually furnish the principal part of the material of which craters are built, but the gravel-like kernels, lapilli, may be carried laterally several miles if a wind is blowing, while the dust is frequently showered down on thousands of square miles of land and sea. The solid and usually angular fragments manufactured in this manner vary in temperature, and may still be red hot on falling.
“Volcanoes of the explosive type not uncommonly discharge streams of lava, which may flow many miles. In certain instances these outwellings of liquid rock occur after severe earthquakes and violent explosions, and may have all the characteristics of quiet eruptions. There is thus no fundamental difference between the two types into which it is convenient to divide volcanoes.”
MOUNTAINS BLOW THEIR HEADS OFF
“In extreme examples of explosive volcanoes, the summit portion of a crater, perhaps several miles in circumference and several thousand feet high, is blown away. Such an occurrence is recorded in the case of the volcano Coseguina, Nicaragua, in 1835. Or, an entire mountain may disappear, being reduced to lapilli and dust and blown into the air, as in the case of Krakatoa, in the Straits of Sunda, in 1883.
“The essential feature of a volcano, as stated above, is a tube or conduit, leading from the highly heated sub-crust portion of the earth to the crater and through which molten rock is forced upward to the surface. The most marked variations in the process depend on the quantity of molten rock extruded, and on the freedom of escape of the steam and gases contained in the lava.