The form of typical volcanic mountain is that of a cone, with a circular
basin or depression, called a crater, at its summit. In the center of
the crater is the mouth of a perpendicular shaft or chimney, which emits
clouds of hot vapor and gases; and in periods of greater activity,
ejects ashes, fragments of heated rock, and streams of fiery lava.
Volcanic ashes, when examined under a microscope, are found to be simply
pulverized lava, frequently in minute crystals, and bear no resemblance
to ashes in the ordinary sense of the term.
The lava stream, when flowing white hot from the crater, is not unlike a
jet of melted iron escaping from a furnace, and moves at first with
considerable rapidity. It soon cools on the surface, and becomes covered
with a hard, black, porous crust, while the interior remains melted and
continues to flow. If the stream is thick, the lava may be found still
warm after ten or even twenty years.
The amount of matter ejected by volcanoes is very great. The whole
island of Hawaii, the largest of the Hawaiian Islands, seems to be only
an accumulation of lava thrown out by its four craters. All high oceanic
islands are of the same character. Iceland, with an area of forty
thousand square miles, is a vast table-land from three thousand to five
thousand feet in elevation, composed of volcanic rock similar to the
lavas still ejected by its numerous volcanoes.