What are sometimes called volcanic bombs, but which are more properly volcanic vesicles, are produced by small fragments of lava which are thrown up in the air for only a moderate height and, on cooling, assume pear-like forms. [Fig. 25] represents the appearance of volcanic vesicles. The direction in which these vesicles moved through the air while in a molten state is indicated by their shape, the blunt end being the end towards which the particles were projected.
Fig. 25. Volcanic Vesicles
From Dana's Manual of Geology
But by far the greater portion of the hardened lava; i. e., the coarser, heavier particles, fall back on the mountain, and collecting around the crater build up volcanic cones, as already described in the case of mountains of the Vesuvian type.
There are two different ways in which the melted lava is broken up into fine particles when it is thrown upwards from the crater of the volcano. Nearly all lava contains large quantities of steam that are shut up, or occluded in the mass, being prevented from escaping by reason of the pressure to which the lava is subjected. The lava is released from this pressure as it is thrown out of the crater. The steam or gases escape explosively and thus break the lava into fine liquid spray, which rapidly hardens.
There is another way in which small particles of lava are formed. Sometimes large pieces of hardened lava are shot upwards into the air with a velocity as great as that with which a heavy projectile leaves the muzzle of a large gun. These heavy particles striking against one another, either while rising or falling, are broken into smaller fragments. Sometimes, indeed, these fragments fall back again into the crater from which they are again violently thrown out, and are again broken into smaller fragments either while rising or falling.
You will, probably, remember several instances of volcanic eruptions where masses of rock were thrown violently up into the air out of the crater. These larger masses are known as volcanic blocks. They probably consist of masses of hardened lava that have collected in the tube of the volcano during some of its periods of inactivity. Sometimes, however, they consist of fragments of rocks that are not of volcanic origin. Cases are on record where volcanic blocks have been thrown out of the craters in so great quantities as to cover the surface of many square miles of land with fragments hundreds of feet deep.
There is sometimes formed on the surface of a pool of lava as it collects in the craters of such volcanoes as Mt. Loa or Kilauea, when the volcanoes are not in eruption, a material resembling froth or scum. The same thing sometimes occurs on the surface of some kinds of lava as it runs down the side of the mountain. In this way a very light variety of highly cellular lava known as pumice stone is produced. The action which thus takes place is not unlike that which occurs during the raising of a lot of the dough from which bread is made, where the carbonic acid gas which is formed during the raising of the dough expands, and produces the well-known open cellular structure of well-raised bread. In the case of pumice stone, however, this raising goes on to such an extent that the mass consists often of less than 2% of solid matter, the remainder being a tangled mass of air.