The Wonder Book of Volcanoes and Earthquakes

by Edwin J. Houston

When the mass of molten rock or lava that comes out of the crater of a volcano is thrown upwards in the air the condition it assumes by the time it falls back again to the earth depends on the height it reaches. If this height is great the lava chills or hardens before reaching the earth, and assumes various forms according to the size of the fragments. The largest of these fragments are called cinders; the finer particles volcanic dust; while most of those of intermediate particles are known among other things as volcanic ashes.

We have already seen that when an explosive volcanic eruption occurs there is suddenly thrown out of the crater of the volcano a huge column of various substances that rises sometimes as high as 30,000 feet or even more. The smaller fragments of lava are quickly cooled and form volcanic ashes, sand, cinders, or dust. These are rapidly spread out by the wind in the form of a black cloud, that not only covers the mountain but reaches out over the surrounding country, completely shutting off the light of the sun. From this cloud particles of red hot ashes, cinders, sand, etc., begin to fall, the largest particles near the crater of the volcano, and the smaller particles at much greater distances. In very powerful explosive volcanic eruptions such as Krakatoa, the finer dust may be carried to practically all parts of the world.

Volcanic ashes consist of a fine, light, gray powder. These particles take the name ashes from their resemblance to the ashes left after the burning of pieces of wood or coal in an open fire. The name, however, as Geicke points out, is unfortunate, since it is apt to lead one to suppose that volcanic ashes consist of some burned material. Such an idea is erroneous, however, since ashes do not consist of anything that is left after burning, but merely of fine particles of molten rock that have hardened by cooling. When in the shape of what is known as volcanic dust these particles are so exceedingly small that they can readily make their way through the smallest openings in a closed room just as does the finest dust in the rooms of our houses when they are shut up. There are cases on record where people have been suffocated by the entrance of volcanic dust in closed rooms to which they have fled for safety during volcanic eruptions.

Volcanic sand consists of the coarser particles of chilled lava that are partly round and partly angular. They are of various sizes up to that of an ordinary pea. Volcanic sand is formed by the breaking up of the lava by the explosion of the vapors as they escape from the lava on relief from pressure. Volcanic dust when examined by the microscope is found to consist of very small particles that are more or less crystalline.

But besides the above there are larger fragments known as lapilli, consisting of rounded or angular bits of lava varying in size from that of a pea to an ordinary black walnut. These sometimes consist of solid fragments, but are usually porous, sometimes so much so that they readily float on water.

A curious form sometimes assumed by lava consists of what are called volcanic bombs. These are formed during explosive eruptions, when masses of liquid lava are hurled high up into the air. During their flight they take on a rotary motion, which tends to make them globular, so that cooling, while still revolving, they assume the form of a more or less spherical mass. At times, however, they are still sufficiently soft when they strike the earth to be flattened out in the form of flat cakes. When of a spherical form these are very properly called volcanic bombs.

That volcanic bombs have actually been subjected to a spinning motion while in the air can sometimes be shown by the fact that masses of scoriæ are frequently found in the interior with air cells largest at the centre of the bomb.

Volcanic bombs are sometimes thrown from the crater to great distances. During one of its recent eruptions, Cotopaxi threw out bombs that fell at a distance of nine miles from the crater.

According to Dana another form of lava bombs is sometimes found on the slopes of the active volcanoes of Hawaii, where masses of lava acquire a ball-like shape while rolling down an inclination.