VIEW IN ACTIVE CRATER.
Scoria or cinders differ from ordinary lava only in the peculiarity of having partially crystallized in some portions and then stiffened or solidified while large bubbles were yet imprisoned or in the act of bursting; thus leaving the mass very ragged and cellular. But if the lava contains no readily formed crystals, the imprisoned bubbles of steam slowly rise to the surface, and being greatly elongated by the flowing of the lava, produce the beautiful material known as “pumice.” It is to lava exactly what froth or foam is to water. Usually it is much lighter colored than the lava on which it floats, for the same reason that well-worked molasses candy is nearly white: they both contain a vast number of minute air bubbles. Pumice floats on water, and its decomposition being generally very slow, it drifts about the sea currents, and is often found thousands of miles from any volcanic region. In the immediate neighborhood of volcanoes it often accumulates on the sea to such an extent that vessels can hardly force their way through it. In the Sunda Islands it has been seen on the sea three feet in depth. During the year 1878 the accumulation of pumice near the Solomon Isles was so great that it took ships three days to force their way through. Sometimes such masses accumulate along the coast line to such an extent that a person can not readily tell where the shore line is. One may land and walk about on the great floating raft of pumice, unable to guess within even a mile of the actual shore. Deep sea soundings show that the entire ocean bottom is covered more or less with the pulverized pumice and volcanic dust. From the wide distribution it is not probable that the layer attains any great thickness.
The Mangaians of the South Pacific told the earlier missionaries of a feat of one of their heroes which at first was unaccountable. This demigod, Maui, a sort of Pacific Hercules, raised the sky to its present position. Not getting it high enough to suit him, he put his head between the legs of his old father Ru, and heaved him and the half-raised sky up together. Ru stuck fast among the stars, and Maui left him there till his body dropped to pieces and his bones fell over the ground below. To prove the truth of their story they brought the missionaries bits of pumice, which they said were the bones of Ru. Singularly enough the white porous stone looked much like bone. The myth had been invented by the simple folk to explain the origin of pumice.
The ashes, or volcanic dust, is excessively minute, and in consequence readily penetrate crevices that are hardly visible. Professor Bonney, examining dust thrown out by Cotopaxi, has calculated that it would require from four thousand to twenty-five thousand particles to make up a grain in weight. The substance known as tufa is merely volcanic dust or ash upon which rain has fallen while the former was still hot. The resultant paste solidifies into a porous and loosely compacted rock.
Why some volcanoes nearly always throw out dust and fragments, while others throw out mere molten streams, was for a time not clearly understood. Some suggested the dust was the result of the continual collision of fragments as they rose and fell, and hence would increase in quantity as the eruption continued. This clearly would not meet the case of volcanoes which, without showing any previous signs of activity, burst into action with tremendous volumes of dust.
It was at length noticed that dust and fragments always were accompanied by tremendous explosions; while eruptions of melted lava were far more quiet, shocks being few and the explosions insignificant as compared with the former. This gave a clue to the mystery.
Many liquids and solids have the power of absorbing vast quantities of gas. Under pressure their absorbing powers may be vastly increased. Sometimes the property appears only at high temperatures. Silver when melted absorbs twenty-two times its volume of oxygen. If suddenly cooled the oxygen is given off with a rapidity verging on explosion. This is called the “spitting” of silver. Tiny cones and melted streams appear on the cooled surface—volcanoes in miniature. The same property belongs to the oxide of lead, and some other metals.
Now water can be made to absorb more than a thousand times its bulk of ammonia; more than five hundred times its bulk of hydrochloric acid. Alcohol may absorb three hundred times its volume of sulphurous acid. Charcoal may absorb one hundred times its volume of ammonia, eighty-five times its volume of hydrochloric acid, sixty-five times its volume of sulphuretted hydrogen, fifty-five times its volume of sulphurous and thirty-five times its volume of carbonic acid. Iron, steel and melted sulphur absorb many gases.
We have already seen that immense volumes of gases are thrown off in volcanic action. Now if a column of lava rises comparatively slowly in its “chimney,” the imprisoned gases rapidly escape, producing violent boiling, but not a positive explosion. But if it rises very rapidly, the sudden removal of the pressure causes so sudden an expansion of the compressed gases in its upper portion as to amount to a tremendous explosion, which reduces the lava to microscopic dust.