Volcanoes: What They are and What They Teach

by John W. Judd

This power of absorption of gases varies in different solids and liquids according to the conditions to which they are subjected. Dr. Henry showed it to be a general law in liquids that, as the pressure is augmented, the weight of the gas absorbed is proportionately increased.

Sometimes this absorption of gases takes place only at high temperatures. Thus silver in a state of fusion is able to absorb 22 times its volume of oxygen gas. When the metal is allowed to cool this gas is given off, and if the cooling takes place suddenly a crust is formed on the surface, and the phenomenon known as the 'spitting of silver' is exhibited. Sometimes during this operation miniature cones and lava-streams are formed on the surface of the cooling mass, which present a striking resemblance to those formed on a grand scale upon the surface of the globe. Similar phenomena are exhibited by several other metals and by the oxide of lead.

The researches of Troost and others have shown that molten iron and steel possess the property of absorbing considerable quantities of oxygen, hydrogen, carbonic acid, and carbonic oxide, and that these gases are given off in the operation known as 'seething,' when either the pressure or the temperature is diminished.

Hochstetter has shown that in the process of extracting sulphur from the residues obtained during the manufacture of soda, some very interesting phenomena are manifested. The molten sulphur is exposed to a temperature of 262° Fahrenheit, and a pressure of two or three atmospheres, in the presence of steam; under these circumstances it is found that the sulphur absorbs a considerable quantity of water, which is given off again with great violence from the mass as it undergoes solidification. The hardened crust which forms on the surface of the molten sulphur is agitated and fissured, miniature cones and lava-streams being formed upon it, which have a striking resemblance to the grander phenomena of the same kind exhibited upon the crust of the globe.

The observations which we have described prove conclusively that many liquids and solids in a molten condition have the power of absorbing many times their volume of certain gases, and that this action is aided by heat and pressure.

That the molten materials which issue from volcanic vents have absorbed enormous quantities of steam and other gases, we have the most undisputable evidence. The volume of such gases given off during volcanic outbursts, and while the lava-streams are flowing and consolidating, is enormous, and can only be accounted for by supposing that the masses of fluid rock have absorbed many times their volume of the gases. But we have another not less convincing proof of the same fact in the circumstance that volcanic materials which have consolidated under great pressure—such as granites, gabbros, porphyries, &c.—exhibit in their crystals innumerable cavities containing similar gases in a liquefied state.

It is to the violent escape of these gases from the molten rock-masses, as the pressure upon them is relieved, that nearly all the active phenomena of volcanoes must be referred; and it was the recognition of this bet by Spallanzani, while he was watching the phenomena displayed in the crater of Stromboli, which laid the foundations of the science of Vulcanology.

SOURCE OF THE ABSORBED GASES.

But here another question presents itself to the investigator of the phenomena of volcanoes: it is this. At what period did the molten rock-masses issuing from vents absorb those gaseous materials which are given off so violently from their midst during eruptions? Two different answers to this question have been suggested. It may be that the original materials of which our globe was composed consisted of metallic substances in a state of fusion which had absorbed many gases, and that, in the fluid masses below the solid crust, vast quantities of vapour and gas are stored up, which are being gradually added to the atmosphere during volcanic outbursts. The fact that meteorites, which, as we have seen, in all probability closely resemble the materials forming the earth's interior, sometimes yield many times their volume of hydrogen and other gases, may be thought to lend some support to this idea. If it be the correct one, we must regard our globe as gradually parting with its pent-up stores of energy, in those absorbed gases and vapours held in bondage by the solid and fluid materials of its interior.

But there is another hypothesis which is, to say the least, equally probable. Water containing various gases in solution is continually finding its way downwards by infiltration into the earth's crust. Much of this water, after passing through pervious beds, reaches some impervious stratum and is returned to the surface in the form of springs. But that some of this percolating water penetrates to enormous depths is shown by the fact that the deepest mines and borings encounter vast underground supplies of water. When we remember that nearly three-fourths of the earth's surface is covered by the waters of the ocean, and that the average depth of these oceanic waters is more than 10,000 feet, we may easily understand how great a portion of the earth's crust must be penetrated by infiltrating waters which can find no outlet in springs. The penetration of the waters of the ocean into the earth's crust will be aided, too, by the enormous pressure amounting to not less than several tons to the square-inch upon the greater part of the ocean-floor. It might be thought that this downward penetration of water would be counteracted by the upward current of steam that would be produced as these subterranean waters reach the hotter portions of the earth's crust. But the experiments of Daubrée have conclusively shown that the penetration of water through rocks takes place in opposition to the powerful pressure of steam in the contrary direction. Hence, we may assume that certain quantities of water, containing various gases and solids in solution, are continually finding their way by capillary infiltration from the surface to the deeply seated portions of the earth's crust, there to undergo absorption by the incandescent rock-masses and to produce oxidation of some of their materials.