Volcanoes: What They are and What They Teach

by John W. Judd

From these great intrusive masses of highly crystalline rocks there proceed in every direction great spurs or dykes, which are evidently the radiating fissures formed during the outwelling of igneous materials from below, injected by these fluid substances. The rock forming these dykes is often less perfectly crystalline than that which constitutes the centre of the mass, and we may indeed detect among the materials of these dykes examples of every variety of structure, from the perfectly crystalline granite to the more or less glassy substance of lavas. Besides the vertical or oblique dykes we also find horizontal sheets, which, passing from these central masses, have penetrated between the surrounding strata, often, as we have seen, to enormous distances.

For the sake of simplicity, we have spoken of these ground-plans, or basal wrecks of volcanoes, as constituting a flat plain; as a matter of fact, however, the unequal hardness of the materials composing volcanic mountains causes them to assume, under the influence of denuding agencies, a very rugged and uneven surface. The hard crystalline materials filling the central vent stand up as great mountain groups; each large dyke, by the removal of the surrounding softer materials, is left as a huge wall-like mass, while the remnants of lava-streams are seen constituting a number of isolated plateaux.

The great Island of Skye is the basal wreck of another volcano which was also in eruption during Tertiary times; probably, many millions of years ago. This immense volcano had originally a diameter at its base of about thirty miles, and a height of 12,000 to 15,000 feet, and must have been comparable to Etna or Teneriffe in its dimensions. At the present time, there is nothing left of this vast pile but the highly crystalline granites and gabbros filling up the great fissures through which the eruption of igneous materials took place. These, worn by denudation into rounded dome-like masses and wild rugged peaks, constitute the Red Mountains and Coolin Hills of Skye, which rise to the height of more than 3,000 feet above the sea-level. From these great, central masses of crystalline rocks, innumerable radiating dykes may be found rising through the surrounding rock-masses, with isolated patches of the scoriæ and lapilli ejected from the volcano, which have here and there escaped removal by denudation. Along what were the outskirts of this great mountain-mass are found flat-topped hills, built up of lava-streams, only small portions of which have escaped removal by denudation.

RESERVOIRS BENEATH VOLCANOES.

But this wearing away of the structure of a volcanic cone by the denuding forces may proceed even one stage farther, and we may then have revealed for our inspection and study the mass of originally fluid materials, from which one or more volcanoes have been fed, cooled and consolidated in their original reservoir. There are many examples of masses of granitic or highly crystalline rocks, having precisely the same composition as the different varieties of lavas, which are found lying in the midst of the sedimentary rocks, and sending off into these rocks veins and dykes of the same composition with themselves. No one who has carefully studied the appearances presented by volcanic mountains in different stages of dissection, by the action of denuding forces, can avoid recognising these great granitic masses as the cooled reservoirs from which volcanoes have in all probability been supplied during earlier periods of the earth's history.

The eruption of these great masses of incandescent rock, impregnated with water and acid gases, through strata of limestone, sandstone, clay, coal, &c., may be expected to produce striking and wonderful chemical changes in the latter. Nor are we disappointed in these anticipations. Whenever we examine the sedimentary materials around volcanic vents, we find that, in contact with the once-fused materials, they everywhere exhibit remarkable evidences of the chemical action to which they have been subjected. The limestones are converted into statuary marble, the sandstones pass into quartzite, the days assume the hardness and lustre of porcelain, while the coals have lost their volatile ingredients and assumed a form like coke or graphite. And these changes are found to extend in many cases to the distance of many hundreds of yards from the planes of junction between the igneous and the sedimentary materials.

Among the most interesting effects resulting from the extrusion of masses of incandescent rock, charged with water and various gases, through beds of limestone, clay, sandstone, &c., we may mention the production of those beautiful crystalline minerals which adorn our museums and are so highly prized as gems. By far the larger part of these beautiful minerals have been formed, directly or indirectly, by volcanic agencies.

These gems and beautiful minerals are, for the most part, substances of every-day occurrence, which entirely owe their beauty to the crystalline forms they have assumed. The diamond is crystallised carbon, the ruby and sapphire are crystallised alumina, the amethyst and a host of other gems are crystallised silica; and in almost all cases the materials of gems are common and widely diffused, it is only in their finely crystalline condition that they are rare and therefore valuable.

FORMATION OF VOLCANIC MINERALS.

Crystals are formed during the slow deposition of a substance, either by the evaporation of a liquid in which it is dissolved, by its volatilisation, or its cooling from a state of fusion. In many cases it can be shown that the formation of large and regular crystals is aided if the work goes on with extreme slowness and under great pressure. By sealing up various substances in tubes containing water which can be kept at a high temperature, minute crystals of many well-known minerals have been artificially formed by chemists. Part of the water converted into steam has formed a powerful spring, which, reacting upon the remainder of the liquid in the tube, has subjected it to enormous pressure, and under these conditions of extreme pressure and temperature, chemical actions take place of which we have no experience under ordinary circumstances. The experiments of Mr. Hannay seem to prove that when carbon is separated from certain organic substances at a high temperature and under great pressure, it may crystallise in the form of the diamond. And the recent discovery of diamonds in the midst of materials filling old volcanic vents in South Africa seems to show that this was in many cases the mode in which the gem was originated. Even under the conditions which prevail at the earth's surface, however, minute and unnoticed chemical actions taking place during long periods of time, produce most remarkable results. This has been well illustrated by M. Daubrée, who has shown that in the midst of masses of concrete which the Romans built up around the hot springs of Plombières and other localities, many crystalline minerals have been formed, in the course of 2,000 years, by the action of the waters upon the ingredients of the concrete.