Volcanoes: What They are and What They Teach

by John W. Judd

EFFECTS OF INTRUSION BENEATH CONES.

That great upheaving forces have operated on volcanoes, subsequently to the accumulation of their materials, we have sufficient evidence in the Val del Bove of Etna, the Caldera of Palma, the Corral of Madeira, &c. In all of these cases we find a radial fissure ('barranco') leading into a great crateral hollow; and these radial fissures are of such width and depth that their origin can only be referred to a disruptive force like that which would be exercised by the intrusion of masses of more or less imperfectly fluid material between the subjacent strata. These facts, of course, lend no countenance to the views formerly held by many geologists, both in Germany and France, that the materials of which volcanoes are built up were deposited in an approximately horizontal position, and were subsequently blown up like a gigantic bubble. In Etna, Palma, and Madeira we find abundant proofs that the mass existed as a great volcanic cone before the production of the fissures (barrancos), which we have referred to the force exercised during the intrusion of great igneous masses beneath them.

But besides the horizontally-disposed intrusive sheets and laccolites, great, radiating, vertical fissures are produced by the heaving forces acting beneath those volcanic centres which have been closed up and 'cicatrised' by the exudation from them of subterranean materials. These vertical intrusions, which we call dykes, like the horizontal ones, differ in character, according to the nature of the materials of which they are composed. Dykes of acid and intermediate lava are usually of considerable width, and do not extend to great distances from the centres of eruption. Dykes composed of the more-liquid, basic lavas, on the other hand, may extend to the distance of hundreds of miles from the central vent. The way in which comparatively narrow, basaltic dykes are found running in approximately straight lines for such enormous distances is a very striking fact, and bears the strongest evidence to the heaving and expanding forces at work at volcanic centres, during and subsequently to the extrusion of the igneous products at the surface.

These basaltic dykes occur in such prodigious numbers around some volcanic vents, that the whole of the stratified rocks in the immediate vicinity are broken up by a complete network of them, crossing and interlacing in the most complicated fashion. Farther away from the vents, similar dykes are found in smaller numbers, evidently radiating from the same centre, and sometimes extending to a distance of more than a hundred miles from it. Nowhere can we find more beautiful illustrations of such dykes than in the Western Isles of Scotland. When composed of materials which do not so easily undergo decomposition as the surrounding rocks, they stand up like vast walls; but when, on the other hand, they are more readily acted on by atmospheric moisture than are the rocks which enclose them, they give rise to deep trenches with vertical sides, which render the country almost impassable.

STRUCTURE OF INTRUSIVE MASSES.

The lava consolidating in these horizontal intrusions (sheets and laccolites), and the vertical intrusions (dykes), is usually more crystalline in structure than the similar materials poured out at the surface. In the same dyke or sheet, when it is of great width, we often find every variation—from a glassy material formed by the rapid cooling of the mass where it is in contact with other rocks, to the perfectly crystalline or granitic varieties which form the centre of the intrusion. It is in these dykes and other intrusions that we find the most convincing evidence of the truth of the conclusions, which we have enunciated in a former chapter, concerning the dependence of the structure of an igneous rock upon the conditions under which it has consolidated. One material is found, under varying conditions, assuming the characters of obsidian, rhyolite, quartz-felsite, or granite; another, under the same set of conditions, taking the form of tachylyte, basalt, dolerite, and gabbro.

That these great intrusive masses, sheets and dykes, in their passage between the sedimentary rocks sometimes find places where the overlying strata are of such thinness or incoherence that the liquefied rocks are able to force a way for themselves to the surface, we have the clearest proof. In some dykes we find the rock in their upper portions losing its compact character and becoming open and scoriaceous, showing that the pressure had been so far diminished as to allow of the imprisoned water flashing into steam.

All round great volcanoes which have become extinct we frequently find series of small volcanic cones, which have evidently been thrown up along the lines where the great lava-filled fissures, which we have been describing, have reached the surface and given rise to explosive action there. The linear arrangement of these small cones, which are thrown up in the plains surrounding vast volcanic mountains that have become extinct, is very striking. The numerous 'puys' of the Auvergne and adjoining volcanic regions of Central France are for the most part small scoria- and lava-cones which were thrown up along great lines of fissure radiating from the immense, central, volcanic mountains of the district, after they had become extinct. These scoria-cones and the small lava-streams which flow from them, as was so well shown by Mr. Scrope, mark the latest efforts of the volcanic forces beneath the district before they finally sank into complete extinction. In the Western Isles of Scotland, as I have elsewhere shown, we can study the formation of these later-formed cones in the plains around extinct volcanic mountains, with the additional advantage of having revealed to us, by the action of the denuding forces, their connection with the great radiating fissures.

It has been shown that the several stages in the decline of each volcanic outburst is marked by the appearance at the vent of certain acid gases. In the same way, after the ejection of solid materials from a volcanic vent has come to an end, certain gaseous substances continue to be evolved; and as the temperature at the vents declines, the nature of the volatile substances emitted from them undergoes a regular series of changes.

ORDER OF EMISSION OF VOLCANIC GASES.