Volcanoes: What They are and What They Teach

by John W. Judd

Fig. 56.—Sheets of Igneous Rock (Basalt) intruded between beds of sandstone, clay, and limestone. (Island of Skye.)

That in the case of most great volcanic mountains, or systems of mountains, vast reservoirs of liquid lava must exist in the earth's crust far below the surface, there can be little room for doubt. Whether such fluid masses are in direct or indirect communication with a great central reservoir, even supposing such to exist, is a totally different question. In many cases the outburst of volcanoes in more or less close proximity has been observed to take place simultaneously, while in others the commencement of the eruption of one volcano has coincided with the lapse into quiescence of another in its vicinity. On the other hand, the remarkable case of the volcanoes of Hawaii seems to indicate that two vents in close proximity may be supplied from perfectly distinct reservoirs of lava. The active craters of Mauna Loa and Kilauea are situated at the heights of 14,000 and 4,000 feet respectively above the sea level; yet the former is sometimes in a state of violent activity, with which the latter shows no signs of sympathy whatever. We shall, in a future chapter, adduce evidence that the liquid lavas in underground reservoirs may undergo various stages of change in the enormous periods of time during which habitual volcanic vents are supplied from them.

We have already shown that the character assumed by a mass of fused material in cooling varies greatly according as the cooling takes place rapidly at the surface or slowly under enormous pressure. In the former case a glassy base is formed containing a greater or smaller number of crystallites or embryo crystals, in the latter the whole rock is converted into a mass of fully-developed crystals.

CONSOLIDATION OF LAVAS AT GREAT DEPTHS.

The lavas which are poured out at the surface consist, as we have seen, of a glassy magma in which a greater or smaller number of crystals are found which have been borne up from below. The great dykes and intrusive sheets consist for the most part of a mass of small or imperfectly developed crystals in which a number of large and perfectly formed crystals are embedded. Such rocks are said to have a 'porphyritic' structure. The rocks formed by the consolidation of the liquid masses in the underground reservoirs are found to be perfectly crystallised, the crystals impressing one another on every side and making up the whole mass to the exclusion of any paste or magma between them. The crystals in those rocks which have consolidated at these vast depths exhibit evidence, in their enclosed watery solutions and liquefied carbonic acid, of the enormous pressures under which they must have been consolidated. The lavas, the more or less porphyritic rocks of the dykes and sheets, and the perfectly crystalline (granitic) rocks of the underground reservoirs pass into one another, however, by the most insensible gradations.

We sometimes find examples of volcanoes which, by the action of denuding forces, have had their very foundations exposed to our view. Such examples occur in the Western Isles of Scotland, in the Euganean Hills near Padua in Northern Italy, and in many other parts of the earth's surface. In these cases we are able to trace the ground-plan of the volcanic pile, and to study the materials which have consolidated deep beneath the surface in the very heart of the mountain.

In studying these 'basal wrecks' of old volcanoes it is always necessary to bear in mind that the appearance and general characters of a volcanic rock may be completely disguised by chemical changes going on within it. It is through want of attention to this fact that so many mistakes were made by the Wernerian school of geologists who declared that they could find no analogy between the basaltic rocks of the globe and the products of active volcanoes, and were hence led to refer the origin of the former to some kind of 'aqueous precipitation.'

Many of the hard and crystalline marbles which are employed as ornamental stones were originally loose masses of shells and corals, as we easily perceive when we examine the polished faces. But these incoherent heaps of organic débris have been converted into a compact and solid rock in consequence of the mass being penetrated by water containing carbonate of lime in solution. Crystals of this substance were deposited in every cavity and interstice of the mass, and thus the accumulation of separate organisms was gradually transformed to a material of great solidity and hardness.

FORMATION OF AMYGDALOIDS.

In precisely the same way loose heaps of scoriæ, lapilli, or pumice may, by the passage through them of water containing various substances in solution, have their vesicles filled with crystals, and thus be converted into the hardest and most solid of rock-masses. Similarly the scoriaceous portions of lava-streams have their vesicles filled with crystalline substances deposited from a state of solution, and are thus converted into a solid mass which may at first sight appear to offer but little resemblance to the vesicular materials of recent lava-streams. To these vesicular rocks which have their cavities filled with crystalline substances geologists apply the name of amygdaloids (L. amygdalus, an almond). The cavities in lava-rocks are usually more or less elongated, owing to the movement of the mass while in a still plastic state, and the crystalline materials filling these cavities take the almond-like shape; hence the name.