Volcanoes: What They are and What They Teach

by John W. Judd

When the amygdaloids and altered fragmentary ejections of volcanoes are studied microscopically, their true character is at once made manifest. The exposure of faces of these altered volcanic rocks to the weathering influences of the atmosphere, in many cases also causes their true nature to be revealed, the crystalline materials filling the interstices and vesicles of the mass are dissolved away by the rain-water containing carbonic acid, and the rock regains its original cavernous structure and appearance. But this repeated passage of water through volcanic rock-masses may result in the removal of so large a portion of their materials that the remainder crumbles down into the condition of a clay or mud.

In the basal wrecks of volcanoes, of which we have spoken, we usually find only small and fragmentary remains of the great accumulations of loose and scoriaceous materials which originally constituted the bulk of the mountain mass. In the centre of the ground-plan of such a denuded volcano we find great masses of highly crystalline or granitic rock, which evidently occupy vast fissures broken through the sedimentary or other rocks upon which the volcanic pile has been reared. These highly crystalline rocks exhibit, as we have shown, clear evidence of having been consolidated from a state of fusion with extreme slowness and under enormous pressure, but their ultimate chemical composition is identical with that of the lavas which have been ejected from the volcano.

When, as frequently happens, the volcano, after pouring out one kind of lava for a certain period, has changed the nature of its ejections, and given rise to materials of different composition, we find clear evidence of the fact in studying the basal wreck or ground-plan of the volcano. A great intrusive crystalline mass, of the same chemical composition as the first-extruded lava, is found to be rent asunder and penetrated by a similarly crystalline mass having the composition of the lavas of the second period. Thus, in the volcanoes of the Western Isles of Scotland, which are reduced by the action of denudation to this condition of basal wrecks, we find that rhyolites, trachytes, and andesites were ejected during the earlier periods of their history, and basalts during the later periods.

Fig. 57.—Plan of the Dissected Volcano of Mull, in the Inner Hebrides.

Fig. 58.—Section of the Volcano along the line A B.

a Rocks on which the Volcano has been built up. b Great intrusive masses of acid and intermediate rocks. c Lara currents of basalt which have flowed from d. d Intrusive masses of gabbros & dolerite. e Lava currents which have flowed from b. f Volcanic tuffs and agglomerates.

ANCIENT VOLCANO OF MULL.

We perceive on studying the ground-plan of these volcanoes that great masses of granite, syenite, and diorite—the crystalline representatives of the first-extruded lavas—are penetrated by intrusions of gabbro—the granitic form of the later-ejected lavas. These features are admirably illustrated by the ruined volcano now constituting the Island of Mull, one of the Inner Hebrides, a plan of which is given in [fig. 57], and a section in [fig. 58]. This volcano probably had a diameter at its base of nearly thirty miles, and a height of from 10,000 to 12,000 feet, but is now reduced to a group of hills few of which exceed 3,000 feet in height.