Volcanoes: What They are and What They Teach

by John W. Judd

Fig, 65.—A primary Parasitic Cone with a secondary one at its base—Ischia.
a. Monte Rotaro. b. Monte Tabor. c. Lava-stream flowing from the latter.

In one case we find that a parasitic cone, the Monte Rotaro, has itself a similar smaller cone, which is parasitic to it, at its foot; this secondary parasitic cone gives off a small lava-stream of trachyte, which has flowed down to the sea. (See [fig. 65].)

Fig. 66.—Scoria-cone near Auckland, New Zealand, with a lava-current flowing from it.
The strata beneath the volcanic cone are exposed in the sea-cliff, and exhibit proofs of depression having taken place.

Fig. 67.—Section of rocks below the ancient triassic volcano of Predazzo in the Tyrol.
The position of the strata a b c, etc., indicates a central subsidence.

SUBSIDENCE BENEATH VOLCANIC VENTS.

Most great volcanic mountains exhibit a tendency towards a subsidence of their central portions, which may take place either during or subsequently to their period of activity. When we examine the strata upon which a volcano has been built up, but which are now exposed to our study by denuding forces, we usually find that they incline towards the centre of the eruptive activity. (See figs. [66] and [67].) Two causes may contribute to bring about this result. In the first instance, we must remark that the piling up of materials around the volcanic vent causes the subjacent strata to be subjected to a degree of pressure far is excess of that which acts upon the surrounding rocks. And secondly, it must be borne in mind that the continual removal of material from below the mountain must tend to the production of hollows, into which the overlying strata will sink. The effect of this central subsidence is to give to the flanks of volcanic cones those beautifully curved outlines which constitute so striking a feature in Vesuvius (see [fig. 17], p. 87), Fusiyama (see [fig. 77], No. 1, facing p. 178), and many other volcanic mountains.

There seems, at first sight, to be scarcely any limit to the dimensions which these great composite volcanic cones may attain: the lateral eruptions tending to enlarge the area of the base of the mountain, and, by the injection of the fissures, to knit together and strengthen its structure, while the central eruptions continually increase the elevation of the mass. Great, however, as is the force which is concerned in the production of our terrestrial volcanoes, it has its limits; and, at last, the piling up of materials will have gone on to such an extent, that the active forces beneath the volcano are no longer competent either to raise materials to the elevated summit of the mountain or to tear asunder its strengthened and fortified flanks. Under these circumstances, the volcanic forces, if they have not already exhausted themselves, will be compelled to find weak places in the district surrounding the volcano, at which fissures may be produced and the phenomena of eruption displayed.

SHIFTING OF VOLCANIC FOCI.