Volcanoes: What They are and What They Teach

by John W. Judd

Fig. 17.—Outlines of Vesuvius, showing its Form at different periods of its history.

CHANGES IN OUTLINE OF VESUVIUS.

In the series of outlines in [fig. 17], we have endeavoured to illustrate the succession of changes which has taken place in Vesuvius during historical times. In the year 79 one side of the crater-wall of the vast mountain-mass was blown away. Subsequent ejections built up the present cone of Vesuvius within the great encircling crater-wall of Somma, and the form of this cone and the crater at its summit have been undergoing continual changes during the successive eruptions of eighteen centuries.

What its future history may be we can only conjecture from analogy. It may be that a long continuance of eruptions of moderate energy may gradually raise the central cone till its sides are confluent with those of the original mountain; or it may be that some violent paroxysm will entirely destroy the modern cone, reducing the mountain to the condition in which it was after the great outburst of 79. On the other hand, if the volcanic forces under Vesuvius are gradually becoming extinct (but of this we have certainly no evidence at present), the mountain may gradually sink into a state of quiescence, retaining its existing form.

The series of changes in the shape of Vesuvius, which are proved by documentary evidence to have been going on during the last 2,000 years, probably find their parallel in all active volcanoes. In all of these, as we shall hereafter show, the activity of the vents undergoes great vicissitudes. Periods of continuous moderate activity alternate with short and violent paroxysmal outbursts and intervals of complete rest, which may in some cases last for hundreds or even thousands of years. During the periods of continuous moderate activity, the crater of the volcano is slowly filled up by the growth of smaller cones within it; and the height of the mountain is raised. By the terrible paroxysmal outbursts the mountain is often completely gutted and its summit blown away; but the materials thus removed from the top and centre of the mass are for the most part spread over its aides, so that its bulk and the area of its base are thereby increased. During the intervals of rest, the sides of the mountain which are so largely composed of loose and pulverulent materials are washed downwards by rains and driven about by winds. Thus all volcanoes in a state of activity are continually growing in size every ejection, except in the case of those where the materials are in the finest state of subdivision, adding to their bulk; the area of their bases being increased during paroxysmal outbursts, and their height during long-continued moderate eruptions.

DEVIATIONS FROM CONICAL FORM.

We have pointed out that the conical form of volcanic mountains is due to the slipping of the falling materials over one another till they attain the angle at which they can rest. There are, however, some deviations from this regular conical form of volcanoes which it may be well to refer to.

The quantity of rain which falls during volcanic eruptions is often enormous, owing to the condensation of the great volumes of steam emitted from the vent. Consequently the falling lapilli and dust often descend upon the mountain, not in a dry state but in the condition of a muddy paste. Many volcanic mountains have evidently been built up by the flow of successive masses of such muddy paste over their surfaces. Some volcanic materials when mixed with water have the property of rapidly 'setting' like concrete. The ancient Romans and modern Italians, well acquainted with this property of certain kinds of volcanic dust and lapilli, have in all ages employed this 'puzzolana,' as it is called, as mortar for building. The volcanic muds have often set in their natural positions, so as to form a rock, which, though light and porous, is of tolerably firm consistency. To this kind of rock, of which Naples and many other cities are built, the name of 'tuff' or 'tufa' is applied. A similar material is known in Northern Germany as 'trass.'

The cause of the 'setting' of puzzolana and tufa is that rain-water containing a small proportion of carbonic acid acts on the lime in the volcanic fragments, and these become cemented together by the carbonate of lime and the free silica, which are thus produced in the mass.