Volcanoes: What They are and What They Teach

by John W. Judd

ANCIENT VOLCANOES IN OTHER DISTRICTS.

If we were to study any other part of the earth's surface, we should arrive at precisely the same conclusion as those to which we have been conducted by our examination of the British Islands—namely, that during past geological times the subterranean forces had made themselves felt in the area, had gradually attained a maximum, and then as gradually declined, passing through all those varied cycles which we have described in a former chapter. And we should also find that these periods of volcanic activity alternated with other periods of complete quiescence which were of longer or shorter duration. But on comparing two different districts, we should discover that what was a period of volcanic activity in the one was a period of repose in the other, and vice versâ.

From these facts geologists have been led to the conclusion which we have already enunciated—namely, that the subterranean forces are in a state of continual flux over the surface of the globe. At one point of the earth's crust these forces gradually gather such energy as to rend asunder the superincumbent rock-masses and make themselves manifest at the surface in the series of phenomena characteristic of volcanic action. But after a longer or shorter interval of time—an interval which must probably be measured by millions of years—the volcanic forces die out in that area to make their appearance in another.

Hence, although we may not be able to prove the fact by any mathematical demonstration, a strong presumption is raised in favour of the view that the subterranean energy in the earth's crust is a constant quantity, and that the only variations which take place are in the locality of its manifestation.

Upon this question whether the amount of this subterranean energy within the earth's crust is at the present time increasing, stationary, or declining, we are not altogether destitute of evidence. There are some considerations connected with certain astronomical hypotheses, to which we shall hereafter have to refer, that might lead us to entertain the view that the subterranean activity was once far greater than it is at present, and that during the long periods of the earth's past history it has been slowly and gradually declining. And those who examine the vast masses of igneous materials which have been poured out from volcanic vents during the earlier periods of the earth's history may be inclined, at first sight, to point to them as affording conclusive proof of this gradual decline.

SUPPOSED DECLINE OF VOLCANIC ACTION.

But a more careful study of the rocks in question will probably cause a geologist to pause before jumping to such a conclusion. If we look at the vast masses of volcanic materials erupted in Miocene times in our own island and in Ireland, for example, we might be led to imagine that we have the indications of a veritable 'Reign of Fire,' and that the evidence points to a condition of things very different indeed from that which prevails at the present day. But in arriving at such a conclusion we should be neglecting a most important consideration, the disregard of which has been the fertile parent of many geological errors. Many independent lines of evidence all point to the inference that these volcanic ejections are not the result of one violent effort, but are the product of numerous small outbreaks which have been scattered over enormous periods of time.

When we examine with due care the lavas, tuffs, and other volcanic ejections which constitute such mountain-masses as those of the Hebrides, of the Auvergne, and of Hungary, we find clear proofs that the ancient Miocene volcanoes of these districts were clothed with luxuriant forests, through which wild animals roamed in the greatest abundance. The intervals between the ejections of successive lava-streams were often so great, that soils were formed on the mountain-slope, and streams cut deep ravines and valleys in them.

The island of Java is situated near the very heart of what is at the present day the most active volcanic centre on the face of the globe, yet vegetable and animal life flourish luxuriantly there, and the island is one of the richest and most fertile spots upon the face of the globe. Not all the terrors of occasional volcanic outbreaks will ever drive the Neapolitan vine-dressers from the fertile slopes of Vesuvius, for its periods of repose are long, and its eruptions are of short duration.

These considerations lead the geologist to conclude that the evidence afforded by the ancient volcanic rocks is clear and positive in support of the view that the manifestations of the subterranean forces in the past agree precisely in their nature and in their products with those taking place around us at the present time. On the question of great secular changes having occurred in the amount of volcanic energy in past geological periods, the evidence must be pronounced negative, or at the best doubtful.