No volcano has been so carefully watched for a long time as Vesuvius. Its history illustrates the phases we have just mentioned. The first recorded eruption is that of A. D. 79, a very severe one of the violent type, by which Herculaneum, Pompeii, and Stabiæ were buried. We have an interesting account by the younger Pliny. Before this great eruption took place, Vesuvius had been in a state of repose for eight hundred years, and if we may judge from the Greek and Roman writings, was not even suspected of being a volcano. Then followed an interval of rest until the reign of Severus, the second eruption taking place in the year 203. In the year 472, says Procopius, all Europe was covered more or less with volcanic ashes. Other eruptions followed at intervals, but there was complete repose for two centuries; that is, until the year 1306. In 1500 it was again active, then quiet again for one hundred and thirty years. In 1631 there took place another terrific outburst. After this many eruptions followed, and they have been frequent ever since. Vesuvius is therefore now in the second stage of moderate activity.
But geologists can take a wider view than this. They can sum up the history of a volcanic region of the earth; and the result is somewhat as follows: Volcanoes, like living creatures, go through different periods or phases, corresponding roughly to youth, middle age, old age, and finally decay. The invasion of any particular area of the earth's surface by the volcanic forces is heralded by underground shocks, or earthquakes. A little later on cracks are formed, as indicated by the rise of saline and hot springs, and the issuing of carbonic acid and other gases at the surface of the earth. As the underground activity becomes greater, the temperature of the springs and emitted gases increases; and at last a visible rent is formed, exposing highly heated and glowing rock below. From the fissure thus formed, the gas and vapours imprisoned in the molten rocks escape with such violence as to disperse the latter in the form of pumice and volcanic ash, or to cause them to pour out as lava-streams.
The action generally becomes confined to one or more points along the line of action (which is a line of fissures and cracks). In this way a chain of volcanoes is formed, which may become the seat of volcanic action for a long time.
When the volcanic energies have become somewhat exhausted, so that they cannot raise up the lava and expel it from the volcanic crater, nor rend the sides of the volcano and cause minor cones to grow up on their flanks, small cones may be formed at a lower level in the plains around the great central chain. These likewise are fed from fissures.
Later on, as the heated rock below cools down, the fissures are sealed up by lava that has become solid; and then the volcanoes fall, as it were, into the "sere and yellow leaf," and remain in a peaceful, quiet state befitting their old age.
After this they begin to suffer from long exposure to the atmospheric influences of decay, and rain and rivers wash them away more or less completely.
But still the presence of heated rocky matter at no great depth below is proved by the outbursts of gases and vapours, the forming of geysers and ordinary hot springs. Gradually, however, even these signs of heat below disappear; and the cycle of volcanic phases is at an end. Such a series of changes may require millions of years; but by the study of volcanoes in every stage of their growth and decline it is possible thus to sketch out an outline of their history.
It must be confessed that in the present state of scientific knowledge no full and complete explanation of volcanic action is possible. Geologists and others are as yet but feeling their way cautiously towards the light which, perhaps before long, will illumine the dark recesses of this mysterious subject. Many theories and ideas have been put forward, but in the opinion of the writer the most promising explanation is one that may be briefly expressed as follows:
There are below the crust of the earth large masses of highly heated rock that are kept solid by the enormous pressure of the overlying rocks, or otherwise they would melt,—for it is a known fact that pressure tends to prevent the melting of a solid body. But when earth-movements taking place within the earth's crust—such as the upheaving of mountain-chains—take off some of the weight, the balance between internal heat and the pressure from above is no longer maintained; and so these highly heated rocks run off into the liquid state, and finding their way to the surface through the fissures mentioned above, give rise to volcanic action. There is much to be said in favour of this view. It rightly connects volcanic action with movements of upheaval, with mountain-chains and lines of weakness in the earth's crust.
There is very good reason to believe that the earth was once in a highly heated state, and has been slowly cooling down for ages. The increase of temperature observed in penetrating mines tells us that it still retains below the surface some of its old heat. We need not therefore be surprised at the existence of heated masses of rock down below, or seek, as some have done, an entirely different source for the origin of volcanic heat than that which remains from the earth's once molten condition. It would take too long to state the reasons on which this idea of the former state of our planet is based, and moreover, it would bring us into the region of astronomy, with which we are not concerned at present.