We do know that the temperature increases as we go farther down from the surface, but the increase is very different in different districts—in some places being five times greater than it is in others at an equal depth, and it is always greatest in localities where volcanoes have been active not long before. Now if there were an ocean of melted rock at a certain distance down below the crust all over the globe, there could scarcely be such a great difference between one place and another, and for this and many other reasons geologists are inclined to think that, from some unknown cause, great heat is developed at special points below the surface at different times. This would account for our finding volcanic rocks in almost all parts of the world, even very far away from where there are any active volcanoes now.
But, as I have said, we do not clearly know why great reservoirs of melted rock occur from time to time deep under our feet. We may perhaps one day find the clue from the fact that nearly all, if not all, volcanoes occur near to the water's edge, either on the coast of the great oceans or of some enormous inland sea or lake. But at present all we can say is, that in certain parts of the globe there must be from time to time great masses of rock heated till they are white-hot, and having white-hot water mingled with them. These great masses need not, however, be liquid, for we know that under enormous pressure white-hot metals remain solid, and water instead of flashing into steam is kept liquid, pressing with tremendous force upon whatever keeps it confined.
But now suppose that for some reason the mass of solid rock and ground above one of these heated spots should crack and become weak, or that the pressure from below should become so great as to be more powerful than the weight above, then the white-hot rock and water quivering and panting to expand, would upheave and burst the walls of their prison. Cannot you picture to yourselves how when this happened the rock would swell into a liquid state, and how the water would force its way upwards into cracks and fissures expanding into steam as it went. Then would be heard strange rumbling noises underground, as all these heavily oppressed white-hot substances upheaved and rent the crust above them. And after a time the country round, or the ground at the bottom of the sea, would quake and tremble, till by and by a way out would be found, and the water flashing into vapour would break and fling up the masses of rock immediately above the passage it had made for itself, and following after these would come the molten rock pouring out at the new opening.
Such outbursts as these have been seen at sea many times near volcanic islands. In 1811 a new island called Sabrina was thrown up off St. Michael's in the Azores, and after remaining a short time was washed away by the waves. In the same way Graham's Island appeared off the coast of Sicily in 1831, and as late as 1885 Mr. Shipley saw a magnificent eruption in the Pacific near the Tonga Islands when an island about three miles long was formed.
Another very extraordinary outburst, this time on land, took place in 1538 on the opposite side of the Bay of Naples to where Vesuvius stands. There, on the shores of the Bay of Baiæ, a mountain 440 feet high was built up in one week, where all had before been quiet in the memory of man. For two years before the outburst came, rumblings and earthquakes had alarmed the people, and at last one day the sea drew back from the shore and the ground sank about fourteen feet, and then on the night of Sunday, September 29, 1538, it was hurled up again, and steam, fiery gases, stones, and mud burst forth, driving away the frightened people from the village of Puzzuoli about two miles distant. For a whole week jets of lava, fragments of rock, and showers of ashes were poured out, till they formed the hill now called Monte Nuovo, 440 feet high and measuring a mile and a half round the base. And there it has remained till the present day, perfectly quiet after the one great outburst had calmed down, and is now covered with thickets of stone-pine trees.
These sudden outbursts show that some great change must occur in the state of the earth's crust under the spots where they take place, and we know that eruptions may cease for centuries in any particular place and then begin afresh quite unexpectedly. Vesuvius was a peaceable mountain overgrown with trees and vines in the time of the Greeks till in the year A.D. 79 occurred the terrific outburst which destroyed Herculaneum and Pompeii, shattering old Vesuvius to pieces, so that only the cliffs on the northwest remain and are called Somma (see Fig. 37), while the new Vesuvius has grown up in the lap, as it were, of its old self. Yet when we visit the cliffs of Somma, and examine the old lava streams in them, we see that the ancient peaceful mountain was itself built up by volcanic outbursts of molten rock, and showers of clinkers or scoriæ, long before man lived to record it.
Meanwhile, when once an opening is made, we can understand how after an eruption is over, and the steam and lava are exhausted, all quiets down for awhile, and the melted rock in the crater of the mountain cools and hardens, shutting in once more the seething mass below. This was the state of the crater when I saw it in 1864, though small streams still flowed out of two minute cones; but since then at least one great outburst had taken place in 1867, and now on this November night, 1868, the imprisoned gases rebelled once more and forced their way through the mountain-side.
At this point we can leave off forming conjectures and really study what happens; for we do know a great deal about the structure of volcanoes themselves, and the history of a lava-flow has been made very clear during the last few years, chiefly by the help of the microscope and chemical experiments. If we imagine then that on the day of the eruption we could have seen the inside of the mountain, the diagram (Fig. 39) will fairly represent what was taking place there.
