Every volcano has been built up on a platform of ordinary stratified rocks; and at some period after these had been laid down in water and raised up into dry land, molten rock found its way through them, and so the volcano was built up by successive eruptions during many years. It is probable that earthquake shocks, preceding the first eruption, cracked up these strata, and so made a way for the lava to come up.
The main point we wish to emphasize is that volcanoes are never formed by upheaval. In this way they differ from all other mountains. They have not been made by the heaving up of strata, but have been gradually piled up, something like rubbish heaps that accumulate in the Thames barges as the dustmen empty their carts into them, only in the case of volcanoes the "rubbish" comes from below. It is not necessary to suppose that the reservoir down below, from which the molten rock is supplied, exists at any very great depth below the original land surface on which the volcano grows up.
The old "upheaval theory" of volcanoes, once advocated by certain authorities, instead of being based on actual evidence or on reasoning from facts, was a mere guess. Moreover, if the explanation we have given should not be sufficiently convincing, there is good proof furnished by the case of a small volcano near Vesuvius, the building of which was actually witnessed. It is called Monte Nuovo, or the New Mountain. It is a little cone 430 feet high, on the bank of Lake Averno, with a crater more than a mile and a half wide at the base. It was almost entirely formed during a single night in the year 1538, A. D. We have two accounts of the eruption to which it owes its existence; and each writer says distinctly that the mountain was formed by the falling of stones and ashes.
One witness says,—
"Stones and ashes were thrown up with a noise like the discharge of great artillery, in quantities which seemed as if they would cover the whole earth; and in four days their fall had formed a mountain in the valley between Monte Barbaro and Lake Averno, of not less than three miles in circumference, and almost as high as Monte Barbaro itself,—a thing incredible to those who have not seen it, that in so short a time so considerable a mountain should have been formed."
Another says,—
"Some of the stones were larger than an ox. The mud (ashes mixed with water) was at first very liquid, then less so, and in such quantities that with the help of the afore-mentioned stones a montain was raised one thousand paces in height."
(The writer's astonishment led him greatly to exaggerate the height.)
These accounts are important as showing how in a much longer time a big volcano may be built up. From such small operations we learn how Nature works on a large scale. The great volcano in Mexico known as Jorullo was probably built up in a very similar way. There is a tradition among the natives that it was made in two or three days; but we can hardly believe that. Volcanoes, as they get older, tend to grow taller and bigger; but every now and then a large portion may be blown away by some great eruption, and they have, as it were, to begin again.