Through all its length, while clouds of smoke
The living soul of ether choke.
Virgil: Æneid iii.
In some parts of the world we meet with mountains of a very different kind from any we have yet considered,—mountains that are known at times to send forth fiery streams of glowing lava, and to emit with terrific force great clouds of steam. Such mountains have long been known, in popular but unscientific language, as "burning mountains,"[25]—a term which is unfortunate, because they do not burn in the proper sense of the word, like candles or gas-jets. They are better known as volcanoes. There are about three hundred and fifty known active volcanoes; and if we include all mountains that once were in that state, the number is about one thousand.
Such mountains are connected in a curious way with those upheaved ridges of the world known as mountain-chains (see chap. vi., p. [191]). And not only are many mountains more or less penetrated and intersected by rocks of an igneous origin (see chap. v., p. [155]), but some have been largely formed by the action of old volcanoes. In fact, there are hills in Great Britain and parts of Europe, in America, and other countries, that once were actual volcanoes (see page [277]).
We must briefly consider these strange mountains so different from others, and see what we can find out about them. Let us first inquire how a volcano is made, then consider what a volcano does; that is, we must view it as a geological agent that has a certain definite part to play in the economy of the world. And lastly, we may glance at some of the old volcanoes, and see what they were doing in those long ages of the world during which the great series of the stratified rocks were formed,—which rocks are, as it were, the book in which the earth has written her autobiography.
In old days volcanoes were regarded with superstitious awe; and any investigation of their actions would have been considered rash and impious in the highest degree. Mount Etna, as Virgil tells us, was supposed to mark the spot where the angry gods had buried Enceladus, one of the rebellious giants. Volcano, a certain "burning mountain" in the Lipa Islands, was likewise called the forge, or workshop, of Vulcan (or Volcan), the god of fire. And so it comes about that all "burning mountains" take their name from this one Mediterranean island, and at the same time tell us of the mythological origin of the word. It has been said that words are "fossil thoughts;" and we have here an old and very much fossilised thought,—a kind of thought long since extinct among civilised peoples, and one which is never likely to come to life again.
A volcanic mountain consists of alternating sheets of lava and volcanic ashes, mantling over each other in an irregular way, and all sloping away from the centre. In the centre is a pit or chimney, widening out towards the top so as to resemble a funnel or cup; hence the name "crater," which means a cup. In the centre of this crater a very small cone ("minor cone") is frequently found; and it is interesting to find that many of the moon's volcanic craters possess these "minor cones." A number of cracks or fissures intersect the volcano. These frequently spread out from the centre of the mountain in all directions, like the spokes of a wheel. They generally get filled with lava that wells up from below, thus forming "dykes," which may be regarded as so many sheets of igneous rock, such as basalt, that have forced their way while still liquid in among the layers of lava and ashes. The word "ash" is used by geologists in a special sense; and volcanic ash is not, as might be supposed, a deposit of cinders, but mostly of dust of various degrees of fineness, and sometimes it is very fine indeed. Pieces of pumice-stone may be embedded in a layer of volcanic ash, and sometimes great blocks of stone that have been shot out of the volcano as from a big gun, but these only form a small part of the layer. Dykes strengthen the mountain, and tend to hold it together when violently shaken during an eruption.
The shape and steepness of a volcano depend on the nature of the materials ejected. The finer the volcanic ash, the steeper and more conical is the mountain. The building up of a volcano may be fairly illustrated by the little cone of sand formed in an hourglass as the sand-grains fall. These settle down at a certain slope, or angle, at which they can remain, instead of falling down to the bottom, as they do directly this slope is exceeded. Some volcanoes are built up almost entirely of volcanic ash and its embedded blocks. Vesuvius, Teneriffe, Jorullo, in Mexico, and Cotopaxi, in the Andes, are examples of steep volcanic cones built up in this way. Others, less steep and more irregular in shape, are chiefly formed of successive lava-flows. Little minor cones are frequently formed on the side of a volcano; and these during an eruption give rise to small outbursts of their own. They are easily accounted for by the dykes which are mentioned just now; for when molten rock forces its way through fissures, it sometimes finds an outlet at the surface, and being full of steam, as soda-water is full of gas, it gives rise to an eruption. The great opening in the centre of a volcano, with its molten lava, is like a very big dyke that has reached the surface and so succeeded in producing an eruption.
The opening of a soda-water bottle not infrequently illustrates a volcanic eruption; for when the pent-up carbonic acid cannot escape fast enough, it forces out some of the water, even when the bottle is held upright.