We have already remarked that the lava, which in a fluid state fills the crater and the internal vent or chimney of the volcano, is forced upwards by gaseous fluids, and by the steam which has been generated from the water, entangled with the lava. In some cases the mechanical force of this vapour is so great as to drive the lava over the edge of the crater, when it forms a fiery torrent, spreading over the sides of the mountain. This only happens in the case of volcanoes of inconsiderable height; in lofty volcanoes it is not unusual for the lava thus to force an outlet for itself near the base of the mountain, through which the fiery stream discharges itself over the surrounding country. In such circumstances the lava cools somewhat rapidly; it becomes hard and presents a scoriaceous crust on the surface, while the vapour escapes in jets of steam through the interstices. But under this superficial crust the lava retains its fluid state, cooling slowly in the interior of the mass, while the thickening stream moves sluggishly along, impeded in its progress by the fragments of rock which this burning river drives before it.
The rate at which a current of lava moves along depends upon its mass, upon its degree of fluidity, and upon the inclination of the ground. It has been stated that certain streams of lava have traversed more than 3,000 yards in an hour; but the rate at which they travel is usually much less, a man on foot being often able to outstrip them. These streams, also, vary greatly in dimensions. The most considerable stream of lava from Etna had, in some parts, a thickness of nearly 120 feet, with a breadth of a geographical mile and a half. The largest lava-stream which has been recorded issued from the Skaptár Jokul, in Iceland, in 1783. It formed two currents, whose extremities were twenty leagues apart, and which from time to time presented a breadth of from seven to fifteen miles and a thickness of 650 feet.
A peculiar effect, and which only simulates volcanic activity, is observable in localities where mud volcanoes exist. Volcanoes of this class are for the most part conical hills of low elevation, with a hollow or depression at the centre, from which they discharge the mud which is forced upwards by gas and steam. The temperature of the ejected matter is only slightly elevated. The mud, generally of a greyish colour, with the odour of petroleum, is subject to the same alternating movements which have been already ascribed to the fluid lava of volcanoes, properly so called. The gases which force out this liquid mud, mixed with salts, gypsum, naphtha, sulphur, sometimes even of ammonia, are usually carburetted hydrogen and carbonic acid. Everything leads to the conclusion that these compounds proceed, at least in great part, from the reaction produced between the various elements of the subsoil under the influence of infiltrating water between bituminous marls, complex carbonates, and probably carbonic acid, derived from acidulated springs. M. Fournet saw in Languedoc, near Roujan, traces of some of these formations; and not far from that neighbourhood is the bituminous spring of Gabian.
IV.—Mud volcano at Turbaco, South America.
Mud volcanoes, or salses, exist in rather numerous localities. Several are found in the neighbourhood of Modena. There are some in Sicily, between Aragona and Girgenti. Pallas observed them in the Crimea—in the peninsula of Kertch, and in the Isle of Tamàn. Von Humboldt has described and figured a group of them in the province of Cartagena, in South America. Finally, they have been observed in the Island of Trinidad and in Hindostan. In 1797 an eruption of mud ejected from Tunguragua, in Quito, filled a valley 1,000 feet wide to a depth of 600 feet. On the opposite page is represented the mud volcano of Turbaco, in the province of Cartagena ([Plate IV.]), which is described and figured by Von Humboldt in his “Voyage to the Equatorial Regions of America.”
In certain countries we find small hillocks of argillaceous formation, resulting from ancient discharges of mud volcanoes, from which all disengagement of gas, water, and mud has long ceased. Sometimes, however, the phenomenon returns and resumes its interrupted course with great violence. Slight shocks of earthquakes are then felt; blocks of dried earth are projected from the ancient crater, and new waves of mud flow over its edge, and spread over the neighbouring ground.
To return to ordinary volcanoes, that is to say, those which eject lava. At the end of a lava-flow, when the violence of the volcanic action begins to subside, the discharge from the crater is confined to the disengagement of vaporous gases, mixed with steam, which make their escape in more or less abundance through a multitude of fissures in the ground.