LAVA, a stream of melted minerals which runs out of the mouths, or bursts out through the sides, of burning mountains, during the time of an eruption. See ÆTNA, VESUVIUS, HECLA; and see also VOLCANO, GEOLOGY Index.
The lava at its first discharge is in a state of prodigious ignition, greatly superior to any thing we can have an idea of from the small artificial furnaces made by us. Sir William Hamilton informs us, that the lava of Vesuvius, at the place whence it issued (in the year 1767), "had the appearance of a river of red hot and liquid metal, such as we see in the glass houses, on which were large floating cinders half lighted, and rolling over one another with great precipitation down the side of the mountain, forming on the whole a most beautiful and uncommon cascade." Now, if we consider the materials of which the lava consists, which undoubtedly are the common matters to be found everywhere in the earth, namely, stones, metallic ores, clay, sand, &c. we shall find that our hottest furnaces would by no means be able to bring them into any degree of fusion; since the materials for glass cannot be melted without a great quantity of very fusible salts, such as alkalies, nitre, &c. mixed along with them. The heat of a volcano must therefore be immense; and besides its heat, it is sometimes attended with a very uncommon circumstance; for Sir William Hamilton informs us, that "the red-hot heat of stones thrown up by Vesuvius on the 31st of March 1766, were perfectly transparent;" and the like remark he makes on the vast stream of lava which issued from the same volcano in 1779. This we cannot look upon to be the mere effect of heat; for mere heat with us will not make a solid body transparent; and these stones, we are sure, were not in a state of fusion, or the resistance of the air would have broke them all to pieces, even supposing them, which is very improbable, to have been in that state detached from the rest of the lava. For the transparency, therefore, (according to some authors) we must have recourse to electricity; which in some of our experiments hath the property of rendering opaque bodies transparent*. Indeed it is scarcely possible but the lava and every other matter thrown out of a volcano must be in the highest degree electrical, if the fire itself takes its rise from electricity.
The lava, after having once broke out, does not constantly continue running from the same vent, but often has intermissions, after which it will burst out sometimes at the same place, and sometimes at another. No real flame ever appears to come from the lava. In the day time its progress is marked by a thick white smoke, from which the light of the red-hot matter being reflected in the night time, makes it appear like flame. But if, during its progress, it meets with trees or other combustible substances, which it frequently does, a bright flame immediately issues from
its surface, as hath also been remarked by Sir William Hamilton.—This liquid substance, after having run pure for about 100 yards (more or less, no doubt, according to different circumstances), begins to collect cinders, stones, and a scum is formed on the surface. Our author informs us, that the lava which he observed, with its scum, had the appearance of the river Thames, as he had seen it after a hard frost and a great fall of snow, when beginning to thaw, carrying down vast masses of snow and ice. In some places it totally disappeared, and ran in a subterraneous passage formed by the scum for several paces; after which it came out pure, having left the scum behind, though a new one was quickly formed. This lava at the farthest extremity from its source did not appear liquid, but like a heap of red-hot coals, forming a wall in some places 10 or 12 feet high, which rolling from the top soon formed another wall, and so on.—This was the appearance also put on by the lava which issued in the great eruption of 1783 in Iceland; with this difference, that the wall was at one time 210 feet high, and the general thickness of it was more than 100: (See HECLA). While a lava is in this state, Sir William is of opinion, that it is very practicable to divert it into another channel, in a manner somewhat similar to what is practised with rivers. This he was afterwards told had been done with success during the great eruption of Ætna in 1669: that the lava was directing its course towards the walls of Catania, and advancing very slowly, when they prepared a channel for it round the walls of the town, and turned it into the sea. A succession of men, covered with sheep skins wetted, were employed to cut through the tough flanks of lava, till they made a passage for that in the centre, which was in perfect fusion, to discharge itself into the channel prepared for it. But this, it is evident, can only take place in small streams of this burning matter; with that above mentioned it would have been impossible. It hath been also observed of the lavas of Ætna, that they do not constantly fall down to the lowest places, but will sometimes ascend in such a manner as to make the valleys rise into hills. On this Sir William Hamilton has the following note: "Having heard the same remark with regard to the lavas of Vesuvius, I determined, during an eruption of that volcano, to watch the progress of a current of lava, and I was soon enabled to comprehend this seeming phenomenon, though it is, I fear, very difficult to explain. Certain it is, that the lavas, while in their most fluid state, follow always the laws of other fluids; but when at a great distance from their source, and consequently encumbered with scorice and cinders, the air likewise having rendered their outward coat tough, they will sometimes (as I have seen) be forced up a small ascent, the fresh matter pushing forward that which went before it, and the exterior parts of the lava acting always as conductors (or pipes, if I may be allowed the expression) for the interior parts, that have retained their fluidity from not being exposed to the air."
From the year 1767 to 1779, this gentleman made many curious observations on the lavas of Vesuvius. He found, that they constantly formed channels in the mountain as regular as if they had been made by art; and that, whilst in a state of perfect fusion, they continued their course in those channels, which were
sometimes full to the brim, and at others more or less so according to the quantity of matter thrown out. These channels, after small eruptions, were generally from two to five or six feet wide, and seven or eight in depth. They were often hid from the sight by a quantity of scorice that had formed a crust over them, and the lava, having been conveyed in a covered way for some yards, came out again fresh into an open channel. Our author informs us, that he had walked in some of these subterraneous galleries, which were exceedingly curious, the sides, top, and bottom, being exceedingly smooth and even: others were incrustated with what he calls very extraordinary scorice, beautifully ramified white salts in the form of dropping stalactites, &c.
On viewing a stream of lava while in its fluid state in the month of May 1779, he perceived the operation of it in the channels above described in great perfection. After quitting them, it spread itself in the valley, and ran gently like a river that had been frozen, and had masses of ice floating upon it. The wind happening then to shift, our traveller was so incommoded by the smoke, that the guide proposed to cross it, which was instantly put in execution without any other inconvenience than the violent heat with which the legs and feet were affected. The crust was so tough, that their weight made no impression upon it, and the motion so slow that they were in no danger of falling. This circumstance, according to Sir William, points out a method of escape should any person happen to be enclosed betwixt two lavas, but ought never to be tried except in cases of real necessity; and indeed, if the current of melted matter was very broad, must undoubtedly be attended with extreme danger, both from the heat of the upper crust and the chance of its breaking and falling down with the passenger into the burning liquid below. That which Sir William Hamilton crossed was about 50 or 60 feet broad.
Having passed this burning stream, our travellers walked up along the side of it to its very source. Here they saw it boiling and bubbling violently up out of the ground, with a hissing and crackling noise like that which attends the playing off an artificial fire work. A hillock of about 15 feet high was formed by the continual splashing up and cooling of the vitrified matter. Under this was an arched hollow, red hot within, like a heated oven; the lava which ran from it being received into a regular channel raised upon a sort of wall of scorice and cinders, almost perpendicularly, of about the height of 8 or 10 feet, and much resembling an ancient aqueduct. On quitting this fountain of lava, they went quite up to the crater, where as usual they found a little mountain throwing up stones and red-hot scorice with loud explosions; but the smoke and smell of sulphur were so intolerable, that they were obliged to quit the place with precipitation.
By the great eruption in August 1779, the curious channels above mentioned were entirely destroyed, the cone of the mountain was covered with a stratum of lava full of deep cracks, from whence continually issued a sulphureous smoke that tinged the scorice and cinders with a deep yellow, or sometimes white tint. The lava of this eruption appeared to be more perfectly vitrified than that of any former one he had observed.