Читать книгу Wonders of Creation: A Descriptive Account of Volcanoes and Their Phenomena - Unknown - Страница 2

Volcanoes of Iceland—Mount Hecla—Earliest Eruption—Great Eruption in 1845—Skaptàr Yökul—Terrible Eruption in 1783—Rise and Disappearance of Nyoë—Katlugaia—The Geysers—A very hot Bath —Californian Geysers—Iceland-spar—Jan Mayen

We shall begin with the volcanoes of Iceland, of which the most interesting and active is Mount Hecla. The annexed woodcut will give you an idea of its appearance. You will observe the column of volcanic vapour ascending from the snow-clad summit of the cone, and how dreary and desolate is the aspect of the country at its base.

The earliest recorded eruption of Mount Hecla took place in the ninth century of the Christian era; but probably there had been many before that date. Since then there have been between twenty and thirty considerable eruptions of this mountain, and it has sometimes remained in a state of activity for upwards of six years with little intermission. It took a long rest, however, of more than sixty years' duration, prior to the year 1845, when it again burst forth. After a violent storm on the night of the 2nd of September in that year, the surface of the ground in the Orkney Islands was found strown with volcanic dust. There was thus conveyed to the inhabitants of Great Britain an intimation that Hecla had been again at work. Accordingly, tidings soon after arrived of a great eruption of the mountain. On the night of the 1st of September, the dwellers in its neighbourhood were terrified by a fearful underground groaning, which continued till mid-day on the 2nd. Then, with a tremendous crash, there were formed in the sides of the cone two large openings, whence there gushed torrents of lava, which flowed down two gorges on the flanks of the mountain. The whole summit was enveloped in clouds of vapour and volcanic dust. The neighbouring rivers became so hot as to kill the fish, and the sheep fled in terror from the adjoining heaths, some being burnt before they could escape.

On the night of the 15th of September, two new openings were formed—one on the eastern, and the other on the southern slope—from both of which lava was discharged for twenty-two hours. It flowed to a distance of upwards of twenty miles, killing many cattle and destroying a large tract of pasturage. Twelve miles from the crater, the lava-stream was between forty and fifty feet deep and nearly a mile in width. On the 12th of October a fresh torrent of lava burst forth, and heaped up another similar mass. The mountain continued in a state of activity up to April 1846; then it rested for a while, and began again in the following month of October. Since then, however, it has enjoyed repose.

The effects of these eruptions were disastrous. The whole island was strown with volcanic ashes, which, where they did not smother the grass outright, gave it a poisonous taint. The cattle that ate of it were attacked by a murrain, of which great numbers died. The ice and snow, which had gathered about the mountain for a long period of time, were wholly melted by the heat. Masses of pumice weighing nearly half a ton were thrown to a distance of between four and five miles.

Mount Hecla is not the only volcano in Iceland. There are several others; and from one of them, named Skaptàr Yökul, there was, in the year 1783, an eruption still more violent than that from Hecla above described. It began on the 8th of June, and raged with little abatement till the end of August, whence onward it continued, but with less violence, till the following year. The lava, in this case, poured from numerous openings; but these rivulets ultimately united themselves into two large currents, which flowed onwards to the sea. In their progress, these burning torrents filled up the beds of two considerable rivers. The greater of the two streams, after it had ceased to flow and had become a solid mass of rock, measured fifty miles in length, and between twelve and fifteen miles in breadth. Its average depth on the plains was about a hundred feet; but in the bed of the river, which it had filled, it was not less than six hundred feet. The snow and ice, which had previously covered the mountain, were not only melted, but the water that flowed from them was raised to the boiling point, and poured down with destructive effect on the plains. The dust and ashes thrown into the air darkened the sun; and they were then strown over the surface of the island, destroying all the pastures, so that many thousands of cattle, horses, and sheep perished. But worse than that, upwards of nine thousand persons lost their lives by this dreadful catastrophe.

About a month before this great eruption of Skaptàr Yökul, a volcanic island was thrown up from the sea, at a distance of about seventy miles from Iceland. So great was the quantity of ashes and dross ejected from its crater, that it overspread the sea to a distance of a hundred and fifty miles, forming a crust which obstructed the progress of ships. Portions of this crust floated as far as the Shetland and Orkney islands. The King of Denmark named this fiery apparition "Nyoë," or "New Island," and doubtless prided himself not a little on this addition to his limited dominions. But, alas, for human ambition! About a year after the date of its first appearance, Nyoë sank into the depths out of which it arose, and its position is now marked only by a moderate shoal.

It is not by their ejected lavas alone that the volcanoes of Iceland produce their destructive effects. Disastrous consequences have frequently resulted from the sudden melting of their snows and glaciers, on which the volcanic fires operate far more rapidly than does the heat of the sun. It is chiefly by the vast quantities of earth, sand, stones, and broken fragments of rock, which they hurry along with them in their wild career, that the waters, so suddenly freed, produce the greatest amount of damage. During an eruption of Katlugaia, one of the southern Icelandic volcanoes, in 1756, the mass of material thus carried down by the melted snows and glaciers was so great, that, advancing several leagues into the sea, it formed three parallel promontories, which rose above the sea-level, where there had formerly been a depth of forty fathoms of water. Vast ravines were, at the same time, scooped out of the sides of the mountain by the erosion of the waters. Another eruption of this volcano in 1860 produced similar results.

Still more interesting than the volcanic mountains of Iceland are its Geysers, or intermittent springs of boiling water. The chief of these is the Great Geyser. A jet rises to a vast height, and is accompanied by much steam. Indeed, it is quite at the boiling-point.

The little mound, from the top of which the jet appears to rise, is composed of a substance named siliceous sinter, and is a deposit from the water of the fountain. At the top of this mound, which is between six and seven feet in height, there is an oval basin, measuring about fifty-six feet in one direction, and about forty-six in the other; its average depth is about three feet. In the centre of this basin is a round hole, about ten feet in diameter, out of which the water springs. This hole is the mouth of a circular well, between seventy and eighty feet in depth. It is down this well that the jet retires on its disappearance; and it drags along with it all the water out of the basin, leaving both basin and well quite empty, without even a puff of steam coming out of the hole. In this state of emptiness the basin and well remain for several hours. Suddenly the water begins to rise in the well, overflowing till it fills the basin. Loud explosions are heard from below, and the ground trembles. Then, with amazing violence, up springs a vast column of boiling water, surmounted by clouds of steam, which obscure the air. This first jet is followed by several others in rapid succession, to the number of sixteen or eighteen; the last jet being usually the greatest of all, and attaining a height of nearly a hundred feet. In some instances it has risen to a height of a hundred and fifty feet; and one particular jet was measured which rose to the amazing height of two hundred and twelve feet.

The action of the fountain seldom continues more than about five minutes at a time, and then a repose of several hours ensues. If left to itself, the periods of the fountain's activity, though not quite regular, generally recur at intervals of six or seven hours. But they may be hastened by throwing big stones down the well. This not only hurries the eruption of the jet, but increases its energy, and the stones are thrown out with great force by the column of boiling water; the loudness of the explosions being also considerably augmented.

There are several other geysers in the island besides this big one. Their jets are smaller, but to compensate this deficiency, they are more frequent in their ascent; so that travellers who are too impatient to await the eruptions of the Great Geyser, content themselves with visiting the little ones.

Would it not be very convenient to live near a geyser? We might have our victuals cooked by it, and have pipes led from it all round our house, to keep us comfortable in winter; and we might have nice hot baths in our dressing-rooms, arid even a little steam-engine to roast our meat and grind our coffee. But perhaps you may think it might not be altogether pleasant to be kept so continually in hot water.

Were any of the water from the geyser to fall on your hands, you would doubtless feel it rather sore; still more so, were you to be so rash as to thrust your hand fairly into the jet of boiling water, as it ascends into the air. Nevertheless, strange as it may seem, it would be possible for you, without feeling any pain or sustaining any injury, to thrust your hand right into the glowing lava as it flows from the crater of Hecla. The only precaution needful to be observed, is first to plunge the hand into cold water, and then dry it gently with a soft towel, but so as to leave it still a little moist. This discovery was made by a French philosopher, M. Boutigny, and has been practically proved both by him and M. Houdin, the celebrated conjuror, by thrusting their hands into molten iron, as it flowed from the furnace. The latter describes the sensation as like what one might imagine to be felt on putting the hand into liquid velvet1. The reason why this experiment proves so harmless is that between the skin and the glowing substance there is formed a film of vapour, which acts as a complete protection. It is this elastic cushion of vapour which imparts that feeling of softness described by M. Houdin; for it is with it alone that the hand comes into contact.

Geysers have been recently discovered in California; but the jets do not rise higher than twenty or thirty feet. They are, however, very numerous, there being upwards of a hundred openings within a space of half a mile square. The vapour from the whole group rises to upwards of a hundred and fifty feet into the air. The boiling water issues from conical mounds, with great noise. The whole ground around them is a mere crust, and when it is penetrated the boiling water is seen underneath. The Californian geysers, however, are impregnated, not with silica, like those of Iceland, but with sulphur, of which they form large deposits. The sulphurous vapours from the water corrode the rocks near the fountains; nevertheless trees grow, without injury to their health, at a distance from them of not more than fifty feet.

Besides obsidian, already mentioned as a product of its volcanoes, Iceland is famed for another mineral of great scientific value. It is that fine variety of carbonate of lime named Iceland-spar. Transparent and colourless, like glass, this mineral possesses the property of double refraction—any small object viewed through it in a particular direction appearing double. It is much used for optical purposes—especially for obtaining polarized light.

There is another volcano lying far to the northward of Iceland. It is in the island of Jan Mayen, off the coast of Greenland, and has on its summit a vast crater, 2000 feet in diameter, and 500 in depth.