Читать книгу The Wonder Book of Volcanoes and Earthquakes - Edwin J. Houston - Страница 9

"The surface of the lake is covered over with a black solidified crust showing a rim of fire all around its edge. At numerous points at the edge of the crust jets of fire are seen spouting upwards, throwing up a spray of glowing lava drops, and emitting a dull, simmering sound. The heat for the time being is not intense. Now and then a fountain breaks out in the middle of the lake and boils freely for a few minutes. It then becomes quiet, but only to renew the operation at some other point. Gradually the spurting and fretting at the edges augment. A belch of lava is thrown up here and there to the height of five or six feet, and falls back upon the crust. Presently, near the edge, a cake of the crust cracks off, and one edge of it bending downwards descends beneath the lava, and the whole cake disappears, disclosing a naked surface of liquid fire. Again it coats over and turns black. This operation is repeated edgewise at some other part of the lake. Suddenly a network of cracks shoots through the entire crust. Piece after piece of it turns its edge downwards and sinks with a grand commotion, leaving the whole pool a single expanse of liquid lava. The lake surges feebly for awhile, but soon comes to rest. The heat is now insupportable, and for a time it is necessary to withdraw from the immediate brink."

It is very curious to think of cakes of hardened lava floating on the surface of molten lava, but, of course, this is just as natural as cakes of ice floating on the surface of water; for a cake of hardened lava is, as you will understand, only a cake of frozen lava, and, being lighter than the molten lava, must, of course, float on its surface.

The disappearance of these cakes of frozen lava and their remelting is still more curious, and can be explained as follows: The frozen or solidified mass of black basalt is a trifle lighter than the lava on which it is floating only while its temperature is high, and therefore expanded by heat. As soon as it cools, its density increases, and when it becomes a little greater than that of the liquid lava, it begins to sink and soon disappears.

Fig. 10. Sections of Kilauea at Different Periods

From Dana's Manual of Geology

Professor Dana, who has made a careful study of the phenomena of Kilauea, shows in Fig. 10, a cross section of Kilauea at different times. Before the eruption of 1823, the depth of the crater was from 800 to 1,000 feet. At the eruption the bottom 600 to 800 feet, making the depth of Kilauea over this deeper central part about 1,500 feet. The varying depths at different dates are clearly marked on the drawing.

The eruptions of Kilauea generally occur as follows:

First there is a slow rising of the lava in the crater. This rising continues until the pressure is so great that the mountain is ruptured at some lower place. Next a discharge of the lava and a sinking to a level in the conduit that will depend on the position of the crevice. Then a gradual falling in of the hardened floor of the lake, a portion of the horizontal walls remaining on the sides of the caldera.

The eruption of Kilauea, however, has not always been of the quiet type. There was an eruption in the year 1789 that would appear to have been of the explosive variety. The following account is given by Dana as taken from a history of the Sandwich Islands by the Rev. I. Dibble, published in 1843:

"The army of Keoua, a Hawaiian chief, being pursued by Kamehamoha, were at the time near Kilauea. For two preceding nights there had been eruptions, with ejections of stones and cinders. The army of Keoua set out on their way in three different companies. The company in advance had not proceeded far before the ground began to shake and rock beneath their feet, and it became quite impossible to stand. Soon a dense cloud of darkness was seen to rise out of the crater, and, almost at the same instant, the thunder began to roar in the heavens and the lightning to flash. It continued to ascend and spread around until the whole region was enveloped, and the light of day was entirely excluded. The darkness was the more terrific, being made visible by an awful glare from streams of red and blue light, variously combined through the action of the fires of the pit and the flashes of lightning above. Soon followed an immense volume of sand and cinders, which were thrown to a great height, and came down in a destructive shower for many miles around. A few of the forward company were burned to death by the sand, and all of them experienced a suffocating sensation. The rear company, which was nearest the volcano at the time, suffered little injury, and after the earthquake and shower of sand had passed over, hastened on to greet their comrades ahead on their escape from so imminent a peril. But what was their surprise and consternation to find the centre company a collection of corpses! Some were lying down, and others were sitting upright, clasping with dying grasp their wives and children, and joining noses (the mode of expressing affection) as in the act of taking leave. So much like life they looked that at first they supposed them merely at rest, and it was not until they had come up to them and handled them that they could detect their mistake." Mr. Dibble adds: "A blast of sulphurous gas, a shower of heated embers, or a volume of heated steam would sufficiently account for this sudden death. Some of the narrators who saw the corpses, affirm that though in no place deeply burnt, yet they were thoroughly scorched." As you will see in Chapter XI, this sudden and awful death due to highly heated air and dust particles, caused even a greater loss of life in the catastrophic eruption of Pelée, in Martinique on May 8, 1902.

By reason of its situation at a lower level on the slopes of Mt. Loa, Kilauea was at one time thought to be one of the craters lower down on the slopes of Loa. This was the opinion of Professor Dana when he examined the district in 1840. Since this time the region has been more carefully studied, and Mt. Loa and Kilauea, are now generally regarded as separate and independent volcanoes, neither of which acts as a safety valve for the other.

We shall not attempt in this chapter to say anything concerning the sources or places from where these great supplies of lava have been drawn. This will be left to some subsequent chapter, after we have described still other volcanoes.

The outlines of mountains like Mt. Loa or Kilauea differ greatly from mountains like Vesuvius; their slopes, like the slopes of all other Hawaii volcanoes, have an inclination which does not exceed 10°. The lava streams, therefore, as they flow down the mountains, move more slowly than they would were the slopes more precipitous, as in mountains like Vesuvius.

There have been many eruptions of Kilauea. That which occurred in the year 1840, was of great magnitude (see map, Fig. 6), and began in a fissure southwest of the crater. The principal eruption, however, broke out about twelve miles from the sea coast, and about twenty-five miles east of Kilauea. Here an enormous mass of lava forming a stream nearly three miles wide reached the ocean at Nanawale.

When an eruption takes place on Mt. Loa through a fissure at the height of 10,000 to 13,000 feet the length of the lava streams is frequently as great as twenty-five to thirty miles. Often the lava though hardening at the surface will continue to flow underneath through huge tunnels, of which the top and sides are composed of solidified parts of the same lava stream. After the flow has ceased long hollow tunnels often remain. If the lower end of such a tunnel containing molten lava is momentarily closed, the pressure of the lava above may not only burst through the obstruction, but may even throw the lava upwards in jets 300 to 700 feet high. Probably most of you have seen illumined fountains where jets of water are beautifully lighted up by different colored electric lights placed below them. Such fountains, however, can but poorly compare either in beauty or grandeur with these wonderful lava fountains, common on the slopes of Mt. Loa during an eruption.