Читать книгу The Wonder Book of Volcanoes and Earthquakes - Edwin J. Houston - Страница 7

"I have lived in Anjer all my life, and little thought the old town would have been destroyed in the way it has. I am getting on in years, and quite expected to have laid my bones in the little cemetery near the shore, but not even that has escaped and some of the bodies have actually been washed out of their graves and carried out to sea. The whole town has been swept away, and I have lost everything except my life. The wonder is that I escaped at all. I can never be too thankful for such a miraculous escape as I had.

"The eruption began on the Sunday afternoon. We did not take much notice at first, until the reports grew very loud. Then we noticed that Krakatoa was completely enveloped in smoke. Afterwards came on the thick darkness, so black and intense that I could not see my hand before my eyes. It was about this time that a message came from Batavia inquiring as to explosive shocks, and the last telegram sent off from us was telling you about the darkness and smoke. Towards night everything became worse. The reports became deafening, the natives cowered down panic-stricken, and a red, fiery glare was visible in the sky above the burning mountain. Although Krakatoa was twenty-five miles away, the concussion and vibration from the constantly repeated shocks were most terrifying. Many of the houses shook so much that we feared every minute would bring them down. There was little sleep for any of us that dreadful night. Before daybreak on Monday, on going out of doors, I found the shower of ashes had commenced, and this gradually increased in force until at length large pieces of pumice stone kept falling around. About six a. m. I was walking along the beach. There was no sign of the sun, as usual, and the sky had a dull, depressing look. Some of the darkness of the previous day had cleared off, but it was not very light even then. Looking out to sea I noticed a dark, black object through the gloom, travelling towards the shore.

"At first sight it seemed like a low range of hills rising out of the water, but I knew there was nothing of the kind in that part of the Sunda Strait. A second glance—and a very hurried one it was—convinced me that it was a lofty ridge of water many feet high, and worse still, that it would soon break upon the coast near the town. There was no time to give any warning, and so I turned and ran for my life. My running days have long gone by, but you may be sure that I did my best. In a few minutes I heard the water with a loud roar break upon the shore. Everything was engulfed. Another glance around showed the houses being swept away and the trees thrown down on every side. Breathless and exhausted I still pressed on. As I heard the rushing waters behind me, I knew that it was a race for life. Struggling on, a few yards more brought me to some rising ground, and here the torrent of water overtook me. I gave up all for lost, as I saw with dismay how high the wave still was. I was soon taken off my feet and borne inland by the force of the resistless mass. I remember nothing more until a violent blow aroused me. Some hard, firm substance seemed within my reach, and clutching it, I found I had gained a place of safety. The waters swept past, and I found myself clinging to a cocoanut palm-tree. Most of the trees near the town were uprooted and thrown down for miles, but this one fortunately had escaped and myself with it.

"The huge wave rolled on, gradually decreasing in height and strength until the mountain slopes at the back of Anjer were reached, and then, its fury spent, the water gradually receded and flowed back into the sea. The sight of those receding waters haunts me still. As I clung to the palm-tree, wet and exhausted, there floated past the dead bodies of many a friend and neighbor. Only a mere handful of the population escaped. Houses and streets were completely destroyed, and scarcely a trace remains of where the once busy, thriving town originally stood. Unless you go yourself to see the ruin you will never believe how completely the place has been swept away. Dead bodies, fallen trees, wrecked houses, an immense muddy morass and great pools of water, are all that is left of the town where my life has been spent. My home and all my belongings of course perished—even the clothes I am wearing are borrowed—but I am thankful enough to have escaped with my life and to be none the worse for all that I have passed through."

As is common in cases of earthquake waves a great depression in the level of the sea occurred at places great distances from Krakatoa. For example, at the harbor of Ceylon, the water receded so far that for about three minutes the boats were left high and dry, and then a huge wave carried them with it as it rushed over the land.

Perhaps one of the best evidences of the immense power of ocean waves is to be seen in the massive blocks of white coral rock that were washed up by the waves, on parts of the coast of Java for distances of from two to three miles from the ocean. Many of these blocks weighed from twenty to thirty tons. Indeed, some of them reached the weight of from forty to fifty tons.

It is probable that the island of Krakatoa and its neighboring smaller islands formed portions of a huge cone about eight miles in diameter, that has been broken up at some very remote but unknown time by, perhaps, a greater catastrophe than that of August, 1883.

In the Straits of Sunda the water was raised fifty feet to eighty feet above the ordinary level, and produced tremendous destruction especially on the coasts of Java and Sumatra, sweeping away many villages and drowning many thousands of people. The wave had a velocity of progression of nearly 400 miles per hour, or eight times faster than an ordinary express train.

When it is said that the velocity of progression of the wave was nearly 400 miles per hour, it is not meant that a body floating on the ocean, such, for example, as a ship, would have been carried forward at this high velocity, but would merely rise and fall in a to-and-fro swing to about the height of the wave; that is, fifty to eighty feet according to what may have been the height. As in the case of the sound waves these motions of water covered or passed over nearly all the waters of the earth. The waves progressing toward the west, crossed the Indian Ocean reaching to the coast of Hindostan, and Madagascar, and sweeping around the southern part of Africa, finally reached the coasts of France and England, as well as the eastern part of North and South America. Sweeping towards the east, they reached the coasts of Australia, New Zealand, and crossing the vast Pacific Ocean were felt at Alaska and the western coasts of North and South America.

But besides the enormous waves caused by the eruption, there were marked changes in the level of the land. Large portions of the coast of Sumatra and Java were almost annihilated, much of the original surface near the coast being submerged, and places that were formerly dry land are now covered with water to a depth of from 600 to 900 feet.

The enormous amount of material thrown into the air by the forces of the eruption is especially characteristic of this phenomenon. Such quantities of pumice stone and ashes fell from the clouds that, sinking in the water and collecting on the bed of the channel, they changed the depth of the water, so as to render navigation dangerous. Indeed, the Sebesi Channel, lying on the north of the island of Krakatoa was completely blocked by a huge bank of volcanic material, portions of which projected above the water, forming two smaller islands. These, however, have since been washed away by the waves.

We will not attempt to give at present any explanations as to the causes of this great volcanic eruption, since the different theories as to the cause of volcanoes will be better understood when other volcanic eruptions have been described. It is sufficient here to say that if a large quantity of water should have suddenly reached a great mass of molten rock, frightful explosive eruptions would have occurred, and if the island was resting on a submerged crater its sudden disappearance may be explained.

Another great wonder connected with the explosive eruption of Krakatoa was the enormous heights to which the fine dust was thrown up into the air. It has been asserted that during the most intense of these eruptions the particles reached elevations of perhaps more than twenty-five miles above the level of the sea. Carried by the winds, the fine particles remained suspended in the air for many months, and gave rise to magnificent sunlight effects, such as early dawn, lengthened twilights, lurid skies, and gorgeous sunsets of a reddish tint. There were also caused curious haloes, as well as green and blue moons.

The fine dust particles consisted of minute crystals of feldspar and other minerals, and when examined under the microscope presented the appearance shown in Fig. 4.

These mineral substances permitted a portion of the light to pass through them, thus producing wonderful optical effects in the atmosphere either because they acted like minute prisms and so produced rainbow colors, or because they turned the rays of light out of their course as to produce what is called interference by color effects of a nature similar to the colors seen in mother-of-pearl, rainbow coal, or in the wing cases of many beetles. The explanations of these phenomena are too difficult for a book of this character.

An explosive volcanic eruption is a very terrifying and wonderful phenomenon. Frightful roaring sounds are suddenly heard, the earth shakes for many miles around, when suddenly a vast quantity of molten rock, and sometimes huge stones, are thrown out of the crater high up into the air. So great is the force that throws these materials out of the opening that heavy masses of rocks often are ejected very much faster than the projectiles from the largest guns that are used in any of the navies of the world.

Fig. 4. Volcanic Dust as it Appears Under the Microscope

As the molten lava cools and falls in the form of prodigious clouds of ashes, cinders and dust, for many miles around the volcano, even the light of the sun is obscured, and one cannot see the hand before the face. Some of the materials in these clouds are so light that they remain suspended in the air for many hours, often indeed for many days, and sometimes even for years. The heavier particles, however, soon begin to fall, and before long the earth's surface both around the volcano, and often at considerable distances from it, is covered with a thick layer of ashes.

The sounds accompanying a volcanic eruption are often terrifying. Amid shakings and tremblings of the earth's crust, known as earthquakes, there are occasionally heard noises like the explosion of huge guns. Sometimes these sounds follow one another so rapidly that they produce an almost continuous roar. Through the roar of the explosion a curious crackling noise can be heard, due to the fragments of stone hurled out of the crater striking against one another, especially as the stones which are thrown out of the crater and have commenced to fall back again to the earth, are struck by others that are still rising.

Immense quantities of ashes, stones, vapor and gases are thrown upwards for great distances into the air, while, at the same time, a lava stream pours over the lowest side of the crater. As the column of ashes and cinders reaches its greatest height in the air, it begins to spread outward on all sides, rapidly growing like a huge dark mushroom. This soon shuts out the light of the sun, and from it showers of red hot ashes and cinders fall to the earth.

It would be extremely dangerous to be on the side of the volcanic mountain during an explosive eruption; for, even should you escape falling into an opening in the side of the mountain, you might be killed by the huge stones that are constantly falling on all sides around the opening, or might be buried under the vast showers of red hot ashes that are poured down from the dense clouds overhanging the mountain, or suffocated by clouds of sulphur vapor that rush down its sides.

When at a safe distance the sight is certainly magnificent. There is no light from the sun. All would be in pitch darkness but for the reddish glare thrown upwards by the red hot lava, by the glowing showers of ashes that are being rained down on the sides of the mountain, or by terrific lightning flashes, due to the discharge of the immense quantities of electricity produced by the forces of the eruption.

Naturally a great volcanic eruption can cause a considerable loss of life and property. When a large lava stream begins to flow down the sides of the mountain, it cannot be stopped, and should it flow toward a village or town it is likely to destroy the town completely. Besides this, the vegetation of the country for many miles around is destroyed by the showers of red hot ashes that fall from the sky. The houses of neighboring cities are similarly ruined by the great conflagrations thus set up. Further destruction is also caused by large streams of mud that rush down the slopes of the mountain, or by huge waves set up in the ocean. If the volcano is situated, as most volcanoes are, near the coast, the showers of ashes and falling stones may set fire to vessels in the neighborhood, or the progress of such vessels may be seriously retarded by layers of ashes or pumice stone that float on the surface. Sometimes these layers are so thick as actually to bring ships to a complete standstill.

It must not be supposed that volcanoes are in a constant state of eruption. On the contrary, nearly all volcanoes, after an eruption, become quiet or inactive. The air soon clears by the ashes settling, and the sunlight again appears. A crust forms over the surface of the lava, which rapidly becomes hard enough to permit one to walk over it safely. The vegetation, which has been destroyed by the hot ashes, again springs up, and, if the volcano happens to be situated within the tropics, where there is an abundance of moisture, the land soon again becomes covered by a luxuriant vegetation. Most of the people, who have escaped sudden death during the eruption, return to the ruins of their houses; for it is a curious fact that no matter how great has been a volcanic eruption, or how far-reaching the ruin, the survivors, as a rule, do not appear to hesitate to return to their old neighborhood. In a few years the fields are re-cultivated, the villages are rebuilt, and the people apparently forget they are living over a slumbering volcano, which may at any time again burst forth in a dangerous eruption.

A volcano that throws out molten rock, vapor and gases is known as an active volcano. An active volcano, however, is only correctly said to be in a state of eruption when the quantity of the molten rock, lava or vapor it throws out is greatly in excess of the ordinary amount.

Sometimes the volcanic activity so greatly decreases that the molten rock or lava no longer rises in the crater, but, on the contrary, begins to sink, so that the top of the lava in the crater is often at a considerable distance below its edges. The lava then begins to harden on the surface, and, if the time is sufficient, the hardened part extends for a considerable distance downward. In this way the opening connecting the crater with the molten lava below becomes gradually closed, the volcano being thus shut up, or corked, just as a bottle is tightly closed by means of a cork driven into the opening at its top so as to prevent the escape of the liquid it contains.

It may sound queer to say that a volcano has its crater so corked up as to prevent the escape of the lava, but the idea is nevertheless correct and helpful. To realize the size of these huge volcanic corks one must remember that the craters of some volcanoes are several miles across. A volcano thus choked or corked up is said to be extinct.

When we speak of an extinct volcano we do not mean that the volcano will never again become active. A volcano does not cease to erupt because there are no more molten materials in the earth to escape, but simply because its cork or crust of hardened lava has been driven in so tightly that the chances of its ever being loosened again seem to be very small. But small as the chances may seem we must not forget that the volcano may at any time become active, or go into its old business of throwing out materials through its crater. A volcano in an extinct condition is not unlike a steam boiler, the safety valve of which has been firmly fixed in place. If the steam continues to be generated in the boiler, it is only a matter of time when the boiler will blow up, and the explosion will be all the greater because the safety valve did not allow the steam to escape earlier.

Sometimes an intermediate class of volcanoes called dormant is introduced between active volcanoes on the one hand and extinct volcanoes on the other. The name dormant volcano, or, as the word means, sleeping volcano, is objectionable, since it might lead one to think that an extinct volcano is not sleeping but dead, and this is wrong.

Since the plug of hardened lava in the volcanic crater is generally at a much lower level than the top of the crater, the crater will soon become filled to a greater or less depth with water, produced either by the rain, or by the melting of the snow that falls on the top of the mountain. Crater lakes, often of very great depths, are common in extinct volcanoes.

Of course, when an extinct volcano again becomes active, two things must happen if the eruption is explosive. In the first place, the force of the explosion must be sufficiently great to loosen the stopper or plug of hardened lava which stops it. In doing this the mass is broken into a number of fragments that are thrown forcibly upwards into the air. After rising often for great heights they soon fall again on the sides of the mountain.

But besides the breaking up of the stopper, the lake in the crater of the volcano is thrown out along with the cinders or ashes, producing very destructive flows of what are called aqueous lava or mud streams. These streams flow down the sides of the mountain, carrying with them immense quantities of both the ashes thrown out during the eruption, or those that have collected around the sides of the crater during previous eruptions. Very frequently, these streams of aqueous lava produce greater destruction than the molten lava.

If you have ever watched common ants at work clearing out or enlarging their underground homes, in a piece of smooth gravel walk in your garden, you can form some idea why the mountain immediately around a volcanic crater is conical in shape. If the colony of ants happens to be fairly large, you can see an almost unbroken stream of these industrious little animals, each bearing in its mandibles a small grain of sand or gravel brought up from some place below the surface.

Carrying it a short distance from the opening, it throws it on the ground, rapidly returning for another load. In this way there is heaped up around all sides of the opening a pile of sand or gravel, the outward slopes of which gives the pile a conical form. You have, probably, noticed that the steepness of the slopes depends on the size of the grains; for the larger these grains the sharper or steeper the slopes, the very fine grains producing flat mounds or cones.

It is the same with a volcanic cone. The materials that are thrown upwards into the air, falling again on the mountain, collect around the crater on all sides, thus giving it the characteristic cone-like shape of the volcanic mountain. Where nothing occurs to disturb the formation of the cone its height above the level of the sea will gradually increase. Very frequently, however, during explosive eruptions, a large part of this cone will be blown away by the force of the eruption only to be again built up during some later eruption. Indeed, in the case of volcanic islands, the force of a great volcanic eruption is sometimes so great that not only is a large volcanic mountain blown entirely away, but a hole is left, where it had been standing, that extends further downwards below the level of the sea than the top of the mountain extended previously above it. The above are some, but by no means all, the wonders attending volcanic eruptions. We shall refer to others in subsequent chapters in describing particular eruptions.