Читать книгу The Wonder Book of Volcanoes and Earthquakes - Edwin J. Houston - Страница 5

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Krakatoa is a little island in the Straits of Sunda, about thirty miles west of the island of Java, and nearly the same distance east of the island of Sumatra. It is uninhabited and very small, measuring about five miles in length and less than three miles in width. Its total area is only thirteen square miles. This little piece of land made itself famous by what took place on it during the month of August, 1883.

Krakatoa is one of the many islands that form the large island chain known as the Sunda Islands. The most important islands of this chain are Sumatra, Java, Sumbawa, Flores, and Ceram. Between Sumatra and Java, the largest two of these islands, there is a channel called the Straits of Sunda that connects the waters of the Indian Ocean with those of the Pacific Ocean. The Straits of Sunda is an important piece of water that forms one of the great highways to the East. Shipping is, therefore, always to be found in its waters.

As can be seen by the map, Krakatoa is not far from the Equator, being situated in lat. 6° 7' S. and long. 105° 26' E. from Greenwich. Since there are about sixty-nine miles in every degree of latitude, Krakatoa is about 420 miles south of the Equator, and is about twenty-five miles from Java. Java is part of the Dutch East Indies, which includes Java, Celebes, the Spice Islands, and parts of Borneo and Sumatra. Batavia, the principal seaport of Java, near the northwest coast, is a great shipping centre, visited by vessels from nearly all parts of the world. It has, however, no harbor, but is approached from the ocean by means of a canal two miles in length, the sides of which are provided with massive brick walls. Besides Batavia, which is situated about one hundred English miles east of Krakatoa, there are many smaller towns or villages, the most important of which is Anjer, a thriving seaport town, where sailing vessels obtain their supplies of food and fresh water. Before the eruption of Krakatoa, Anjer was provided with a strong, stone lighthouse.

Java is especially noted for its production of coffee, in which it is second only to Brazil. Its area is about the same as that of the State of New York. Java is one of the most densely populated parts of the world, containing nearly four times as many people as the whole State of New York.

These facts about the situation and surroundings of Krakatoa are necessary to an understanding of the wonderful thing that happened on it during the month of August, 1883. In that month Krakatoa suffered a most tremendous explosive volcanic eruption, for it is a volcano.

Fig. 1. The Sunda Islands

A volcano is a mountain or hill, generally conical in shape, having at the top a nearly central opening, called a crater, from which at times melted rock and lava, vapor and gases escape. The lava either flows down the side of the mountain in a liquid condition, or is thrown upwards into the air. If the distance the lava is thrown upwards is sufficiently great the melted matter solidifies before it falls to the earth. In such cases the largest fragments form what are called volcanic cinders, the smaller pieces, volcanic ashes, and the extremely small particles, volcanic dust. If, however, the lava is thrown to a comparatively small height, it is still melted when it falls, and is then known as volcanic drops or driblets.

Fig. 2. Krakatoa Before the Eruption

It is not surprising that Krakatoa is a volcanic island, since it lies in one of the most active belts of volcanic islands in the world, and near the coasts of the most active of these islands; i.e., Java. This belt, as shown in the map, includes, besides the Sunda Island chain, parts of Gilolo, Celebes, Mindanao and the Philippine Islands. These islands lie between Asia on the northwest and Australia on the southeast.

Fig. 3. Krakatoa After the Eruption

There is no other part of the world with, perhaps, the single exception of Japan, where so many active volcanoes are crowded in so small a space. The island of Java, small as it is, has nearly fifty volcanoes, of which at least twenty-eight are active. They are situated in a lofty range running from east to west, some of the peaks of which are more than 10,000 feet above the level of the sea. Volcanic eruptions are so frequent that the island is seldom free from them.

As will be seen from the map shown in Fig. 2, Krakatoa consists of three groups of volcanic mountains, the southern group giving the name of Krakatoa to the island. Strictly speaking, this mountain was called Rakata, but as it is now generally known as Krakatoa, it would be unwise to attempt to call it by any other name. The central mountain or group of mountains is known as Danan, and consists largely of part of an old crater. The group of mountains which lies near the northern end of the island was known as Perboawatan. From the centre of this latter group of mountains are several old lava streams consisting of a variety of lava resembling a dark-colored glass, known to mineralogists as obsidian, or volcanic glass.

Although Krakatoa was always a volcano, yet between the years 1680 and 1883, it was in the condition of a sleeping or extinct volcano. There had been a severe explosive eruption in the year 1680, that caused great loss of life and property, but ever since that time all activity had ceased and it seemed that the volcano would never again burst out. In other words, it was generally regarded as a trustworthy, sedate, quiet, inoffensive and perfectly safe volcano, that had become extinct.

The long continued quiet of Krakatoa was broken on the 20th of May, 1883, when the inhabitants of Batavia on the island of Java were terrified by noises like the firing of great guns, that were first heard between ten and eleven o'clock in the morning. These noises were accompanied by the shaking of the ground and buildings. The sleeping volcano of Krakatoa was evidently growing restless, but no great damage was done and soon all was again quiet. The disturbances were merely the forerunner of the terrible eruption soon to follow, and confidence was soon restored. But suddenly, on Sunday, August 26th, 1883, almost without any further warnings, Krakatoa burst into terrible activity and began an explosive eruption that has never been equalled in severity in the memory of man.

That memorable Sunday of August 26th, 1883, came during a season of the year known as the dry monsoon, a name given the season of the periodical winds from the Indian Ocean. Batavia, and the surrounding country, greatly needed rain, for in this part of the world it seldom rains from April to October, although the air is very moist and damp. For this reason the beginning of the wet season is always welcomed. When, therefore, the rumbling sounds of the approaching catastrophe of Krakatoa were heard in Batavia, the people, believing that the noises were due to peals of thunder, rejoiced, for all thought they heralded an earlier setting in of the wet monsoon. But when the rumbling sounds increased and reports were heard like heavy artillery, it was clear that the sounds were the beginning of a volcanic eruption, a phenomenon with which they were only too well acquainted, but, as volcanic eruptions were far from being uncommon in Java, no one was very greatly frightened.

But this time the noises increased to such an extent that the people became alarmed. Throughout the night the appalling sounds continued and were accompanied by shakings of the earth sufficiently strong to shake the houses violently. Sleep was out of the question. Many of the people left their houses and remained all night in the open air, fearing the shocks would bring the houses down over their heads.

The morning instead of heralding the dawn of a beautiful tropical day, with its bright, cheerful sunlight, brought with it skies covered with gray clouds that completely hid the sun. The rumbling sounds, however, had decreased, and the people were beginning to congratulate themselves that the dangers were over, when suddenly, the sky grew darker, and there began a shower of ashes that soon covered the streets and houses of the city. About seven o'clock on the morning of August 27th, a most tremendous crash was heard. The sky rapidly became so dark that it was soon necessary to light the lamps in the houses of Batavia, and some of the neighboring towns in the western part of Java. In addition to this the air was filled with vapor, while every now and then earthquake shocks were again felt. These shocks were accompanied by terrific noises like those produced by the explosion of heavy artillery. The noises rapidly increased in number and intensity until they produced a nearly continuous roar, the nature of which it is almost impossible to describe since it is probable that such sounds had never been heard before by man. It is a curious fact, which, I believe, has never been satisfactorily explained, that in most cases the people in the immediate neighborhood of the volcano, as, for example, those on board vessels in the Straits of Sunda, did not hear the terrific noises at all. Possibly they were too loud and simply gave a single inward impetus to the drum of the ear and then held it in position.

Probably some of my readers may remember that witty description given by Dr. Oliver Wendell Holmes of an alleged effort made by all the people of the world to find out whether or not there is a man in the moon. This wonderful plan was as follows:

Careful calculations were made to ascertain when it would be the same time over all the earth so that all the people of the earth could simultaneously shout at the top of their voices. In this way it was hoped that the man in the moon, if there were such a person, would notice the noise.

The story goes on to tell how when the time approached for the great experiment, and all were ready to shout as loud as they could, that each person reasoning to himself or herself, that amid so great a noise no one could notice whether his or her voice was omitted, determined to remain silent, so as to be able to hear the noise and the better to observe what the man in the moon would do when the sound reached him. The result was that every person on the earth remained silent and simply listened, so that the earth was never so quiet before.

Had Oliver Wendell Holmes, or any other person conceiving the witty idea, lived during the time of the great explosive eruption of Krakatoa, on that memorable August 27th, 1883, he might have taken the opportunity of observing the man in the moon, had he not been frightened by what was occurring, for certainly never before were such tremendous or terrifying sounds produced, for these sounds, as we shall see shortly, were actually heard for distances of more than 3,000 miles from the volcano.

There were two different kinds of waves produced in the air by the tremendous forces at work in the eruption of Krakatoa; namely, atmospheric waves and sound waves.

The atmospheric waves showed their presence in the air by means of changes produced in the atmospheric pressure. Now, while these changes cannot readily be felt by man, yet their presence can be easily shown by the use of instruments called barometers.

There are in different parts of the world, buildings called meteorological observatories, that are provided, among other instruments, with recording barometers. These instruments caught the great atmospheric waves that were produced by the eruption of Krakatoa. In this manner, the astounding fact was learned that the waves starting from the volcano travelled no less than seven times around the world. When we say astounding, it must not be understood that the formation of such waves was at all contrary to the known laws of physics. On the contrary, provided the force of the eruption was sufficiently great, such waves must have been produced in the great ærial ocean. The astonishing, or wonderful thing, was that the force setting up these waves was so great that it caused them to move seven times around the globe.

The atmospheric waves were so powerful that it will be worth our while to describe them in detail. Starting from the volcano of Krakatoa, as a centre, these waves moved outwards in all directions, becoming gradually larger and larger until they reached a point halfway round the globe, or 180° from Krakatoa. The waves did not, however, stop here, but continued moving onward, now growing smaller and smaller until they reached a point in North America, immediately opposite Krakatoa. Such a point on a globe is called an antipodal point.[1] The waves did not stop at this point, but again advanced moving toward Krakatoa, growing larger and larger until they again reached a point halfway around the globe, or 180° from Krakatoa, when they again continued moving but now continually growing smaller and smaller, until they reached Krakatoa. Here they again began moving completely around the globe, and this was continued for as many as seven times. It must not be supposed that the waves ceased on the seventh time around. On the contrary, they, probably, kept on moving for many additional times, but they were then so feeble that even the sensitive recording barometers were unable to detect their presence.

There was another kind of waves in the atmosphere that did not require barometers for their detection. These were the sound waves, and can readily be detected by the human ear.

Now, in the case of the great eruption of Krakatoa, the intensity of the sounds was so great that the sounds could be heard distinctly at distances of several thousand miles from Krakatoa.

The sound waves so closely resembled the explosion of artillery that at Acheen, a port on the northern coast of Sumatra, 1,073 miles from Krakatoa, the authorities, believing that an attack was being made on the port, placed all their troops under arms to repel the invaders. The sounds were also distinctly heard at Bangkok, in Siam, a distance of 1,413 miles from the volcano. They were also heard at the Chagos Islands, a group of islands situated in the Indian Ocean about 2,267 miles from Krakatoa.

Two steamers at Singapore, 522 miles distant, were despatched to find the vessel that was believed to be firing guns as distress signals.

The sounds were distinctly heard in parts of South Australia, 2,100 miles distant, and in Western Australia, at 1,700 miles distance.

But it will be unnecessary to give any further details of the great distances at which these sounds were actually heard. It will suffice to say that they were heard as far off as about 3,000 miles.

It is difficult to picture to one's self such great distances. Assuming the greatest distances to be in the neighborhood of 3,000 miles, it would be as if a sound produced, say, in Boston, New York, or Philadelphia, was so loud that it could be heard in Amsterdam, London, or Paris.

Some idea of the intensity of these sounds can be had from the fact that in Batavia, when, in accordance with usage, a gun was fired from one of the forts at eight o'clock in the morning, two hours before the greatest intensity of the sounds had been reached, the sound of the gun could scarcely be heard above the continuous roar.

While, of course, the principal reason the sound waves were carried so far was the great force causing the eruption, yet these distances were increased by the fact that the explosion occurred in a region almost entirely surrounded by great bodies of water. The waves could, therefore, be readily carried along the surface of the sea. Had there been a high mountain wall, like the Andes of South America, on one side of the volcano they would probably have been shut off in this direction a short distance from where they were produced.