Read Ebook: The Wonder Book of Volcanoes and Earthquakes by Houston Edwin J Edwin James

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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.

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.

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.

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.

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.

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.

THE VOLCANIC ISLAND OF HAWAII

The volcanic island of Hawaii, the largest of the Sandwich Island chain, is situated in the mid Pacific, south of the Tropic of Cancer. As shown in Fig. 5, this island chain consists of Hawaii, Maui, Molokai, Oahu, Kauai, Nihau, and about eight large islands, together with numerous small islands, extending in a general northwest direction from Hawaii to Nihau, a distance of about 400 miles. Like most volcanic islands they lie in more or less straight lines, probably along fissures, in this case in two nearly parallel lines. The island of Nihau, however, is an exception, the direction of the greatest length being almost straight across the two parallel lines.

The Sandwich Islands lie 2,000 miles from San Francisco in deep water, between 2,000 and 3,000 fathoms, or between 12,000 and 18,000 feet in depth. This island chain consists of great volcanic mountains, that had, at one time, fifteen active volcanoes of the first class. These are now all extinct but three, and all of these are on the island of Hawaii.

In his report to the United States Geological Survey for 1882-83, Dutton states that the summit of Mt. Haleakala on East Maui is 10,350 feet above the sea level. Oahu has peaks on its eastern side 2,900 feet high, and peaks on the western side 3,850 feet high. The summit of Kauai is probably 6,200 feet above the sea.

It can be shown by deep-sea soundings that all these volcanic piles are the summits of a gigantic mountain mass that rises abruptly from the bed of the Pacific. There are reasons for believing that this submarine chain continues for many hundreds of miles in the same direction beyond Kauai.

The extinct volcano, Haleakala, on East Maui appears to have been in eruption at a much later day than Mt. Kea, which is also an extinct volcano. But the natives have no traditions of any eruptions.

The volcanoes on the other islands have been extinct for a very long time judging from the extent of their erosion. Dutton is of the opinion that the western islands of the chain have been extinct for much longer times than the remaining islands.

The Sandwich Islands, also known as the Hawaiian Islands, are one of the colonial possessions of the United States. The island of Hawaii is about 2,000 miles from San Francisco. Honolulu, on the island of Oahu, the principal seaport of the chain, has a pleasant climate, and is an important coaling station for warships, commercial vessels, whalers, and trading ships generally.

The principal product of the island is sugar cane.

The island of Hawaii, as shown in map, Fig. 6, consists of five volcanic mountains and some small coral reefs. These mountains are: Mt. Kea, on the north, 13,805 feet in height; Mt. Haulalai, in the west central part of the island, 8,273 feet in height; Mt. Loa, in the south central part of the island, 13,675 feet in height; Mt. Kilauea, twenty miles east of the crater of Loa, 4,040 feet high at the Volcano House, and 4,158 feet on the highest point on the west, and Kohala, 5,505 feet in height, running through the northwestern part of the island, and the Kohala mountains in the northwestern part.

Of these mountains, Mt. Loa and Kilauea are the only active volcanoes, and are in frequent eruption. Mt. Haulalai was in eruption during 1804. Mt. Kea has not been active during historical times, while Mt. Kohala has been inactive for so long a time that its slopes are deeply gullied wherever the rivers flow down them.

As you can see from the map, Hawaii is very large. It has a length of ninety-three miles from north to south, and a breadth of eighty miles from east to west, its area is about 6,500 square miles. With the exception of small patches of coral reefs, Hawaii is formed entirely of lava, and is the largest pile of lava in the world with the single exception of Iceland.

Where the islands of the Hawaiian chain have coral reefs extending off their coasts, excellent harbors are found in the deep waters between the islands and the reefs. Hawaii, however, has no extended reefs of this character, and, consequently, no first-class harbors. Hilo, on the eastern coast, is the best harbor, and is, therefore, the principal settlement.

A very brief examination of the map of Hawaii will show you that there are no rivers on the island, except on the sides exposed to the wind, that is, on the northern and northeastern slopes. Since the yearly rainfall on Hawaii is large, being in the neighborhood of a hundred inches, you will understand that considerable rain water falls on the island. In those parts of the island where it does not run off the surface it must drain downward through the loose piles of broken rocks or cinders. A rainfall of one hundred inches a year means that if all the rain which falls on each square foot of surface was collected in a flat vessel one foot square with vertical sides it would fill the vessel to the depth of one hundred inches, or over eight feet. The drainage of the rainwater downwards through these parts of the island, must, therefore, be large.

Another curious fact you can notice on the map, is that the lava streams of the past fifty years from Mt. Loa indicated by heavy dotted lines, in no cases begin at the crater, but start at fairly considerable distances from it. Later on in this chapter we shall explain the reason for this curious fact.

Since practically the whole of Hawaii has been formed from the streams of lava that have flowed at one time or another, you can understand how great these flows must have been. But to do this fully you must not only take into consideration the portions of the island that lie above the ocean and reach into the air at its greatest height to 13,805 feet above its surface, you must also remember that this mountain rises from a deep ocean, so that if all the water were removed, you would see Hawaii towering up above the former level of the sea to the height of about 31,000 feet, or higher than Mt. Everest, the highest point on the earth above the present sea level. This would be, approximately, five and eight-tenths miles. You can understand, therefore, how great the flow of lava must have been.

We shall begin the description of Hawaii with the active volcano of Mt. Loa, or, as it is sometimes called in Hawaii, "The White Mountain."

You will remember that the eruption of Krakatoa was of the explosive type. Practically no melted rock or lava escaped from the crater. Indeed, if it had escaped it would not have been seen; for, not only the cone near the crater, but also much of the mountain itself was blown completely out of sight and covered by the waters of the ocean.

The eruptions of Mt. Loa are of an entirely different type. In Loa there are no explosions, the eruptions being what are called the non-explosive or quiet volcanic eruption type. It will be necessary to explain some of the peculiarities of this kind of eruptions.

There is a great difference in the liquidity or the ease with which different kinds of lava flow. Some lava is very thick or viscid, or is sticky like thick molasses or tar, and therefore flows very sluggishly. Other lava is thin or mobile, more closely resembling water in the ease with which it flows. Now, in the case of a volcanic mountain of fairly considerable height, where the lava possesses marked liquidity, the lava as it rises from great depths in the tube of the volcano seldom flows over the top or rim of the crater. This is not because the force that brings the lava up is unable to carry it a few thousand feet higher, so that it can run over the brim of the crater, but because the walls of the volcanic mountains are unable to stand the great pressure which the mass of liquid lava exerts against their sides.

It can be shown that a column of liquid lava 500 feet high, will exert a pressure on the walls of the crater of about 625 pounds to the square inch. Therefore, in very high volcanic mountains, long before the lava can reach the edge of the crater and overflow, the pressure becomes so great, that cracks or fissures are made in the sides of the mountain, through which the lava is quietly discharged; when, of course, the level of the lava in the crater falls considerably. In volcanoes of the explosive type, no matter what may be the condition of lava, should a large quantity of water suddenly find an entrance to a large body of molten lava at some distance below the surface, the lava would be suddenly thrown explosively into the air, where being chilled, it would afterwards descend in showers of ashes, cinders, or volcanic dust.

At the bottom of the large pit or caldera on the summit of Mt. Loa can be seen the level floor formed of hardened lava. This floor is surrounded by vertical walls on which can be seen the broken edges of the old lake bed.

Captain Dutton, in a paper on Hawaiian volcanoes, prepared for the United States Geological Survey, and published in its Fourth Annual Report for 1882-83, thus describes the appearance at the great crater as it was in 1882.

"The summit of Mauna Loa , is a broad and large platform about five miles in length and four miles in width, within which is sunken the great caldera called Mokuaweoweo. The distance from the point where we first reach the summit to the brink of the pit is about a mile and a half. The surface of the platform is much more rugged than the slopes just ascended. It is riven with cracks, and small faults, and piles of shattered rock are seen on every hand. Nowhere is there to be seen the semblance of a cinder cone. Doubtless many eruptions have broken forth from the various fissures on this summit, but only here and there can insignificant traces of such catastrophes be definitely distinguished. The absence of fragmental ejecta is extraordinary. The shattered blocks, slabs, and spalls which everywhere cumber the surface appear to have resulted from the spontaneous shivering and shattering of the lava sheets by their own internal tensions as they cooled.

Fig. 7, taken from Dutton's report, gives the general shape of this great caldera. Dutton's description of the same is as follows:

"The length of the main caldera is a little less than three miles and its width about a mile and three-quarters. Its floor, viewed from above, appears to be composed of a series of flat surfaces occupying two distinct levels, the higher upon the surface of the black ledge, the lower lying within the ledge. Upon the western side is a small cinder cone standing close upon the border of the black ledge. It is the only one visible, either within the caldera or upon the surrounding summit. Its height is about 125 or 130 feet. It was seen in operation, throwing up steam, clots of lava, and lapilli in the year 1878. The only other diversifications of the floor are many cracks which traverse it, the larger of which are distinctly visible from above. Some of them are considerably faulted. There is no difficulty in recognizing the fact that the whole floor has been produced by the sinkage of the lava beds which once continued over the entire extent of the depression, their undersides having been melted off most probably by the fires beneath. The lava beds in the immediate vicinity of the brink upon the summit platform wear the aspect of some antiquity. They have become brown and carious by weathering, and, although no soil is generated, little drifts of gravel are seen here and there mixed with pumice. Since the caldera was formed there is no indication that the lavas have anywhere overflowed its rim. And yet it is a very strange fact that within a half mile, and again within a mile to a mile and a half, lavas have been repeatedly erupted within the last forty years from the summit platform, and have outflowed at points situated from 700 to 900 feet above the level of the lava lake within. Traces may also be seen, at varying distances back of the rim, of very many eruptions in which the rocks betoken great recency, although no dates can be assigned to their occurrence."

During his visit to this great pit, Captain Dutton succeeded in climbing down the almost vertical walls on the side of the crater, and, reaching the surface of the hardened lake, walked over it. It must have required no little courage to thus venture on the thin floor of a lake which he knew was filled to great depths with red hot boiling lava, for he was walking over the surface of a slumbering volcano, that might at almost any moment awaken, and opening, swallow him and his companions. Through enormous cracks in the floor, he could feel the heat from the molten mass, while, through the same openings came suggestive whiffs of sulphur vapor.

During the eruption of this mountain, on January 23d, 1859, the light from the glowing lava streams was bright enough to read fine print at Hilo, a distance of thirty-five miles.

During the eruption of 1852, a stream of white-hot lava was thrown up into the air from one of the fissures to a height of from 300 to 400 feet.

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