In the history of earthquakes nothing is more remarkable than the extreme fewness of those recorded before the beginning of the Christian era, in comparison with those that have been registered since that time. It is to be borne in mind, however, that before the birth of Christ only a small portion of the globe was inhabited by those likely to make a record of natural events. The vast apparent increase in the number of earthquakes in recent times is owing to a greater knowledge of the earth’s surface and to the spread of civilization over lands once inhabited by savages. The same is to be said of volcanic eruptions, which also have apparently increased greatly since the beginning of the Christian era. There may possibly have been a natural increase in these phenomena, but this is hardly probable, the change being more likely due to the increase in the number of observers.
The structure of a volcano is very different from that of other mountains, really consisting of layers of lava and volcanic ashes, alternating with each other and all sloping away from the center. These elevations, in fact, are formed in a different manner from ordinary mountains. The latter have been uplifted by the influence of pressure in the interior of the earth, but the volcano is an immediate result of the explosive force of which we have spoken, the mountain being gradually built up by the lava and other materials which it has flung up from below. In this way mountains of immense height and remarkable regularity have been formed. Mount Orizabo, near the City of Mexico, for instance, is a remarkably regular cone, undoubtedly formed in this way, and the same may be said of Mount Mayon, on the Island of Luzon.
In many cases the irregularity of the volcano is due to subsequent action of its forces, which may blow the mountain itself to pieces. In the case of Krakatoa, in the East Indies, for instance, the whole mountain was rent into fragments, which were flung as dust miles high into the air. The main point we wish to indicate is that volcanoes are never formed by ordinary elevating forces and that they differ in this way from all other mountains. On the contrary, they have been piled up like rubbish heaps, resembling the small mountains of coal dust near the mouths of anthracite mines.
It is to the burning heat of the earth’s crust and the influence of pressure, and more largely to the influx of water to the molten rocks which lie miles below the surface, that these convulsions of nature are due. Water, on reaching these overheated strata, explodes into volumes of steam, and if there is no free vent to the surface, it is apt to rend the very mountain asunder in its efforts to escape. Such is supposed to have been the case in the eruption of Krakatoa, and was probably the case also in the recent case of Mt. Pelee.
GENERAL DESCRIPTION OF ERUPTIONS
If we should seek to give a general description of volcanic eruptions, it would be in some such words as follows: An eruption is usually preceded by earthquakes which affect the whole surrounding country, and associated with which are underground explosions that seem like the sound of distant artillery. The mountain quivers with internal convulsions, due to the efforts of its confined forces to find an opening. The drying up of wells and disappearance of springs are apt to take place, the water sinking downward through cracks newly made in the rocks. Finally the fierce unchained energy rends an opening through the crater and an eruption begins. It comes usually with a terrible burst that shakes the mountain to its foundation; explosions following rapidly and with increasing violence, while steam issues and mounts upward in a lofty column. The steam and escaping gases in their fierce outbreaks hurl up into the air great quantities of solid rock torn from the sides of the opening. The huge blocks, meeting each other in their rise and fall, are gradually broken and ground into minute fragments, forming dust or so-called ashes, often of extreme fineness, and in such quantities as frequently to blot out the light of the sun. There is another way in which a great deal of volcanic dust is made; the lava is full of steam, which in its expansion tears the molten rock into atoms, often converting it into the finest dust.
The eruption of Mt. Skaptar, in Iceland, in 1783, sent up such volumes of dust that the atmosphere was loaded with it for months, and it was carried to the northern part of Scotland, 600 miles away, in such quantities as to destroy the crops. During the eruption of Tomboro, in the East Indies, in 1815, so great was the quantity of dust thrown up that it caused darkness at midday in Java 300 miles away and covered the ground to a depth of several inches. Floating pumice formed a layer on the ocean surface two and a half feet in thickness, through which vessels had difficulty in forcing their way.
The steam which rises in large volumes into the air may become suddenly condensed with the chill of the upper atmosphere and fall as rain, torrents of which often follow an eruption. The rain, falling through the clouds of volcanic dust, brings it to the earth as liquid mud, which pours in thick streams down the sides of the mountain. The torrents of flowing mud are sometimes on such a great scale that large towns, as in the instance of the great city of Herculaneum, may be completely buried beneath them. Over this city the mud accumulated to the depth of over 70 feet. In addition to these phenomena, molten lava often flows from the lip of the crater, occasionally in vast quantities. In the Icelandic eruption of 1783 the lava streams were so great in quantity as to fill river gorges 600 ft. deep and 200 ft. wide, and to extend over an open plain to a distance of 12 to 15 miles, forming lakes of lava 100 feet deep. The volcanoes of Hawaii often send forth streams of lava which cover an area of over 100 square miles to a great depth.
GREAT OUTFLOWS OF LAVA
In the course of ages lava outflows of this kind have built up in Hawaii a volcanic mountain estimated to contain enough material to cover the whole of the United States with a layer of rock 50 feet deep. These great outflows of lava are not confined to mountains, but take place now and then from openings in the ground, or from long cracks in the surface rocks. Occasionally great eruptions have taken place beneath the ocean’s surface, throwing up material in sufficient quantity to form new islands.