Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

Ætna is, as we have noticed, a much more powerful volcano than Vesuvius. Its outbreaks are more vigorous, its emanations vastly greater in volume, and the mass of its constructions many times as great as those accumulated in any other European cone. There are, however, a number of volcanoes in the world which in certain features surpass Ætna as much as that crater does Vesuvius. Of these we shall consider but two—Skaptar Jokul, of Iceland, remarkable for the volume of its lava flow, and Krakatoa, an island volcano between Java and Sumatra, which was the seat of the greatest explosion of which we have any record.

The whole of Iceland may be regarded as a volcanic mass composed mainly of lavas and ashes which have been thrown up by a group of volcanoes lying near the northern end of the long igneous axis which extends through the centre of the Atlantic. The island has been the seat of numerous eruptions; in fact, since its settlement by the Northmen in 1070 its sturdy inhabitants have been almost as much distressed by the calamities which have come from the internal heat as they have been by the enduring external cold. They have, indeed, been between frost and fire. The greatest recorded eruption of Iceland occurred in 1783, when the volcano of Skaptar, near the southern border of the island, poured forth, first, a vast discharge of dust and ashes, and afterward in the languid state of eruption inundated a series of valleys with the greatest lava flow of which we have any written record. The dust poured forth into the upper air, being finely divided and in enormous quantity, floated in the air for months, giving a dusky hue to the skies of Europe, which led the common people and many of the learned to fear that the wrath of God was upon them, and that the day of judgment was at hand. Even the poet Cowper, a man of high culture and education, shared in this unreasonable view.

The lava flow in this eruption filled one of the considerable valleys of the island, drying up the river, and inundating the plains on either side. Estimates which have been made as to the volume of this flow appear to indicate that it may have amounted to more than the bulk of the Mont Blanc.

This great eruption, by the direct effect of the calamity, and by the famine due to the ravaging of the fields and the frightening of the fish from the shores which it induced, destroyed nearly one fifth of the Icelandic people. It is, in fact, to be remembered as one of the three or four most calamitous eruptions of which we have any account, and, from the point of view of lava flow, the greatest in history.

Just a hundred years after the great Skaptar eruption, which darkened the skies of Europe, the island of Krakatoa, an isle formed by a small volcano in the straits of Java, was the seat of a vapour explosion which from its intensity is not only unparalleled, but almost unapproached in all accounts of such disturbances. Krakatoa had long been recognised as a volcanic isle; it is doubtful, however, if it had ever been seen in eruption during the three centuries or more since European ships began to sail by it until the month of May of the year above mentioned. Then an outbreak of what may be called ordinary violence took place, which after a few days so far ceased that observers landed and took account of the changes which the convulsion had brought about. For about three months there were no further signs of activity, but on the 29th of August a succession of vast explosions took place, which blew away a great part of the island, forming in its place a submarine crater two or three miles in diameter, creating world-wide disturbances of sea and air. The sounds of the outbreak were heard at a distance of sixteen hundred miles away. The waves of the air attendant on the explosion ran round the earth at least once, as was distinctly indicated by the self-recording barometers; it is possible, indeed, that, crossing each other in their east and west courses, these atmospheric tides twice girdled the sphere. In effect, the air over the crater was heaved up to the height of some tens of thousands of feet, and thence rolled off in great circular waves, such as may be observed in a pan of milk when a sharp blow pushes the bottom upward.

The violent stroke delivered to the waters of the sea created a vast wave, which in the region where it originated rolled upon the shores with a surf wall fifty or more feet high. In a few minutes about thirty thousand people were overwhelmed. The wave rolled on beyond its destructive limits much in the manner of the tide; its influence was felt in a sharp rise and fall of the waters as far as the Pacific coast of North America, and was indicated by the tide gauges in the Atlantic as far north as the coast of Europe.

Owing to the violence of the eruption, Krakatoa poured forth no lava, but the dust and ashes which ascended into the air—or, in other words, the finely divided lava which escaped into the atmosphere—probably amounted in bulk to more than twenty cubic miles. The coarser part of this material, including much pumice, fell upon the seas in the vicinity, where, owing to its lightness, it was free to drift in the marine currents far and wide throughout the oceanic realm. The finer particles, thrown high into the air, perhaps to the height of nearly a hundred thousand feet—certainly to the elevation of more than half this amount—drifted far and wide in the atmosphere, so that for years the air of all regions was clouded by it, the sunrise and sunset having a peculiar red glow, which the dust particles produce by the light which they reflect. In this period, at all times when the day was clear, the sun appeared to be surrounded by a dusky halo. In time the greater part of this dust was drawn down by gravity, some portion of it probably falling on every square foot of the earth. Since the disappearance of the characteristic phenomena which it produced in the atmosphere, European observers have noted the existence of faint clouds lying in the upper part of the air at the height of a hundred miles or more above the surface. These clouds, which were at first distinctly visible in the earliest stage of dawn and in the latest period of the sunset glow, seemed to be in rapid motion to the eastward, and to be mounting higher above the earth. It has been not unreasonably supposed that these shining clouds represent portions of the finest dust from Krakatoa, which has been thrown so far above the earth's attraction that it is separating itself from the sphere. If this view be correct, it seems likely that we may look to great volcanic explosions as a source whence the dustlike particles which people the celestial spaces may have come. They may, in a word, be due to volcanic explosions occurring on this and other celestial spheres.

The question suggested above as to the possibility of volcanic ejections throwing matter from the earth beyond the control of its gravitative energy is one of great scientific interest. Computations (not altogether trustworthy) show that a body leaving the earth's surface under the conditions of a cannon ball fired vertically upward would have to possess a velocity at the start of at least seven miles a second in order to go free into space. It would at first sight seem that we should be able to reckon whether volcanoes can propel earth matter upward with this speed. In fact, however, sufficient data are not obtainable; we only know in a general way that the column of vapour rises to the height of thirty or forty thousand feet, and this in eruptions of no great magnitude. In an accident such as that at Krakatoa, even if an observer were near enough to see clearly what was going on, the chance of his surviving the disturbance would be small. Moreover, the ascending vapours, owing to their expansion of the steam in the column, begin to fly out sideways on its periphery, so that the upper part of the central section in the discharge is not visible from the earth.

It is in the central section of the uprushing mass, if anywhere, that the dust might attain the height necessary to put it beyond the earth's attraction, bringing it fairly into the realm of the solar system, or to the position where its own motion and the attraction of the other spheres would give it an independent orbital movement about the sun, or perhaps about the earth. We can only say that observations on the height of volcanic ejections are extremely desirable; they can probably only be made from a balloon. An ascension thus made beyond the cloud disk which the eruption produces might bring the observer where he could discern enough to determine the matter. Although the movements of the rocky particles could not be observed, the colour which they would give to the heavens might tell the story which we wish to know. There is evidence that large masses of stone hurled up by volcanic eruption have fallen seven miles from the base of the cone. Assuming that the masses went straight upward at the beginning of their ascent, and that they were afterward borne outwardly by the expansion of the column, computations which have a general but no absolute value appear to indicate that the masses attained a height of from thirty to fifty miles, and had an initial velocity which, if doubled, might have carried them into space.

Last of all, we shall note the conditions which attend the eruptions of submarine volcanoes. Such explosions have been observed in but a few instances, and only in those cases where there is reason to believe that the crater at the time of its explosion had attained to within a few hundred feet of the sea level. In these cases the ejections, never as yet observed in the state of lava, but in the condition of dust and pumice, have occasionally formed a low island, which has shortly been washed away by the waves. Knowing as we do that volcanoes abound on the sea floor, the question why we do not oftener see their explosions disturbing the surface of the waters is very interesting, but not as yet clearly explicable. It is possible, however, that a volcanic discharge taking place at the depth of several thousand feet below the surface of the water would not be able to blow the fluid aside so as to open a pipe to the surface, but would expend its energy in a hidden manner near the ocean floor. The vapours would have to expand gradually, as they do in passing up through the rock pipe of a volcano, and in their slow upward passage might be absorbed by the water. The solid materials thrown forth would in this case necessarily fall close about the vent, and create a very steep cone, such, indeed, as we find indicated by the soundings off certain volcanic islands which appear only recently to have overtopped the level of the waters.