The Elements of Geology; Adapted to the Use of Schools and Colleges

by Justin R. Loomis

The force by which mountains are elevated being the elasticity of the vapor diffused through the subjacent lava, it may happen, if the lava have a high degree of fluidity, that this vapor will collect in large masses, and rise as far as the lava is in a fluid state. The irregular flow of lava from craters during an eruption is undoubtedly due to the rapid ascent of such steam bubbles through the lava. Such an accumulation of vapor under a mountain mass, if it cannot escape, would support it as long as the temperature remained unchanged. But, upon a reduction of temperature, the mass which had been upheaved by it would be unsupported, and liable at any time to sink. Instances of subsidence on a comparatively small scale will admit of explanation in this way. Papandayang, one of the loftiest volcanic mountains of Java, sunk down four thousand feet in the year 1772. The area engulfed was sixteen miles long and six broad. The crater of Kilauea, in one of the Sandwich Islands, was evidently formed in this way. It is situated on the side of a mountain, and consists of a chasm eight miles in circumference and a thousand feet in depth. Liquid lava can always be seen boiling in the small craters at the bottom; and at times it rises so as to overflow them, and fill the chasm to within four hundred feet of the top, when lateral subterranean passages are opened, by which it is discharged. The same explanation—a depression of the central portion—may be given of the formation of the large craters in the Canary and Grecian islands. It is also probable that Lake Avernus and others, in Italy, and some in Germany, have had a similar origin.

The subsidence of Papandayang is of importance as a historical fact; and it is not at all unreasonable to suppose that larger chasms of great depth were also sudden subsidences of a similar character. Lake Superior has a depth considerably greater than the elevation of its surface above the level of the sea. The bottom of the Dead Sea is two thousand six hundred feet below the surface of the Mediterranean. And at one place in the Atlantic Ocean a sounding was attempted with more than six miles of line, without reaching bottom. These sunken areas, however, though of great extent, occupy only an insignificant portion of the entire surface of the earth.

6. The Elevation of Continents.—The causes of change of level which have been given will not explain those slow vertical movements which are now taking place in Greenland and the north of Europe, or those by which the present continents have been elevated and the bed of the sea depressed. Any cause which will account for these movements must be one operating for long periods, under large areas, and with great uniformity.

The cause which fulfils all these conditions most satisfactorily is a variation of temperature in the mass of rock underlying the portion of the surface whose level is changing. It has before been shown that the temperature increases as we descend below the surface; but there is also reason to suppose that it undergoes great variations. The volcanic grits interstratified with the silurian rocks of England show that at the silurian period volcanic fires were active below that portion of the surface. When the early fossiliferous rocks of this country were deposited, the Alleghany Mountains had not been elevated; but before the tertiary period they had taken nearly their present form. Some portion of the intermediate period was therefore one of volcanic upheaval. The trappean rocks are also evidence of intense volcanic action existing here. France, during the tertiary period, was a highly volcanic country; but all volcanic activity has now subsided. The Andes have been mostly elevated since the tertiary period, and are still rising. It is evident, then, that at different periods volcanic heat may vary from its highest to its least degree of activity, below any portion of the earth’s surface.

This variation of temperature must be followed by variation of volume of the earth’s crust; that is, it must produce expansion or contraction. Experiments have been made, under the direction of the United States government, to determine the expansion of the several kinds of rock used in our public works. It was found that granite expands nearly one two hundred thousandth of its length for every degree of increased temperature, limestone somewhat more than that, and sandstone about twice as much. Taking the expansion of the granite as the basis of calculation, and supposing the crust for a hundred miles in thickness to be undergoing change of temperature, there would be a resulting difference of level exceeding two and a half feet for each degree of change in temperature, or more than two thousand five hundred feet for a change of one thousand degrees.

This calculation is made upon the supposition that the law of expansion is the same for all temperatures, and that no new conditions are introduced at high temperatures by the presence of aqueous particles. We know, however, that solids expand more rapidly at high temperatures than at low, and the elasticity of aqueous vapor at high temperatures must increase the rate of expansion of the rock through which it is diffused. Although we are not able to introduce, numerically, the effect of these two circumstances, yet it is obvious that they must be considerable.

The mean elevation of land above the level of the sea is about nine hundred feet, the mountain masses above that level not being included; and the estimated mean depth of the ocean, not including its chasms, does not exceed two thousand six hundred feet. The total elevation of the continental masses, for which it is necessary to account, does not therefore exceed three thousand five hundred feet. This amount of vertical movement may evidently be produced by the expansion and contraction resulting from changes of temperature.

These changes of level must, however, be very gradual. Any diminution of temperature must result from the transfer of heat to the surface; and the conducting power of rocks is very imperfect. The lava in a crater is often so cooled on the surface that it can be walked on, while but a few feet below it is still liquid. Lava currents continue in gradual motion long after the surface is nearly cold. This was the case with one of the currents from Ætna for more than nine months after its eruption, and with another for ten years. Humboldt visited Jorullo forty years after it was thrown up, when the lava around the mountain was still in a heated state, the temperature in the fissures being on the decrease from year to year; but twenty years after its ejection the heat was still sufficient to light a cigar at the depth of a few inches. If so long a period is insufficient to solidify a comparatively small quantity of melted rock when the circumstances for cooling are most favorable, we may well suppose that centuries would be required to abstract sufficient heat from the earth’s crust to produce any material change in the areas of continents.

If this account of the elevation and subsidence of continents is correct, it would seem that they ought to be constantly undergoing change of level. And their apparent stability may be regarded as an objection to it. If in any place there is absolutely no vertical movement, then those conditions must exist in which, for the time being, there is no change of temperature.