ERUPTION OF MOUNT VESUVIUS.
high temperature exists at no very great distance from the surface is beyond a doubt. The observations made in mines and artesian wells show that the average increase in the temperature is one degree for every fifty-five feet in depth. One noted variation exists in the deep wells at Buda-Pesth, in Austria, where the temperature increased up to 3,000 feet; but beyond that depth it became cooler again. The Comstock lode in Nevada, the richest mineral vein in the world, is nearly at the limit of practicable working, the normal temperature being as high as one hundred and fifty degrees.
Even if it be conceded that the material of the earth is a molten mass, there would be two theories to explain it: one, that the earth was originally in a state of fusion, and was slowly cooling; the other, that the great pressure from without keeps an otherwise solid center greatly compressed and heated, and consequently liquid. Either supposition is based on well established facts; but it does not appear clear that the molten globe with a cool shell can settle the entire question.
The objections to this are several. One, the complete absence of uniformity in the increase of heat as we descend. While the total average is as given above, the variations are so many and vast, that there does not seem to be any general law, as there should be if the molten interior possessed the least uniformity. In some shafts the increase is one degree for twenty feet; in others, one for every one hundred; in some, the temperature increases much more rapidly at great depths, in others, much less rapidly.
A second objection is, the vast difference in the character of lavas, even in districts very near each other. Thirdly, there seems no definite connection between volcanoes in the same region. Two adjacent ones may exhibit very different conditions. Mauna Loa is about 10,000 feet above Kilauea, a great crater of the same mountain. Yet the upper is often in a violent state of eruption when the latter is perfectly quiet. It would be difficult to conceive how these are supplied from the same source. If the interior were a molten mass in a state of equilibrium, as would be necessary if the uniformity of its motions in the solar system were to be preserved, any undue pressure would compel the molten matter to escape from the lowest opening. This would be in accordance with the simplest laws of liquids. Then we should find volcanic action most vigorous at the lowest active volcano; but such is not the case. The idea of a uniformly liquid interior seems hardly tenable.
There is still other objection to this theory. Experiments have been made with various materials to ascertain the change affected in them by heat. It is found that a block of granite five feet long, by a change of ninety-six degrees in temperature, is expanded .27792 of an inch; crystalline marble, .03264; sandstone, .0549. If, then, a portion of the earth’s crust ten miles in thickness be heated six hundred degrees, its crust would be raised two hundred feet; or a change of one degree, the rate of expansion being fairly uniform to five hundred or six hundred degrees, would raise the surface four inches. How important this matter is may be better understood when we consider that if the interior of the earth be a uniformly molten mass, with a crust ten miles thick, a contraction of one-twelve thousandth of an inch should force out of the crust a cubic mile of lava. We should find then a change in temperature one forty-eight thousandth of a degree should effect this, if the crust were ten miles thick.
We are then forced to conclude that the earth is not cooling to any appreciable extent; or that the liquid interior is still capable of indefinite compression without necessarily being forced out through orifices in the crust; or that the interior is not a uniformly molten mass.
Such are the arguments against a melted interior.