The Wonder Book of Volcanoes and Earthquakes

by Edwin J. Houston

In a deep well bored in a salt mine at Neusalzwerk, Prussia, a depth of 2,200 feet showed a temperature of ninety-one degrees Fahrenheit at the bottom. This was at the rate of one degree for every fifty feet of descent. At Schladenbach, in Prussia, a well has been dug to the depth of 5,735 feet with a temperature of 134° F. A boring at Wheeling, in West Virginia, reached a depth of 4,500 feet, 3,700 feet below the level of the sea. Here the rate of increase of temperature in the upper half was one degree Fahrenheit for every eighty feet, and in the lower half of one degree for every sixty feet.

It must not be supposed because the rate of increase of temperature is not uniform that the argument of a highly heated interior is weakened. On the contrary, it would be very surprising if the rate continued uniform; for it is evident that the conducting power of different materials in the earth's crust for heat must necessarily make a great difference in the rate at which heat should increase, as we go farther down into the earth. This is so important a matter that I will explain it at somewhat greater length.

Let us suppose that instead of the highly heated interior of the earth, we consider the simple case of a hot stove, the doors or other openings into which are closed so that it is impossible to see the red hot coals inside. Now, suppose holes were bored in the sides of this stove not deep enough to reach the red hot mass within, and that tightly fitting rods or plugs all of the same length and thickness, but of different kinds of materials such as wood, earthenware, glass, iron, copper, silver, and gold, etc., were so placed in the holes as to tightly fit them. Now, under these circumstances the end of all the plugs would be at the same distance from the heated inside. They would not, however, by any means show the same temperatures, the metallic rods would be too hot to touch, while the end of the piece of wood would hardly be hot enough to burn the hand when held against it. The piece of glass and earthenware though less cool would be much less hot than the different rods of metals. Their temperatures would be necessarily affected by their conducting power for heat. The wood, the glass, and the earthenware being poorer conductors than the metals would show much lower temperatures.

Now, the same thing is true with the different materials that constitute the rocks of the earth's crust. Some of these are much better conductors of heat than others, so that the rate of increase of temperature with descent below the surface must necessarily vary with the kind of materials that form the crust of different parts of the earth.

You may, therefore, safely conclude that the entire interior of the earth is in a highly heated condition, and that the source of this heat is to be traced to the heat the earth originally possessed when, in accordance to the nebular hypothesis of LaPlace, it was separated from the sun which gave birth to it, that the present crust of the earth has been formed on the outside by the loss of a portion of this heat.

The rapidity with which a body cools, depends, among other things, on the difference between its temperature and that of the medium in which it is placed. The greater this difference of temperature the greater the rapidity of cooling. Careful measurements made by Tait, the English physicist, show that our earth loses every year from each square foot of surface, an amount of heat that would be able to raise the temperature of one pound of water from the melting point of ice to the boiling point of water, or from 32° F. to 212° F. The rate of loss of heat, must, therefore, have been much greater when the earth was more highly heated than it is now, and will be much smaller than now many years from the present.

Now, let us suppose, what nearly everyone acknowledges to be true, that the earth was originally so hot as to be a molten globe, and that while in this molten condition, it began to revolve or move around the sun. Since the empty space through which the earth moves is very cold, something in the neighborhood of 45° below the zero of the Fahrenheit thermometer scale, the loss of heat would take place very rapidly and a thin crust of hardened materials would be formed on the outside. Now all the time the earth is cooling, it is shrinking or growing smaller.

A very little thought will convince you that this cooling or shrinkage could not go on uninterruptedly; for, while the earth was cooling it was contracting, or growing smaller, and in this way a great pressure, or as it is generally called in science, a great stress was being produced. Every now and then this stress became so great that the crust of the earth was fractured or broken.

At first these fractures would not require a very great amount of stress or force, since the crust of lava was then very thin. After great periods of time, however, the crust grew thicker and thicker, and the amount of force required to break it continually increased, so that the fractures of the crust produced a greater disturbance.