That there is a very hot region somewhere inside the earth is evident, since from some place or places below the surface there come out the immense streams of lava that, continuing to flow at irregular intervals, have at last built up such great masses of land as the island of Hawaii, the still greater island of Iceland, the even greater lava fields of the western United States, and the great plateau of the Deccan in southern Hindustan.
It certainly must have required a great quantity of lava to build up an island like Hawaii with its area of fully 40,000 square miles, for the highest point on the summit of Mt. Kea reaches 13,805 feet above the level of the sea, and, moreover, stands on the bed of the Pacific Ocean in water fully 12,000 feet deep.
But Iceland is only one of many similar cases. Volcanoes are to be found in practically all parts of the earth, not only in the equatorial regions, where they are especially numerous, but also in the frigid and temperate zones. We must also remember the immense lava streams that are known to have come from the interior during the great fissure eruptions of the geological past. When all these facts are taken into consideration, it would certainly seem that there is only one source sufficiently great to supply this wonderful demand, and that is the entire inside of the earth.
But entirely apart from volcanic phenomena there are other proofs that the entire interior of the earth is in a highly heated condition. The differences of temperature caused by the sun during day and night do not affect the earth much below a depth of three feet, while the differences of temperature between summer and winter do not extend much further below the surface than forty feet. Below these depths, in all parts of the earth, the temperature of the crust rises at a rate, which, although not uniform, yet is not far from an increase of one degree of the Fahrenheit thermometer scale for every fifty or sixty feet of descent.
If the above rate of increase continues uniform the temperature of the crust would be sufficiently hot to boil water at a distance of about 8,000 feet below the surface, while at a depth of about thirty miles the temperature would be sufficiently high to melt all known substances at ordinary conditions of atmospheric pressure; that is, to melt all known substances if they were subjected to such a temperature at the level of the sea.
In considering the above we must not lose sight of the fact that this increase in temperature with descent below the surface of the earth's crust occurs, not only in places where there are volcanoes, but over all parts of the earth, thus seeming to point out that there is something hot below the surface which fills the entire inside of the earth.
It is true the greatest distance to which man has actually gone down through the earth's crust is but a few miles. We do not, therefore, know by actual experience that the interior is anywhere in a fused condition, yet the escape of lava or molten rocks in all latitudes, and in the enormous quantities referred to above, seems to show that the entire inside of the earth is at a temperature sufficiently high to melt all known substances under ordinary conditions.
It may be interesting in this connection to examine some of the proofs of this increase in temperature with descent below the surface. The following figures are given by Dana:
Borings to great depths have been made in various parts of the earth, both for artesian wells as well as for the shafts of mines. After passing the line of invariable temperature, the rate of increase for a total distance of 4,000 feet below the surface is in the neighborhood of from one degree for fifty-five to sixty feet, or an average of fifty-seven and a half feet for each degree of heat. In the case of the deep artesian well bored at Grenelle, Paris, where a temperature of eighty-five degrees Fahrenheit was reached at a distance of 2,000 feet, the rate of increase was somewhat more rapid, being one degree Fahrenheit for every sixty feet.