CHAPTER XIX

THE EARTH'S HEATED INTERIOR, THE CAUSE OF VOLCANOES

As we have already seen, the nebular hypothesis of LaPlace would seem to make it more than probable that the earth was originally in a highly heated condition, and only reached its present state after long cooling. While this cooling has gone on for probably millions upon millions of years both before and during the geological past, yet in the opinion of perhaps the best geologists the interior of the earth is still very hot, only the outer portions or crust having hardened by loss of heat.

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.

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.

Whenever the earth's crust was fractured in this way the earth was shaken by what are called earthquakes, while a part of the molten interior would run out or escape, making volcanoes. In the very early times neither the earthquakes or the volcanoes were as energetic as they were at later periods when the thickness of the earth's crust increased.

Now, having as we believe correctly come to the conclusion that the entire interior of the earth is in a highly heated condition, the next question that arises is as to the present condition of this interior. A long time ago it was believed that the interior of the earth is still melted, and that a cooled portion or crust surrounds a great molten mass that fills all the inside; that it is this mass which supplies the immense quantities of molten rock or lava that escape through the craters of volcanoes or through the fissures in the crust. Without going into this question thoroughly, since it is a very difficult question to understand, it will be sufficient to say that there are many reasons why it is impossible to believe that the interior is still melted.

You will understand that if the interior of the earth were melted like a huge central sea of fire that each volcano would necessarily affect all the others. Now, as we have seen, this is never the case, so that this is one reason we cannot believe in the existence of a melted interior.

Another reason we cannot believe in a molten interior is an astronomical consideration. It can be shown that under the attraction of the sun and moon the earth could not possibly behave as it does if it were still liquid in the interior. That, on the contrary, the behavior of the earth to the attraction of the sun and moon is such as to make it necessary for us to believe that it is as rigid throughout as would be a globe of steel of the same size.

I can easily understand that you find it very difficult to see how it can be believed that the interior of the earth is solid and yet at the same time be sufficiently hot to melt. I can imagine hearing you ask if it is hot enough in the inside to melt any known materials, why it is not melted. The reason, however, is very simple when you come to think it over. For a solid to fuse or become melted, it is not only necessary for it to be heated to a temperature which is different for different substances, but that at the same time it is heated it shall have plenty of room in which to expand or grow bigger. In other words, the temperature required to fuse any substance increases very rapidly with the pressure to which that substance is exposed.

Now, try to think of the pressure to which the materials that fill the inside of the earth are subjected at great distances below the surface. This pressure is enormous, not only by reason of the weight of the many miles of rocks that are pressing down, but also by reason of the enormous stress or pressure caused by contraction or shrinkage. When we say that the interior of the earth is hot enough to melt all known substances we mean hot enough to melt them if they could be brought from great depths to the level of the sea, but not hot enough to melt them when subjected to the great pressure that exists in regions far below the surface of the earth.

Briefly, the condition of things is believed to be as follows: The entire interior is filled with rock hot enough to melt at the level of the sea, but under too great pressure to melt. If this be granted, as it is by perhaps the greatest number of men who are competent to judge, the phenomena of earthquakes can be readily explained, as can, indeed, the phenomena of those great movements whereby great changes of level take place in different parts of the earth.

Now let us see how volcanoes can be explained on the assumption that the interior of the earth is hot enough to melt, but remains solid only because there is no room for the heated mass to expand in. Such a heated interior as we have imagined, must be constantly losing its heat and, therefore, shrinking. Every now and then this shrinkage must produce great fissures or cracks in the solid crust of the earth. Now should such cracks or fissures extend downwards to the heated interior, there must result a decrease in the pressure. The rocks would, therefore, begin to expand and would be forced by the great pressure to rise slowly in such cracks or fissures. The further they rise the greater the relief of pressure, until they at last assume a molten condition in which they are forced out through the craters of volcanoes as molten rocks or lava.

But it is not only volcanoes that seem to indicate a highly heated plastic condition as existing in the earth's interior. As geologists well know, there are to be found in the various strata of the earth places where great fissures have been made at various times during the geological past. These fissures vary in width from a few inches to many hundreds of feet, and are frequently scores of miles in length. Lava either flows out of them, and covers adjoining sections of the country, or simply rises in them and, afterwards cooling, forms dikes. In many instances, however, the lava is forced in between more or less horizontal layers and in some cases has caused these layers to assume the shape of what geologists know as _subtruderant mountains_. Some of the eastern ranges of the Rocky Mountains have been formed in this manner.

We can, therefore, picture to ourselves the following as the manner of formation of an ordinary volcano. A fissure is first formed in the solid crust of the earth, extending downwards to the regions of great heat. There is thus produced a relief of pressure, so that at this point the highly heated rocks begin to be slowly forced up through the fissure. As they rise higher and higher they become less solid and finally expand into fused masses that can flow out of the crater or opening in the earth's surface. In this way a volcano is started.

But for this volcano to continue in eruption, it is necessary that the conditions shall continue that force the molten rock upwards from great depths. It is not enough for the lava to fill the crevice that exists upwards to the surface, it must continue to be forced upwards until it escapes. If it is permitted to remain in the fissure for any time, it hardens, and only great dikes are formed. It would seem, therefore, that some other force must be called into action to keep the fissure open or, in other words, to prevent the chilling of the lava. Now, this force is generally believed to be the expansive force of steam or the vapor of water.

As Dana points out, by far the greater part of the vapor which escapes from the craters of volcanoes consists of steam or the vapor of water. Indeed, it can be shown that for every hundred parts of different vapors, at least ninety-nine of such parts consist of water vapor. It is for the greater part, to the pressure of steam or water vapor that the escape of lava from the tube near the top of the crater is due.

Of course, the question arises as to where the water comes from that produces this steam. There are three possible sources. From the rains; from leakage at the bed of the ocean; and from vapors existing at great depths below the surface.

It is not probable that either rain water, or water from the ocean, penetrates through the earth's crust for distances much greater than a few thousand feet. It is, however, very well known that in all parts of the earth, except in desert regions, whether they are near or far from the ocean, the rocks are always found fully charged with water. When, therefore, the slowly rising lava passes through the moist rocks that everywhere form the crust of the earth, there must be formed in them great quantities of steam under very high pressure. Moreover, many substances, especially those forming lava, possess the power of absorbing large quantities of steam and other gases. Therefore, as the molten material reaches the moist rocks in the earth's crust, it becomes highly charged with steam, and as the lava rises towards the surface this steam expands.

Where the lava is in a very fluid condition the steam quietly escapes, as does the steam from the surface of boiling water. But where the lava is viscous, like tar or pitch, great bubbles are formed, which, on their explosion, throw the lava upwards for great distances into the air.

We can, therefore, account in this manner for both the non-explosive as well as the explosive type of volcanoes.

It must not be supposed, however, that it is the explosive power of steam which is the principal cause of the lava rising upwards from great depths. This is caused by the great pressure or stress set up by the contraction of a cooling crust. The pressure of this steam is added to this pressure which keeps the lava flowing upwards from great depths below.

The objection has sometimes been urged that it is impossible to believe the lava comes from a highly heated interior, because, as is well known, lavas are of different types even when coming from the same volcano at different times of eruption. While such an objection would have weight were it believed that the interior of the earth is still in a molten condition, it loses its weight when one believes that the interior is solid. It must, however, be acknowledged that the largest part of the interior of the earth would probably have the same chemical composition if it had ever been in a completely melted condition throughout.

I do not doubt you have already concluded that the reason the earth's volcanoes are practically limited to the borders of continents, or to the shores of islands, is the leakage of the ocean waters into the crust at these parts. This was at one time believed by most geologists. That sea water has much to do with such volcanoes as Vesuvius there is no doubt, but it is now generally recognized that it is not so much the present outlines of the earth, or the present arrangement of its land and water areas, that determines the distribution of the world's volcanoes. It is rather believed that the location of the lines of fractures along which the earth's volcanoes are found were determined by conditions that occurred long before the earth assumed its present outlines.

But there is another explanation that has been suggested as regards the condition of the interior of the earth. Judd refers to this explanation as follows:

"Some physicists have asserted that a globe of liquid matter radiating its heat into space, would tend to solidify both at the surface and the centre at the same time. The consequence of this action would be the production of a sphere with a solid external shell and a solid central nucleus, but with an interposed layer in a fluid or semi-fluid condition. It has been pointed out that if we suppose the solidification to have gone so far as to have caused the partial union of the interior nucleus and the external shell, we may conceive a condition of things in which the stability and rigidity is sufficient to satisfy both geologists and astronomers, but that in still unsolidified pockets or reservoirs, filled with liquefied rock, between the nucleus and the shell, we should have a competent cause for the production of the volcanic phenomena of the globe. In this hypothesis, however, it is assumed that the cooling at the centre and the surface of the globe would go on at such rate that the reservoirs of liquid material would be left at a moderate depth from the surface, so that easy communication could be opened between them and volcanic vents."

I must caution you, however, not to think that the above theory of volcanoes is accepted by all scientific men. On the contrary, there are many who believe that the earth is solid throughout because it has completely lost its original heat; that it is only comparatively small areas that are to be found filled with molten or at least highly heated material. But these opinions are held largely by those who have given their attention almost entirely to the phenomena of earthquakes, or who base their reasonings on mathematical grounds only and have not sufficiently considered the phenomena of volcanoes. Since, however, they can be better understood after we have explained the phenomena of earthquakes, we will defer their discussion to the last chapters of this book.