When was the Earth created?—How we can tell the age of the Earth—How the Oceans became salt—The birth of the Moon—The inside of the Earth—Eruption of Krakatoa—What we learn from earthquakes—The thickness of the Earth's crust—The evidence of meteorites

Before considering what we have discovered concerning the age of the Earth, it will be of some assistance to picture its early youth. There is no doubt that at one time it was a great mass of incandescent gas. It was not "burning," because there was no air surrounding it at that time. However, it was natural that this great fiery mass should cool down. As it cooled it became smaller and smaller; it passed into the state of a molten liquid and then a white-hot solid. By this time an envelope of water vapour had formed around the solid globe. The pressure of this envelope of steam would exert an immense pressure on the surface of the hot planet, and there is little doubt this pressure played an important part in distorting the surface of the Earth, depressing it at some places and raising it at others. When things had so far cooled down that the water vapour could condense upon the Earth, there were formed boiling and steaming oceans of water lying in the hollows.

The foregoing short description will enable us to appreciate two of the calculations made concerning the age of the Earth. Knowing that the Earth had cooled from a highly heated body to its present condition, the late Lord Kelvin set himself the task of calculating how long it had taken the planet to cool down. The final result was that at least twenty million years had elapsed since the Earth was a molten globe. Since the discovery of Radium in the Earth's crust it is contended that these twenty million years must be very substantially increased, and that now the Earth is increasing in temperature because of these radioactive substances. However, Lord Kelvin's calculation leaves no doubt that this planet upon which we reside has been in existence for many millions of years.

Professor Joly, of Dublin, has put forward a very interesting theory, but one which some geologists do not accept. He maintains that the sea has become salt very gradually by the rivers washing down sodium compounds from the rocks. He has calculated how long it would take the rivers to bring down the necessary amount of sodium compounds to account for the present saltness of the sea. He has calculated that all the rivers of the Earth could bring down about one hundred and sixty million tons of sodium in each year. He finds that the total amount of sodium contained in the oceans of the Earth is at least ninety million times more than these figures, so that the age of the Earth must be at least ninety million years. Some of the geologists claim one thousand million years as necessary for the formation of the Earth's crust.

Yet another method of calculation has been used in trying to fix the age of our planet. We have no doubt that our faithful satellite the Moon was at one time a part of this great planet. The Earth must have been in a very plastic condition when it threw off that portion of itself, which drifted away and became the Moon. The late Sir George Darwin calculated how long ago the birth of the Moon must have occurred, and the figures come out about fifty-six million years. Again we are among the tens of millions, and all those calculations go back only to the time when this world was a molten mass of, say, five thousand degrees temperature. But when the Earth was a mass of incandescent gas, its temperature was that of the stars, the hottest of which is about thirty thousand degrees. There must have been an enormous lapse of time to allow of this great drop of temperature. But if we consider only the time required to form the crust of the Earth, even then we have to count the age of our planet in many millions of years. This will become apparent when we consider the building up of the Earth's crust in a succeeding chapter.

Some people have held very queer ideas about the inside of the Earth. One of the strangest ideas has been that the Earth was hollow, and that the inside was the habitation of living beings. The idea was not that these inhabitants were fiery demons, such as are represented in a pantomime by some Mephistopheles being shot up from the lower regions. The recorded descriptions tell of a beautifully mild climate within the Earth, which was reached by an opening at the North and at the South poles of the Earth. Needless to say that the Arctic and Antarctic explorers did not search for any such entrances. It is quite apparent that the writers of these absurd notions were not scientists, and yet we have such writings put down in all seriousness within the last hundred years.

If one desires to know what is inside anything, the best plan is to make a direct investigation and examine the contents. In the case of the Earth this is impossible; we cannot sink a shaft to any great depth. When one is going down a deep coal-mine for the first time it seems quite a long journey, but the deepest mine ever made goes down only about one mile into the Earth. To reach down to the centre of the planet would mean a journey four thousand times as great. But it would be quite impossible to continue the journey to any great depth. Even if we were able to overcome the difficulties of the increasing temperature and the increasing pressure of air, it would take more than a lifetime to dig a depth of ten miles. Suppose for a moment that it were possible to continue at the same rate of progress, it would take thirty thousand years to reach the centre. But the whole idea is quite ridiculous, so we may give up all hope of ever discovering the contents of the inner Earth by direct investigation. Our discoveries must be made by the study of phenomena occurring in the Earth. We can only question and cross-examine these phenomena as we should do witnesses.

Our conceptions of the inside of the Earth have changed greatly within quite recent years. Some thirty years ago there occurred a tremendous volcanic eruption in the island of Krakatoa. Although this volcanic island is situated far away in the East, the eruption threw such immense volumes of dust particles into the air that for the succeeding three years our sunsets in Great Britain were more than usually red. A great wave also swept through the oceans of the planet. In a public lecture dealing with this great eruption, and delivered some years after the occurrence, an eminent astronomer described the relation of the solid crust of the Earth to be to the molten interior no more than the solid shell is to the liquid of an egg. Since then we have had to alter this picture very materially. Not only does the solid crust extend to nearly one quarter of the four thousand miles, but we have reason to believe that the contents of the Earth are even more solid at the centre. Indeed, that within the great rock mantle there exists a solid metallic core. We have strong evidence in this direction from one witness in particular: the phenomena of earthquakes.

When an earthquake occurs at some great distance, we are able to take a record of it here, and at any other part of the globe where there is a delicate seismograph. One form of such an instrument is a very delicately poised indicator which will move will every tremor of the Earth. The movements of the indicator are recorded by means of a ribbon of photographic paper. There are now about one hundred different seismological or earthquake stations spread over the planet. On comparing the records obtained at different stations we can see how long it has taken the tremors to travel to the different points. The tremors going along the solid crust of the Earth will take a time proportional to the distance travelled by them. But there are other tremors which go right through the Earth from the place of the eruption to the distant recording station. If the Earth were of homogeneous material throughout, then the rate of travel would be proportional to the distance of this short-cut route through the Earth from point to point. But the earthquake records show that if the short-cut, from place to place through the Earth, pass through a greater depth than eight or nine hundred miles, there is a considerable increase in the velocity of the tremors. Indeed, the tremors pass about twice as fast through the core, and the change of velocity is quite sudden at a depth of eight or nine hundred miles. The evidence of this witness is that the core of the Earth is of different materials from its crust. By supposing the centre to be a great metallic core we can get the different records of earthquakes to agree. Indeed, this seems the only satisfactory way in which we can account for the evidence, although all geologists are not agreed upon the point.

It will be of interest to inquire if there is no other witness we can call to give evidence on the question of the metallic core of the Earth. We have discovered the weight of the planet, as we shall see in the succeeding chapter, and this brings us some further evidence. We know that the weight of the whole Earth is five and a half times that bulk of water. But the rock mantle weighs not more than three and a half times the weight of water, therefore the centre is heavier. If we allow even one thousand miles for the depth of this rock mantle, we find that the centre core must be about eight times as heavy as water. It so happens that iron is about eight times heavier than water, but is there any more reason why we should think of the Earth's core being iron in preference to copper or gold? Yes, we have several witnesses to call. The rocks thrown up from a great depth by active volcanoes have been found to be very rich in iron.

Picture the molten Earth cooling, it would be natural for the heavier material, say iron, to fall to the bottom while the lighter material which forms the rock mantle would float to the surface. We know that in the blast furnace the stones or slag float to the surface of the molten iron, and are conveniently drawn off.

Sir Isaac Newton is said to have guessed the density of the Earth to be between five and six times that of water. It was no blind guess. He argued that when the Earth was cooling from a molten to a solid state that the lighter materials would float to the surface, and he made his calculations accordingly. It is remarkable that these figures arrived at some two hundred and fifty years ago are found to be correct.

There is still another witness to give evidence. When lumps of matter have fallen on to our planet from stellar space, we find most of meteorites to be very rich in iron. Again, we shall see in a later chapter how we have discovered the contents of our far-distant Sun, but mean-time it is of interest to note that iron is one of its constituents.

We have seen that the Moon was thrown off by the molten earth during its process of cooling. If at the time of the birth of the Moon the slag were on the surface of the molten metal, we should expect to find that the Moon is made of the rocky material alone. The weight of the Moon corroborates this, for its total density is about the same as the rock mantle of the Earth. The most recent calculations concerning the Earth give a "crustal zone" of about 30 miles in depth, with a mean density of 2.8. Then a "stony zone" or rock mantle estimated to be from 600 to 900 miles in depth, and having a density of 3.4.

It may be that there still remains a layer of molten material within the Earth, but we can no longer picture the centre of the Earth as a fiery mass; we have strong evidence in support of the rock mantle with a solid iron core.

Before considering what we have discovered concerning the crust of the Earth, it will be of interest to see how we discovered the weight of the Earth itself.