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HOW THE CRUST OF THE EARTH WAS
T is a matter of common knowledge that there are very great variations of temperature on the surface of the earth. There are regions where snow is unknown except on the highest mountains, and where at midday a rock on which the sun's rays have been beating will scorch your hand. There are regions where snow and ice are eternal, so cold that no green thing will live. Yet whether you are at the freezing poles or under a scorching tropical sun, you will find the same temperature a hundred feet below the surface. It is always 52° F. Above this level the variations of temperature are due to the sun ; below it the sun's influence does not penetrate. Descend farther and the temperature is found to increase, and increase regularly for any depths to which the thermometer has been carried. This regular increase is 1° F. for every 66 feet of descent-or 80° F. for a mile of descent. Thus at a depth of a little over a mile the temperature would be 132° F.—too hot for us to live in ; at a little over two miles 212° F.--the temperature of boiling water. All borings hitherto made, or likely to be made, are quite insignificant in depth compared with the earth's radius. Two miles is zobo of it. Thus we cannot be sure that the rate of increase in temperature is regular for depths much greater than those to which the thermometer has
been carried, but we are justified in believing that the temperature does increase and that the interior of the earth must be intensely hot.
When a volcanic eruption takes place masses of molten rock are driven through vents in the earth's surface. The heat of the interior, then, is such as to reduce solid rock to the liquid state when the vast pressure of the overlying matter is removed. In normal circumstances, however great the heat, this pressure is probably such as to prevent the expansion which would accompany liquefaction.
This interior heat, which is great enough to melt rock, increases, as we have said, toward the centre, so that it must be leaking outward, and either the earth is cooling or the heat is being continually renewed.
Where did this internal heat come from? It has been said that the earth may possibly have had its origin in a gaseous nebula, or more probably in a swarm of cold solid meteorites.
If the earth began as a mass of luminous gas, it should show signs that the oldest climate of which geologists can find evidence was very hot, for the heat would be very great, the crust would transmit it slowly, and cooling would be gradual. But the study of the rocks that. form the earth's crust shows that, with local variations (i.e. some parts of the earth's surface were hotter than they are now, and some were colder), the oldest surviving layers were formed under much the same conditions of climate as exist to-day.
If, however, the earth was formed by the crowding together of a swarm of meteorites, becoming heated by collision and shrinkage, it would never have had so high a temperature as if it had arisen in a gaseous nebula, and the hot crust would have cooled more quickly.
When the meteorites had all come into contact the mass so formed would go on shrinking, and the motion of shrinking would continue to create heat, which would spread through it until the temperature was the same throughout. The heat would be sufficient to melt some of the materials, but toward the centre they would be kept solid by pressure.
Now the composition of meteorites is known, since fragments have often been found on the surface of the earth and analysed. Many of them are lumps of metallic iron with a small percentage of nickel and earthy minerals of certain kinds. Others consist of earthy minerals of other kinds. They often contain gases, the most important of which are carbon dioxide, carbon monoxide, hydrogen, and nitrogen.
When it is desired to separate the metal in an ore from the earthy materials mixed with it the ore is melted, and the heavier metal then sinks to the bottom of the furnace and is covered by a stony crust composed of the earthy impurities. In the same way the heat of the contracting mass of meteorites would melt the materials in the outer layers of the earth and so bring about the separation of the metals from the earthy constituents, while in the deeper layers the stronger metallic materials would in the process of contraction squeeze out the rest, and thus a metallic core would be formed, surrounded by a rocky crust. We cannot prove the existence of this metallic core by access to it, but it is made more probable by the fact that the earth weighs more as a whole than would be the case if the interior was composed of the same materials as the surface rocks. An interior composed chiefly of metals would account for this. There is other evidence leading to the same conclusion, and we may regard it as almost certain that the earth consists of two main parts: a stony crust and a heavy metallic core.
The increase of temperature as we approach the
interior of the earth shows that the earth is losing heat outward, but, as has been said of the sun, there is no good reason to suppose that the earth is cooling. Shrinkage no doubt goes on, though at a continually slower rate, but this is not the only source of heat. Heat may be continually created in the interior by chemical energy in materials that are radio-active.
As the crust formed and cooled, the water which had been present in vast quantities as vapour in the earth's envelope of gases condensed and occupied the hollows in the surface. These became the ocean basins, and the higher regions the continents, and thus the division into land and water took place, roughly five-sevenths being water and the remaining two-sevenths dry land. Round the globe the envelope of gases formed an atmosphere.
There is enough water in the oceans to cover the whole globe, if the surface were made even ; possibly when it first condensed it did so cover it. Had it continued to do so, the history of the earth would obviously have been very different. But the earth is continually shrinking, the interior more quickly than the solid crust, and while it was still young and shrinkage most rapid the crust would begin to accommodate itself to this diminished interior. After an apple has been picked some time it begins to grow dry and lose bulk, and the skin, which has become loose, wrinkles itself in order to fit it. In the same way, while the crust of the earth was still thin it crumpled everywhere to keep pace with the lessening bulk inside. Thus a division of the surface into land and water would take place, but the arrangement would clearly be very different from that which exists to-day. Later on when the crust was thicker the crumpling took place only in the weaker areas, and as shrinkage proceeded great portions of it sank and formed shallow depressions. Thus the earth to-day is a globe slightly flattened on four faces, and these hollows are occupied by the four great