130 THE EARTH AS MAN'S ABODE It is possible to determine experimentally the present rate at which salt is thus being added to the oceans and also to determine the present total amount therein. On the basis of such figures, the earth has been calculated to be at least 100,000,000 years old. Some of the earth's rock formations were laid down as sediment in lakes and oceans. The maximum thickness of all such sedimentary deposits if laid on top of each other would be about 500,000 feet. It has been estimated that the average rate of sedimentation is about one foot per 880 years. Such figures would place the age of the earth at about 440,000,000 years, but they are based on so many variable factors that the figures have little significance except in verifying the conclusion that the earth is very old. Of the nearly forty methods of estimating the earth's age, only one is potentially accurate. This is the method based on the study of radioactivity. Radioactive elements disintegrate to form lighter elements. The present rate of this disintegration is definitely known. By analysis, the amounts of the original elements and their disintegrated products in a given rock can be determined, and from these data, assuming no initial products of disintegration, the age of the rock can be calculated. A given stratum of rock can be recognized by the fossils which it contains and by other methods, and the age of any stratum which contains igneous rocks can be determined by determining the age of these rocks by the radioactive method. Radioactive examinations of the earth's crust indicate that its age lies between 1^ and 3}/2 billions of years. Einstein by other methods, however, calculates that the earth is 10,000,000,000 years old. Radioactive examination of many meteorites, some probably of interstellar origin, has led some scientists to conclude that the whole universe was born out of what came before it at about the same time that our solar system is thought by some scientists to have originated, about 1800 million years or more ago. These various scientific estimates are based on methods which differ in precision, but they all lead to the conclusion that the earth is very old. Lemaitre's Hypothesis Is That the Universe Started as One Giant Atom about 1800 Million Years Ago. Spectroscopic evidence leads to the theory of an expanding universe because distant nebulae seem to be receding from us at terrific speeds. Calculations based on the measured speeds and directions of motion of many nebulae, assuming that they all started moving from one point at a certain time in the remote past, being launched with their present
THE ORIGIN AND AGE OF THE EARTH 131 speeds and in their present directions by a tremendous explosion, yield the conclusion that the hypothetical starting-time was 1840 million years ago. The agreement of this figure with figures concerning the age of the earth obtained by entirely independent methods is very striking. Lemattre explains this agreement by use of the hypothesis that the universe started out as a giant atom, which broke up some 1800 millions of years ago to form supergalaxies, galaxies, stars, and our solar system. STUDY QUESTIONS 1. Why is the Laplacian hypothesis no longer tenable? 2. State the hypothesis of the origin of the earth which is most acceptable today. 3. Using the hypotheses concerning the origin of the earth as examples, show how scientists view hypotheses. 4. List some of the consequences of the unequal heating of the earth due to its rotation. 5. Define gradation, degradation, and aggradation. 6. Discuss the rate at which changes take place on the earth's surface. 7. List several ways in which the age of the earth has been estimated. What generalization can you derive from these estimates? 8. Explain the formation of mountain ranges. 9. Discuss the theory of an expanding universe. What data are there to support this theory?