166 THK EARTH AS MAN'S ABODE STUDY QUESTIONS 1. What processes offset the results of degradation? 2. What is isostasy? 3. What is an earthquake? 4. Recent data suggest the possibility that some earthquakes occur at depths of five hundred miles, but this depth does not seem likely to be a region of fractures. Two views arise: (1) the earthquakes are caused by fractures, and fractures occur at greater depths than is commonly thought; or else (2) (Suggest another possible alternative.) 5. All the known ice ages occurred either during or soon after continental uplifts and mountain-building. Thus the earth may be said to manufacture its own ages. Volcanic activity was also very great during these mountain-building periods. Is there any evidence in this section that would support this idea that the earth manufactures its own ice ages? 6. Why do earthquakes often occur along certain seacoasts? 7. What is a fault, and how is it produced? 8. Why have the eroding and transporting action of the various degrading factors not brought all land areas below sea level? 9. What processes bring about a shift in weight and pressure on the earth's surface? 10. E.xplain the present gradual elevation in the Great Lakes region. 11. Explain how earthquake centers may be located. 12. Explain the principle of the seismograph. 13. Differentiate between the behavior and nature of the two types of volcanoes. 14. What is the cause of volcanoes? 15. Of what kinds of tremors do earthquakes consist? 16. Is there any reason why earthquake regions should also be volcanic regions? 17. What are the causes of earthquakes? 18. Are there any evidences of present volcanic activity in the United States? 19. What are dikes, sills, and laccoliths? 20. Mention some well-known topographic features that are volcanic in origin. 21. What is diastrophism? 22. Why do earthquakes occur more often in California than in the Mississippi Basin? 23. Give several examples of very destructive explosive \'olcanoes.
UNIT III SECTION 6 THE HISTORY OF THE EARTH IS DIVIDED INTO ERAS, PERIODS, AND EPOCHS Introduction The hills are shadows, and they flow From form to form, and nothing stands; They melt like mists, the solid lands; Like clouds they shape themselves and go. — Source unknown. If we should return to our favorite resort near some mountain stream or lake or seashore in the year 3000, very little change would be noted. We might find that the stream had cut a little deeper channel, that the shores of the lake or ocean had moved a few hundred feet or so, but the general contour of the mountains and the configurations of the coast would be familiar. But if we should return several million years later, nothing would seem to be the same. Where we had known mountains, plains might exist. New mountain ranges might be observed, and the oceans might now be covering large sections which were formerly high dry land. Our dams and skyscrapers might long since have been ground to powder under a glacier a couple of miles thick. Suppose that we were to excavate a portion of a city dump that had been gradually built up over a period of two hundred years. In the lower layers we might find old knives, spinning-wheel parts, horseshoes, and handmade square nails. In other layers, sewing-machine parts, cylindrical phonograph records, tin cans, machine-made wire nails, aluminum utensils, and radio tubes would appear for the first time. A study of the contents of these different strata in the city dump would enable us to piece together a fairly accurate story of the developments which took place during the two-hundred-year period. In this same way archeologists are studying civilizations which existed thou- many of sands of years ago by excavating the ruins of ancient cities, which were found to rest in turn on six or more layers of previous ruins. The geologist obtains a story of the history of the earth by a similar 167