136 THE EARTH AS MAN'S ABODE Inasmuch as warm water is less dense than cold water, the ocean depths are nearly ice-cold (4° C.) even in the tropics. The Earth Is Round. The earth is a typical planet, and other planets are seen to be spherical by observations with a telescope. The earth's shadow on the moon is circular during an eclipse. Aviators and ships have circumnavigated the earth. Photographs of the earth taken from great heights reveal a curvature in all directions. The circumference of the earth as measured checks with the circumference calculated from the curvature obtained by measuring-instruments. The circumference of the earth is about 24,800 miles. It is not strictly spherical but is slightly bulged (to the extent of 27 miles in diameter) at the equator. You will recall that Newton suggested that this bulging is due to centrifugal force produced by the rotation of the earth. The mass of the earth is about 6 X 10^^ tons. The surface area is 197 X 10^ square miles. The Lithosphere Is Thought to Consist of a Dense Core of Metal Surrounded by a Thick Layer of Rock and a Relatively Thin Outside Crust. The average density of the earth is 5.5 times that of water, although the average density of its crust, which is estimated to be about 60 miles thick, is only 2.7 times that of water. It thus appears that the center of the earth is composed of denser material than the crust. Earthquake waves showing abrupt changes in speed at a depth of about 1800 miles suggest that the center of the earth, called the centrosphere, is a sphere of iron having a diameter of about 4400 miles. There is another abrupt change in the speed of earthquake waves at a depth of about 300 miles. One hypothesis is that the earth's crust consists of a layer of rock about 300 miles in depth and that there is a layer of sulfides of nickel and iron about 1500 miles in depth between the rocky crust and the metallic core. Tidal stresses of the sun and moon deform the earth as they would deform a sphere of steel rather than one of rock. Examination of thousands of meteorites, as previously mentioned, shows that they consist of rocks, nickel, and iron, the iron and nickel being found in much larger quantities relative to their masses than are found in the earth's crust. According to the planetesimal theory, the earth and meteorites had a common origin. Where, then, is the
THE NATURE OF THE EARTH 137 earth's allotment of iron and nickel? The present theory, as mentioned above, is that the iron and nickel form the core of the earth and that this core is a rigid solid, hot, perhaps, but not hot enough to be molten. You undoubtedly are thinking to yourself, " If the earth is solid inside then where does the molten lava of volcanoes come from?" This is a subject for a later section; but, in brief, it is now thought that volcanic activities are the result of local heat-producing disturbances. STUDY QUESTIONS 1. Discuss man's attempts to explore the upper atmosphere. 2. Distinguish between the troposphere and the stratosphere. 3. What would you consider to be the advantages and disadvantages of stratosphere airplane travel? 4. What is the tropopause? 5. What is the probable nature of the centrosphere? Give the evidence for this conclusion. 6. Does lava flow out of cracks in the earth's crust from a molten interior? 7. What are the outstanding facts of importance to the geologist concerning the gaseous, liquid, and solid layers of the earth? 8. What is the composition of the dry atmosphere at sea level? 9. Discuss the reasons for concluding that the atmosphere may once have had a diff'erent composition from that which it has today. 10. Discuss the thickness of the atmosphere, stating how thick it is and giving the basis for this statement. 11. Why is it unlikely that man will ever penetrate the ocean depths?