THE CONTINENTS AND MAN The Form of the Solid Part of the Earth.—In the diagram of Human Geography on page 3 is followed by "land forms." Hence these are the subject of the next section of this book. The greatest land forms are the great uplands known as continents between which lie the vast hollows filled with water and called oceans.
This arrangement of continents and oceans is apparently due to the fact that the earth is slowly cooling. Geologists say that the earth is steadily losing heat and therefore contracting. Since the crust is stiff it cannot shrink any more than can the• shell of a nut. If we want to make a nut occupy less space, the only way is to break the shell by shoving it inward. During untold millions of years much the same thing has happened to the earth's crust. It has slowly set tled downward by reason of its own weight. The parts that have fallen inward form the hollows that now contain the oceans, while the parts that have not fallen form the lands.
At first glance there seems to be no system in the distribution of the continental uplands and the oceanic hollows which have thus resulted from cooling. But look at a globe and see how the northern continents form an almost complete band near the arctic circle, and enclose the hollow of the Arctic Ocean. From this band three branches extend southward: (1) North and South America; (2) Europe and Africa; and (3) Asia, the Malay Peninsula and Aus tralia. The Atlantic, Pacific, and Indian oceans fill the hollows between the branches while Antarctica rises where the southern con tinents would meet if prolonged southward.
This distribution of the lands as broad ridges between four chief oceanic hollows makes the solid part of the earth slightly tetrahedral in form. A tetrahedron is a four-sided solid (Fig. 20) resembling the tent of Fig. 21. If a hollow elastic tetrahedron were blown up until it formed a sphere except for broad ridges along the six edges, its shape would roughly correspond to that of the solid earth. If the part cor responding to the floor of the tent were at the north, the northern hol low would be filled by the Arctic Ocean, while the surrounding ridges would correspond to the ring of land formed by Asia, Europe, and northern North America, where the main mountains run nearly east and west. The
crests of the three southward running ridges would correspond to the main mountain systems of the Americas, eastern Asia, and Australia which lie close to the Pacific hol low, and of Africa close to the Indian hol low. The third hollow, though occupied by the Atlantic Ocean, has few mountains parallel to its shores. Far to the south Antarctica represents the meeting place of the three continental ridges.
Because of the earth's tetrahedral shape four-fifths of the lands of the northern hemisphere lie between and from the equator in the latitudes where the variable climate is best for civilization. How important this is may be judged from the fact that all the great powers are in this zone. Four-fifths of the area of the southern pro jections, on the other hand, lie in regions too warm and monotonous to promote human progress, and hence are the home of backward and dependent peoples.
Where Mountain-building, Volcanoes and Earthquakes are Most Active.—The tetrahedral form of the earth also seems to determine the location of the greatest mountain ranges. The Sierras and Rockies, the Andes, the Himalayas and Hindu Kush, the Alps, and the Pyrenees, all run more or less parallel to the edges of the earth's rough tetrahedron, with the continents flanking them on one or both sides. These mountains, because they are edges, are lines of bending and breaking along which volcanoes break out and little movements of the earth's crust keep taking place. Such movements give rise to earthquakes, which are most frequent where high mountains rise abruptly from oceans of great depth, as along the west coast of both Americas and along the opposite side of the Pacific Ocean from Kam chatka to New Zealand. In Figs. 22 and 23, notice how abundantly volcanoes and earthquakes are found in three tongues that extend southward on the east sides of Australia and Africa and on the west side of South America, that is, along the tetrahedral edges. Notice how another volcano and earthquake area also corresponds with a tetrahedral edge, for it extends from south-eastern Asia through the Himalayas and Asia Minor to Vesuvius and Aetna on the northern side of the Mediterranean Sea, and thence skips to Iceland with its craters and volcanic hot springs in the midst of snow and ice.