24. When The Earth Quakes.

   1. On the night of August 17, 1959, at about 20 minutes before midnight, the ground in the vicinity of Yellowstone National Park began shaking violently. At the time there was a rumbling sound, something like a huge truck would make. Both the heaving of the ground and the noise were very frightening but lasted not quite 45 seconds.
   2. What was even more frightening was the sound of huge boulders which began rolling down the steep mountain. In one part of the upper reaches of the Madison River, a whole mountain began shifting, then came crashing down to fill the deep valley and dam the great river with millions of tons rock and trees.
   3. A dozen or more campers along the river were buried deep beneath the great landslide. Others were able to climb to safety, some of them badly hurt, but were trapped by the slide. Finally these people were saved, many of them by helicopter.
   4. This earthquake near Yellowstone Park was just one of nearly a million that happen every year all over the world. And as bad as this quake was, many have been worse. Earthquake experts say that the Yellowstone quake of 1959 was about as bad as the one which hit San Francisco in 1906.But the San Francisco quake caused more damage because it struck in a place where there were so many people living. In San Francisco 700 person lost their lives. An earthquake in Japan in 1923 took 160,000 lives. In china in 1920 an earthquake took 200,000 lives. It is easy to understand why earthquake are so feared.
   5. What causes these terrible shakes of the very ground on which we live?
   6. To answer that question we must first understand some things about the earth itself. Forty miles deep in the earth is the edge of the outer crust of the earth, and there it is so hot that instead of hard rock there is material much like the hot lava that a volcano erupts. It is the earth's 40 mile deep crust with which we are concerned when we seek the cause of earthquake. The earth's crust is formed of many different layers of rocks. The layers of rocks are not laid evenly, as a bricklayer would build a wall. Instead, the earth's crust is made of rock layers that are often uneven and not perfectly balanced. Because of the great weight pressing down on them, these layers tend to fold downward at weak spots, and this finally causes an actual break in the crust. When this break occurs, or when the sides of an old break slip, the earth quakes, or shakes, while the crust is settling into a new position.