Thomas Hines, age 9, of Poland, New York, for his question:
What happens to crevices made by earthquakes?
Most earthquakes are not strong enough to make cracks in the ground. But once in a while, a mighty trembler topples down buildings and mountain tops and cracks open crevices in the ground. When the shaking stops, people get busy repairing the damage and usually the new buildings are better than before. The earth also gets busy repairing the changes made in the ground. And usually the new geography also is better than before.
Our earth is always changing and remodeling itself, though most of the changes are very slow. A mighty earthquake may leave a gash in the ground, maybe several feet deep and many miles long. Dirt and loose stones slide down the sides, sometimes even before the quaking stops. Later, more rockslides tumble down to the bottom of the crevice. Rains and melting snows wash down tons of mud. In desert regions, tons of sand blow in and pile up on the bottom. Sometimes :tumbling streams settle in a hollow and create a lake in the bottom of an old earthquake crevice.
In tine, a crevice of this sort is fillet': up with dirt and rocks. It may take thousands of years to heal the gaping gash, but the earth is very patient. However, before the job is done, there may be other earthquakes in the same place. Each one opens the crack deeper and wider. The patient earth cannot fill up the growing crevice fast enough. After perhaps a few million years it may become a vast ditch, several miles wide and hundreds of miles long. A monstrous earthquake crevice of this sort is called a "rift valley."
The largest rift valley in the world runs from north to south in the eastern part of Africa. This is the Great Rift Valley. It took the might of many big earthquakes to make it and the job most likely went on through many thousands of years. But the patient earth has been toiling to improve it all this time. At present, the great crevice is 30 to 50 miles wide and several thousand miles long. In some places, its rocky cliffs step down in steep terraces. Streams dash down to the bottom and fill seven large lakes with water. Landslides have covered the floor with fertile soil. Warm rains feed green forests that thrive in the crevice and along its sides.
The Great Rift Valley of Africa is a scenic wonderland. Other rift valleys are beautiful in other ways. There is one in Europe along the Valley of the Rhine River. Here the loose dirt makes it possible for the dense Black Forest to grow. But not all rift valleys have forest scenery. In California, earthquakes created two deep crevices side by side. In time, the slab of ground between them sank down below sea level. It became the beautiful desert region we call Death Valley. As usual, the remodeled earthquake crevice was lovelier than before.
An earthquake may happen anywhere in the world, leaving a crevice that heals after a few thousand years. But as a rule, they tend to happen again and again in certain regions. These earthquake zones are faults and weak places in the earth's outer crust. Here a mighty quake may happen every 100 years or so. The same crevice may be dug deeper and deeper, creating a rift valley. Finally, after a few million years, the restless zone stops quaking. And the earth has a chance to put the finishing touches to the remodeled scenery.