Kara Wensel, age 10, of Tucson, Arizona, for her question:
Do earthquake cracks ever close?
Right now, scientists are very interested in the restless movements of the earth's crust. In the last few years, research teams have gathered new evidence that our planet's geography is not nearly as settled as we thought it was. Some o€ the zones being investigated are deep crevices in the ocean floors. Others are cracks and fissurers in the continents. The ones in earthquake zones reveal a lot about the gradual changes of the earth's geography.
A major earthquake may tear a crack in the ground, perhaps many miles long and deeper than ten feet. The top of the crevice may form ,a fairly wide gap and it's natural to wonder whether this wound will heal. It is just possible that future crustal movements may push the two sides together and close the gash. But this is not likely. When the earth alters the landscape, the gashes and piles of debris are left where they are. There may be later changes in the same region, creating more upheavals. But the cracks and bumps gradually become part of the remodeled geography.
Some of the earth's largest cracks are in a region called the Great Rift Valley. Actually, it is a series of valleys extending from southeast Africa northward through the middle East as far as Syria. Parts of the steep sides are a mile high and the floor between them is 20 to 30 miles wide. Sediments have washed down and carpeted the valley floor with rich soil. The Red Sea flowed in to fill .one of the deep wide clefts. Several hollows filled with lake water. The Jordan River and other streams gathered in some of the ditches.
Recent studies in the Great Rift Valley suggest that cracks made by quakes and other crustal movements gradually fill up with soil or water. The studies also reveal that this region is still unsettled. Some time ago, a farmer, his plow and his ox suddenly sank from sight as a new crack opened in the valley floor. Earth scientists suspect that the Great Rift Valley is gradually growing wider and perhaps deeper. In the far future it may sever most of Africa from Asia.
Earthquake zones in other parts of the world are still scarred with cracks and bumps created ages ago. Some of those in California filled with lake water. In one place, a Do earthquake cracks for Saturday, July 24, 1971 crustal slab between two mighty cracks sank down below sea level. After an upheaval of this sort, crustal activity usually shifts to another region. In time, the cracks and other changes are remodeled with lakes and streams and hills to form a new landscape. In California, that crustal slab between the two great cracks became the scenic wonderland of Death Valley.
The earth's deepest cracks are in the sea beds. The ancient ones are partly filled with sediments. Cone samples recently taken from the depths suggest that the floors of the oceans are spreading. This exciting new evidence may explain how the land masses became separated and gradually pushed apart. The cracks in the Great Rift Valley may reveal more of this earth moving story.