Sandra Ketuszenke, age 9, of Norfolk, Massachusetts, for her question:

 Can the earth heal itself after a bad quake?

In 1964, when you were a member of the kindergarten crowd, Alaska was shaken by a major earthquake. The ground and the seabed shuddered for thousands of miles. Gaping cracks were torn in the earth and buildings were buried by tumbling rock slides. These small scars already are healing    but experts say that some of the big changes in the earth are here to stay.

Earthquakes, as we all know, happen in the outside crust of our globe. This rocky skin is from 20 to 40 miles thick. It is the ground on which we walk and solid floor of the ocean beneath the watery waves. Earth scientists are very interested in this rocky crust. By studying the layers of its rocks they can trace back a lot of history. They can trace the patterns of earthquakes that happened ages ago. Each earthquake changes the surface of the ground, often for hundreds of miles. Experts can figure out when a dramatic earthquake happened and trace the changes made to heal its wounds.

In relatively recent history, our western mountains have been shaken by several monstrous earthquakes. One of these cracked the surface rocks for miles and miles. The ground on one side of the crack was pushed downward. On the other side of the crack, the rocky layer was pushed up and left standing in a steep cliff. This earthquake cliff still stands in the Nevada desert. But the long scar will not stay there forever.

It is buffeted by wind and weather. Storms lash its face and tear off chunks of its rock. Blowing sands hurl gritty little bullets that chip away flakes and fragments. A few smaller earth tremors have shaken down rock slides from the top. Gradually the earth is healing this old scar on her face. Sometimes an earthquake tears apart two great slabs of the crustal rock, leaving a plunging ditch in the ground. There are many of these old earthquake rifts in the world. Some of these old scars also are healed by wind and weather. Dust and debris tend to settle in the hollows. Streams flow down and dump layers of soft mud and silty sand. Time and weather, wind and running water gradually fill many old earthquake ditches.

But cliffs and rifts are small wounds. A major earthquake makes changes for thou¬sands of miles on land and under the sea. It may be a part of a huge plan, such as the building of a massive range of mountains. Then an earthquake is soon followed by another and another. Each one lifts a ridge of the earth's crust higher and higher. After each upheaval, the weather starts to fill the cracks and smooth down the ridges. But more trouble strikes before the job is done. After millions of years, the mountains reach their full height. Then the wind and the weather spend more millions of years smoothing out their ups and downs.

The earth is not bothered by scars and wrinkles in her rocky face. But she is always working to change these ups and downs. The job is slow, too slow to change much of the scenery in a thousand years. But this remaking and reshaping of the map never stops. And a lot of the heavy rock shifting work is done by sudden earthquakes.

Some of the old rifts become valleys and some fill with lake water. Some of the cliffs wear down and some rise still higher to become lofty mountain peaks.