What is diastrophism? How do mountains and plateaus form, similarity and differences, the effects on the Earth’s surface.
Geologists have discovered many facts which show that the land does not always remain at the same level above the sea. During the past 100,000 years the northern Atlantic coast has been sinking, or the ocean has been rising. We know this because the valleys of the Saint Lawrence and Hudson rivers extend for hundreds of miles into the ocean beneath the water. Deep, narrow valleys, called gorges, are found in the rock of the ocean floor. Through these gorges the rivers once ran. But the rivers could not possibly have dug valleys beneath the ocean. Instead, the rivers would fiil up the ocean near their mouths with sediment. These undersea valleys must then have been made when the land was above the level of the sea. So either the land sank, or the ocean rose. Scientists have reasons for believing that the land sank.
Other parts of the earth’s surface that were once under water are now above the level of the sea. Near Lake Champlain, in New York State, is a beach 500 feet above the lake. It is not a lake beach, either, because the shells found in the sand are from ocean animals. The skeleton of a whale was dug up in an inlet along this old shoreline. These facts show that much of New York State was once lower than it is now and that the ocean flowed over it. From sedimentary rocks we can teli that other large areas were once under water, because that is where sedimentary rocks are formed. Whenever we see limestone, sandstone, or shale, we know that the place where we see them was once under water.
The rising and sinking of the earth’s surface and other changes in the positions of rocks have been given a special name by geologists. They call all these changes diastrophism. Geologists do not know the exact causes of diastrophism, but they are learning more about it. They have found that the rocks in the deepest parts of the ocean are denser than the rocks of the continents. Perhaps the continents were raised by the sinking of the denser rocks around them. You remember that gravity pulls down harder on denser materials. As the denser rocks were pulled down, they could have pushed the less dense rocks up to higher places.
The forces that cause diastrophism seem to act in sever al ways. Sometimes they raise or lower the rocks in the earth’s crust. At other times, they stretch the layers of rock. But more often the layers are pressed in from the sides. To get an idea of what happens, you can push against the ends of a pile of long sheets of paper. The layers of paper will bend upward in some places and downward in others. They will bend and fold over farther and farther until some are shaped like an S. Layers of rock seem to act in the same way.
As the rocks are bent and folded, breaks may occur so that one side of a rock slides past an-other. Such a break in the rock is called a fault. Sometimes one side is pushed over the other side for miles. At other times rock on one side moves up, while the rock on the other side moves down. Or a block between two faults may be pushed up higher than the rocks on either side. Any of these faults may be hundreds of miles long.
Some earthquakes are caused by volcanic action. Others are caused by diastrophism. When a fault is made, the rocks do not seem to move slowly past each other. Instead, they stick until the force becomes very great. Then the rocks slide past each other with a jerk, which starts the earthquake. One of the most disastrous earthquakes in the United States destroyed a large part of San Francisco in 1906. A crack in the earth 300 miles long showed where the rocks had slipped. Geologists had known that there was a fault along the line of this crack. Fences that crossed the fault were broken. Their ends were moved sideways from each other until they were 20 feet apart.
Now you can understand some of the changes in the earth’s surface that form mountains and plateaus. When a thick layer of rock is bent up-ward, a range of folded mountains is formed. Or a fault may form, and the rocks on one side will be lifted up into a huge sloping ridge. At other times the land between two faults is raised and tilted to form block mountains. When these changes lift a large area above its surroundings, the area is known as a plateau.
The mountains, as we see them today, are not formed by diastrophism alone. As soon as any part of the land begins to be lifted, erosion starts to wear it down and change its shape. Through the millions of years that mountains are slowly rising, erosion is steadily gnawing away at their tops and sides. The rocks that are easily destroyed are broken up and carried away. Layers and ridges of harder rocks are left. Some mountains are only the hard parts of old plateaus or the ridges that were left as streams carved out their valleys. The mountains of the western part of our country are rough and jagged, because they are still comparatively young. The rocks in the mountains of the East show all the folding and wrinkling of rugged mountains, but these mountains are much older than those of the West. So they have been worn down until they are rounded and lower.