Beverly Ann Lee, age 13, of Champaign, I11., for her question:
IS THE EARTH'S CRUST THICK?
Earth's shape is like a giant ball. The continents and the lands beneath the oceans are part of a rocky skin that surrounds the main body of the earth. This skin is called the earth's "crust." Its thickness varies from about five miles under the oceans to about 20 miles under the continents. So, indeed, you could safely say that the earth's crust is thick.
Temperatures within the deepest parts of the crust may reach 1,600 degrees Fahrenheit. This is not enough to melt rock.
The earth's crust is made up of three kinds of rock: igneous, sedimentary and metamorphic.
Igneous rocks were formed when melted rock deep inside the crust cooled and hardened. Sedimentary rocks developed from materials worn away from the land by water and the weather. Metamorphic rocks were formed deep in the crust when igneous and sedimentary rocks were chemically changed by heat and the weight of the crust pressing against them.
Rocks in the earth's crust are made up mostly of two chemicals: silicon and oxygen. These two chemicals account for more than 74 percent of the crust's weight.
Under the continents, the crust is divided into two rock layers. The top layer is called "sial" and the layer beneath it is called "sima." The sima layer extends under the oceans and forms the ocean basins. There is no sial under the oceans.
The bottom of the earth's crust is called the "moho." The moho marks the boundary between the crust and the inner parts of the earth.
Beneath the crust, the earth is a ball of hot rock and metal. By studying the records of earthquakes, scientists have learned that the inside of the earth is divided into three parts: the mantle, the outer core and the inner core.
The mantle is a very thick layer of solid rock below the crust. It goes down about 1,800 miles. The upper part of the mantle has a temperature of about 1,600 degrees and it increases to about 4,000 degrees Fahrenheit.
Below the mantle is the outer core. It begins about 1,800 miles below the earth's surface. Scientists believe the outer core is about 1,400 miles thick and it is made of melted iron and nickel. The temperature of the outer core ranges from about 4,000 degrees in the uppermost parts to about 9,000 degrees Fahrenheit in the deepest parts.
A ball shaped inner core lies within the outer core and makes up the center of the earth. The boundary between the outer and inner cores is about 3,200 miles below the surface of the earth.
The center of the inner core is about 800 miles below this boundary, or about 4,000 miles below the earth's surface. Scientists believe the inner core consists of solid iron and nickel. The temperature there may be as high as 9,000 degrees Fahrenheit. Land makes up only about 30 percent of the earth's surface. About 70 percent of the surface is water, with almost all of it in the oceans.