So much of the Earth is hidden from view. You see some of the rocky crust, but that’s only 1 percent of the Earth’s mass. Beneath the crust is the dense, semisolid mantle, which accounts for 84 percent. The rest of the planet’s mass is the core, with a solid center and a liquid outer layer. The crust and the very top of the mantle make up the lithosphere. This solid portion of the Earth has been identified because it continually moves in slow motion.
The lithosphere is the brittle solid-rock section of the planet, averaging about 100 kilometers (62 miles) deep. It is thinner under the oceans and thicker in mountain areas. The oceanic lithosphere is denser than that of the continents. The rock of the lithosphere is divided into many uneven pieces called tectonic plates. Some, like those under the Pacific Ocean and Antarctica, are enormous; they are thousands of kilometers wide. Others extend just a few hundred kilometers. They shift slowly. Extreme heat of the mantle makes the rock more flexible, so it moves more easily. Millions of years ago, this motion caused one giant land mass to separate into continents.
Considering the Crust
People explore and gather information about sections of the crust using equipment that releases sounds and gathers echoes, which are recorded as images. This procedure is much like medical sonograms used for examining fetuses. Very detailed data is collected this way. Pockets of gas, oil or water can be located. The rock composition, age and history of the crust can be determined. These “seismic reflections” can also be used to find contaminated water underground and help plan for its removal.
Kinds of Crust
The crust is the thinnest of the Earth’s three layers and the top part of the lithosphere. It is only about 8 kilometers (5 miles) thick under the oceans and 32 kilometers (20 miles) beneath continents. The rocks in the crust are primarily made of oxygen, silicon, aluminum and iron. Much of the ocean crust is dense rock like basalt. Less dense material like granite is found beneath land. Continental crust is far older than its ocean counterpart, which is still being made by underwater volcanoes.
More About the Mantle
The crust is an important part of the lithosphere, but below it lies the other component: the top part of the upper mantle. This is denser than the crust. Like the crust, it contains rocks with large amounts of silicon and oxygen, but the mantle also has significant amounts of iron and magnesium. Although the part of the mantle within the lithosphere is solid rock, the lower mantle is so hot that it can move and flow slowly over long periods of time.