The Crust

The crust covers the mantle and is the earth's hard outer shell, the surface on which we are living. Compared to the other layers the crust is much thinner. It floats upon the softer, denser mantle. The crust is made up of solid material but this material is not the same everywhere. There is an Oceanic crust and a Continental crust. The first one is about 4-7 miles (6-11 km) thick and mainly consists of heavy rocks, like basalt. The Continental crust is thicker than the Oceanic crust, about 19 miles(30 km) thick. It is mainly made up of light material like granite.

What does the crust consist of?
The crust consists of two parts: the oceanic and the continental crust.

Oceanic crust:
As the name already suggests, this crust is below the oceans. There, the crust is 4-7 miles (6-11 km) thick. The rocks of the oceanic crust are very young compared with the rocks of the continental crust. The rocks of the oceanic crust are not older than 200 million years. The material of which the oceanic crust consists is for the greater part tholeiitic basalt (this is basalt without olivine). Basalt has a dark, fine and gritty volcanic structure. It is formed out of very liquid lava, which cools off quickly. The grains are so small that they are only visible under a microscope. The average density of the oceanic crust is 3g/cm³.

Full size

Earth cutaway
Here, sections of the Earth
have been removed to show
its internal structure.

Image by:
Colin Rose,
Dorling Kindersley

Continental crust:
When you look at the globe, you see that the surface of the earth consists of a lot of water (71%). The other 29% consists of land. You can divide this land into six big pieces, which are called continents. The different continents - arranged in decreasing order of size - are: Eurasia (Europe and Asia together), Africa, North-America, South-America, Antarctica and Australia. In the past the division of the continents was different (see plate tectonic). The earth's crust is the thickest below the continents, with an average of about 20 to 25 miles (30 to 40 km) and with a maximum of 45 miles (70 km). The continental crust is older than the oceanic crust, some rocks are 3.8 billion years old. The continental crust mainly consists of igneous rocks and is divided into two layers. The upper part mainly consists of granite rocks, while the lower part consists of basalt and diorite. Granite is lightly-colored, coarse-grain, magma. Diorite has the same composition, but it's scarcer than granite and is probably formed by impurities in the granite-magma. The average density of the continental crust is 2.7g/cm³.

The crust itself has no influence on the earth, but the constant moving of the crust does. This moving is caused by the influence of the convection current, or to be more precise, this convection current actually causes the earth plates to move and sometimes touch each other. These movements cause earthquakes and at weak parts of the earth's crust volcanoes can erupt. Because of all these ongoing movements in the last millions of years, mountains and valleys have been formed, and that’s why the surface of the earth looks as it is now. The form of the surface of the earth has its daily influence on the way people live and work. An example: the building of houses. When you build a house in the mountains, you build it in an other way than on flat land. In the mountains the bottom is more solid than on flat land.
Volcanoes and earthquakes also have their direct influence on the people who live near places where they occur. It destroys their houses and many times people are killed or wounded.