The Earth's Crust, Lithosphere and Asthenosphere

This drawing shows the Earth's lithosphere (crust and upper mantle) on top of the asthenosphere. Click on image for full size version (40K GIF)

Crust, the upper layer of the Earth, is not always the same. Crust under the oceans, called oceanic crust, is much thinner than continental crust. It is only about 5 km thick while continental crust can be up to 65 km thick. Also, the two types of crust are not made of the same materials. Oceanic crust is made of a denser collection of minerals than continental crust.

The tectonic plates are made up of the Earth’s crust and the upper part of the mantle layer underneath. Together the crust and upper mantle are called the lithosphere and they extend about 80 km deep. The lithosphere is broken into giant plates that fit around the globe like puzzle pieces. These puzzle pieces move a little bit each year as they slide on top of a somewhat fluid part of the mantle called the asthenosphere. All this moving rock can cause earthquakes.

The asthenosphere is solid even though it is at very hot temperatures of about 1600 C due to the high pressures from above. However, at this temperature, minerals are almost ready to melt and they become ductile and can be pushed and deformed like silly putty in response to the warmth of the Earth. These rocks actually flow, moving in response to the stresses placed upon them by the churning motions of the deep interior of the Earth. The flowing asthenosphere carries the lithosphere of the Earth, including the continents, on its back.