Crust (earth science)

Crust (earth science), outermost layer of Earth. The crust is solid and relatively thin, and it lies below both landmasses and oceans. The dry land of Earth’s surface is called the continental crust. It is about 15 to 75 km (9 to 47 mi) thick. The oceanic crust is thinner than the continental crust. Its average thickness is 5 to 10 km (3 to 6 mi). The crust is very thin in relation to the rest of Earth. If a trip to the center of Earth at 100 km/h (60 mph) were possible, it would take 64 hours, of which only the first 15 to 45 minutes would be in the crust. See Earth: Earth's Surface: Crust.

The crust has a definite boundary. This boundary, called the Mohorovičić discontinuity, or simply the Moho, after the Croatian geologist Andrija Mohorovičić, separates the crust from the underlying mantle. Mohorovičić discovered the boundary in 1909, when he observed that earthquake waves do not pass through Earth’s interior in a straight line but change course at a certain depth below the surface. He believed that the point at which these waves change course marked the boundary between the crust and the mantle. See Earthquake: Studying Earthquakes.

In comparison to the crust, the mantle is much thicker. The mantle extends for about 2,900 km (1,800 mi). It consists of an upper mantle and a lower mantle. The solid, outermost section of the mantle and the solid crust together form the lithosphere. The lithosphere is approximately 65 to 100 km (40 to 60 mi) thick and covers the asthenosphere (see Geology: Guiding Principles of Geology). The asthenosphere is approximately 100 to 350 km (60 to 220 mi) thick. It consists of rocky material that is softer and less rigid than that in the lithosphere. This softer, less rigid state results from higher pressures and temperatures, which cause the rocks partially to melt and become soft.

Oceanic crust and continental crust differ in composition. Oceanic crust consists of dark, dense rocks, such as basalt and gabbro. In contrast, continental crust consists of lighter colored, less dense rocks, such as granite and diorite. Continental crust also includes metamorphic rocks and sedimentary rocks, which the oceanic crust lacks. Metamorphic rocks, which include quartz, marble, and crystal, are formed when high temperatures and pressures at great depths inside Earth transform rock material. Sedimentary rocks form from accumulation and burial of fragments or particles of preexisting rocks. The rocks that make up continental crust possess an average density of 2.8 times the density of water. The rocks that make up oceanic crust are denser, with a density of 2.9 times the density of water.