Earth's Interior

The outermost layer of Earth is the crust, a thin shell of rock that covers the globe. There are two types of crust: (1) the continental crust, which consists mostly of light-colored rock of granitic composition and underlies the continents, and (2) the oceanic crust, which consists mostly of dark-colored rock of basaltic composition and underlies the oceans. The continents have an average elevation of about 2,000 ft (609 m) above sea level, while the average elevation (depth) of the ocean floor is 10,000 ft (3,048 m) below sea level. An important difference between continental and oceanic crust is their difference in density. Continental crust has a lower average density (2.6 g/cm³ ) than does oceanic crust (3.0 g/cm³). This density difference allows the continents to float permanently on the upper mantle, persisting more or less intact for billions of years. Oceanic crust, in contrast, is barely able to float on the mantle (which has a density of about 3.3 g/cm³). As oceanic crust ages, it accumulates a heavy underlayer of cooled mantle rock; the resulting two-layer structure eventually sinks of its own weight into the mantle, where it is melted down and recycled. Because of this recycling process, no oceanic crust older than about 200 million years exists on the surface of the earth. About 16% of the mantle consists of recycled oceanic crust; only about 0.3% consists of recycled continental crust.

Another difference between the oceanic crust and continental crust is their difference in thickness. The oceanic crust is 3–6 mi (5–10 km) thick, while the continental crust averages about 20 mi (35 km) in thickness and can reach 40 mi (70 km) in certain sections, particularly those found under recently elevated mountain ranges such as the Himalayas.

The bottom of the crust (both the oceanic and continental varieties) is determined by a distinct seismic transition zone termed the Mohorovičić discontinuity. The Mohorovičić discontinuity, commonly referred to as "the Moho" or the "M-discontinuity," is the transition or boundary between the bottom of the crust and the solid, uppermost layer of the mantle (the lithospheric mantle). As the thickness of the crust varies, the depth to the Moho varies, from 3–6 mi (5–10 km) under the oceans to 20–40 mi (35–70 km) under the continents.

The Moho was first discovered by the Croatian geophysicist Andrija Mohorovičić (1857–1936) in 1908. On October 8, 1908, Andrija Mohorovičić observed seismic waves from an earthquake in Croatia. He noticed that both the compressional (or primary [P]) waves and the shear (or secondary [S]) waves, at one point in their journey, picked up speed as they traveled farther from the earthquake. This suggested that the waves had been deflected. He noted that this increase in speed seemed to occur at a depth of about 30 mi (50 km). Since seismic waves travel faster through denser material, he reasoned that there must be an abrupt transition at that depth from the material of the crust to denser rocks below. This transition zone was later named for its discoverer. The Moho is a relatively narrow transition zone, estimated to be 0.1–1.9 mi (0.2–3 km) thick. It is defined by the level within the earth where P wave velocity increases abruptly from an average speed of 4.3 mi/sec (6.9 km/sec) to about 5.0 mi/sec (8.1 km/sec).