The Crust

Crust composition. P waves travel faster in oceanic crust (7 kilometers/second) than in continental crust (6 kilometers/second)— these speeds are about the same as those through basalt/gabbro and granite/gneiss, respectively. This suggests that oceanic crust is mostly basaltic and that continental crust is mostly sialic, meaning the rocks, such as granite, contain high amounts of aluminum and silica. Oceanic crustal rocks, such as gabbros and basalts, are high in magnesium and silica (sometimes called sima).

Crust thickness and density. The seismic data also show that the thickness of the continental crust ranges from 30 to 50 kilometers (18−30 miles) and that of the oceanic crust from about 5 to 8 kilometers (3−5 miles). Continental crust is thickest under mountain ranges, where it bulges downward into the mantle, forming a mountain root. Geophysical data also show that continental crust would “float” on oceanic crust because continental crust is less dense (continental crust, 2.7 g/cm 3; oceanic crust, 3.0 g/cm 3).

The Mohorovicic discontinuity. The Mohorovicic discontinuity, or Moho, the first major boundary of the earth's interior, separates the crust from the underlying mantle. It is named for Yugoslavian seismologist Andrija Mohorovicic, who in 1909 presented the first evidence for the layered internal structure of the earth. The Moho occurs at a depth that ranges from 5 to 50 kilometers (3−30 miles) from the surface.