The Web of Life (20) Lithosphere


Earth’s thin outer shell is the lithosphere; a concept developed by American geologist Joseph Barrell. The lithosphere consists of the crust and uppermost mantle. Currently, 1/3rd of the lithosphere is continental, 2/3rds oceanic.

The crust is distinguished from the lithospheric mantle by different mineral composition. This boundary is the Moho discontinuity – named after Croatian seismologist Andrija Mohorovičić. The Moho discontinuity lies 5–10 km below the ocean floor, 20–90 km beneath the surface of a continent, at an average 35 km.

As mineral content differs, oceanic lithosphere is denser than the continental variety. Oceanic crust consists of mafic material: silicate rich in magnesium and iron.

The term mafic is a portmanteau of “magnesium” and “ferric” (referring to iron). Oceanic volcanoes tend to exude mafic magma, which has a lower viscosity and silica content than felsic lava.

Thicker-but-lighter continental crust is felsic: typically granite, but incorporating other silicate materials too, including quartz, muscovite, and feldspar. Felsic is a portmanteau of “feldspar” and “silica.”