An Etymological Dictionary of Astronomy and Astrophysics
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The part of the → Earth's crust which underlies the → continents. Continental crust is more silica-rich and thicker than → oceanic crust, and is on average older. However, it is highly variable in all of these respects. The average thickness of the continental crust is about 40km, but beneath parts of the Andes and the Himalaya mountain ranges the crust is more than 70 km thick. Continental crust is continuously being eroded and turned into sediment. Some of this sediment ends up on the ocean floor where it can be returned to the → Earth's mantle at → subduction zones. The oldest parts of the continental crust include some rocks that are nearly 4 billion years old. New continental crust is produced by the destruction of oceanic crust at subduction zones, a process that continues today.

→ continental; → crust.

Any more or less hard or stiff outer covering or surface. → Earth's crust.

M.E., from O.Fr. crouste from L. crusta "rind, crust, shell, bark;" cf. Skt. krud- "make hard, thicken;" Av. xruzdra- "hard;" Gk. kryos "icy cold," krystallos "ice, crystal;" Lett. kruwesis "frozen mud;" O.H.G. hrosa "ice, crust;" O.E. hruse "earth;" PIE base *kreus- "to begin to freeze, form a crust."

Pusté, → shell.

The rocky outermost layer of the Earth, ranging from about 10 to 65 km in thickness. It is distinguished from the underlying the → Earth's mantle layer by its more → silicon- and → aluminium-rich composition, lower density, and the lower velocity at which it conducts seismic energy. It includes → continental crust (about 40 km thick) and → oceanic crust (about 7 km thick). The crust and the topmost layer of the mantle form the → lithosphere. The five most abundant → chemical elements in the Earth's crust are, in percentage by weight of the Earth's crust: → oxygen (O) 46%, silicon (Si) 28%, aluminium (Al) 8%, → iron (Fe) 5%, and → calcium (Ca) 4%.

→ Earth; → crust.

That part of the → Earth's crust underling most of the Earth's surface which is covered by the oceans. It has a remarkably uniform composition (mostly ~ 49% SiO₂) and thickness (mostly ~ 7 km). The ocean floor is the most dynamic part of the Earth's surface. As a result, no part of the oceanic crust existing today is more than 200 million years old, which is less than 5% of the age of the Earth itself. New oceanic crust is constantly being generated from the → upper mantle by sea-floor spreading at → mid-ocean ridges, while other parts of the oceanic crust are being recycled back into the mantle at subduction zones.

→ oceanic; → crust.