Crust (geology)
In
geology, a
crust is the outer layer of a
planet, part of its
lithosphere. Planetary crusts are generally composed of a less
dense material than that of its deeper layers. The crust of the
Earth is composed mainly of
basalt and
granite. It is cooler and more rigid than the deeper layers of the
mantle and
core.
On stratified planets, such as Earth, the lithosphere is
floating on
fluid interior layers. Because of
convection in the
plastic, although non-molten, upper
mantle and
asthenosphere the lithosphere is broken into
tectonic plates that move. Oceanic crust is different from that of the continents. The
oceanic crust (
sima) is 5 to 10 km thick and is composed primarily of a dark, dense rock called basalt. The
continental crust (
sial) is 20-70 km deep and is composed of a variety of less dense rocks.
The Earth is considered to have divided from a
planetesimal into its
core,
mantle and crust within ~100 million years of the formation of the planet, at 4.4 billion years ago. The primordial crust was very thin, and was likely recycled by much more vigorous
plate tectonics and destroyed by significant
asteroid impacts, which were much more common in the early stages of the solar system. Of particular note is a theory that the
moon was formed by one such very large impact.
The Earth has likely always had some form of basaltic oceanic crust, but there is evidence that it has also had continental style crust for as long as 3.8 to 3.9 billion years. The oldest crust on Earth is the
Narryer Gneiss Terrane in
Western Australia at 3.9
Ga, and certain parts of the
Canadian Shield and the
Fennoscandian Shield are also of this age.
The majority of the current Earth's continental crust was formed primarily between 3.4 billion years and 2.4 billion years before present, in the
Archaean. The vast majority of rocks of this age are located in
cratons where the crust is up to 70km thick, which prevents it being destroyed by
subduction. Crust formation is linked to periods of intense
orogeny or mountain building; these periods coincide with the formation of the
supercontinents such as
Rodinia,
Pangaea and
Gondwana. The crust forms not so much by accumulation of
granite and
metamorphic fold belts, but by depletion of the
mantle to form buoyant lithospheric mantle.
*
Continental drift*
Plate tectonics*
USGS Crust Thickness Mapzh-yue:地殼
