The continental crust is the layer of igneous, sedimentary, and metamorphic
rocks that forms the continents and the areas of shallow seabed close to their
shores, known as continental shelves. This layer is sometimes called sial
because its bulk composition is more felsic, or granitic, in contrast to the
oceanic crust, called sima, whose bulk composition is more mafic or basaltic
rock. Consisting mostly of granitic rock,
continental crust has a density of about 2.7 g/cm3 and is less dense than the
material of Earth's mantle, which consists of ultramafic rock and whose
density is about 3.3 g/cm3. Continental crust is also less dense than oceanic
crust, whose density is about 2.9 g/cm3. Continental crust is considerably
thicker than oceanic crust, mostly 25 to 70 km versus the average thickness of
the oceanic crust of around 7–10 km. About 40% of Earth's surface is now
overlaid by continental crust. Continental crust makes up about 70% of
the volume of Earth's crust. Importance
Because the surface of continental crust mainly lies above sea level, its
existence allowed land life to evolve from marine life. Its existence also
provides broad expanses of shallow water known as epeiric seas and continental
shelves where complex metazoan life could become established during early
Paleozoic time, in what is now called the Cambrian explosion.
Origin There is little evidence of continental
crust prior to 3.5 Ga, and there was relatively rapid development on shield
areas consisting of continental crust between 3.0 and 2.5 Ga. All continental
crust ultimately derives from the fractional differentiation of oceanic
crust over many eons. This process has been and continues today primarily as a
result of the volcanism associated with subduction.
Forces at work In contrast to the persistence of
continental crust, the size, shape, and number of continents are constantly
changing through geologic time. Different tracts rift apart, collide and
recoalesce as part of a grand supercontinent cycle. There are
currently about 7 billion cubic kilometers of continental crust, but
this quantity varies because of the nature of the forces involved. The
relative permanence of continental crust contrasts with the short life of oceanic
crust. Because continental crust is less dense than oceanic crust, when active
margins of the two meet in subduction zones, the oceanic crust is typically
subducted back into the mantle. Continental crust is rarely subducted.
For this reason the oldest rocks on Earth are within the cratons or cores of
the continents, rather than in repeatedly recycled oceanic crust; the
oldest intact crustal fragment is the Acasta Gneiss at 4.01 Ga, while the
oldest oceanic crust is from the Jurassic. Continental crust and the rock
layers that lie on and within it are thus the best archive of Earth's
history. The height of mountain ranges is usually
related to the thickness of crust. This results from the isostasy associated
with orogeny. The crust is thickened by the compressive forces related to
subduction or continental collision. The buoyancy of the crust forces it upwards,
the forces of the collisional stress balanced by gravity and erosion. This
forms a keel or mountain root beneath the mountain range, which is where the
thickest crust is found. The thinnest continental crust is found in rift
zones, where the crust is thinned by detachment faulting and eventually
severed, replaced by oceanic crust. The edges of continental fragments formed
this way are termed passive margins. The high temperatures and pressures at
depth, often combined with a long history of complex distortion, cause
much of the lower continental crust to be metamorphic - the main exception to
this being recent igneous intrusions. Igneous rock may also be "underplated"
to the underside of the crust, i.e. adding to the crust by forming a layer
immediately beneath it. Continental crust is produced and
destroyed mostly by plate tectonic processes, especially at convergent
plate boundaries. Additionally, continental crustal material is
transferred to oceanic crust by sedimentation. New material can be added
to the continents by the partial melting of oceanic crust at subduction zones,
causing the lighter material to rise as magma, forming volcanoes. Also, material
can be accreted horizontally when volcanic island arcs, seamounts or
similar structures collide with the side of the continent as a result of plate
tectonic movements. Continental crust is also lost through erosion and sediment
subduction, tectonic erosion of forearcs, delamination, and deep
subduction of continental crust in collision zones. Many theories of
crustal growth are controversial, including rates of crustal growth and
recycling, whether the lower crust is recycled differently from the upper
crust, and over how much of Earth history plate tectonics has operated and
so could be the dominant mode of continental crust formation and
destruction. It is a matter of debate whether the
amount of continental crust has been increasing, decreasing, or remaining
constant over geological time. One model indicates that at prior to 3.7 Ga ago
continental crust constituted less than 10% of the present amount. By 3.0 Ga ago
the amount was about 25%, and following a period of rapid crustal evolution it
was about 60% of the current amount by 2.6 Ga ago. The growth of continental
crust appears to have occurred in spurts of increased activity corresponding to
five episodes of increased production through geologic time.
See also Terrane
Geologic province Lithosphere
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doi:10.1029/91RG00969. External links
Average composition of Continental Crust Crust 5.1
Evolution of the Continental Crust