In geology, the crust is the outermost solid
shell of a rocky planet, dwarf planet, or
natural satellite.
It is usually distinguished from the underlying
mantle by its chemical makeup; however, in
the case of icy satellites, it may be distinguished
based on its phase (solid crust vs. liquid
mantle).
The crusts of Earth, Moon, Mercury, Venus,
Mars, Io, and other planetary bodies formed
via igneous processes, and were later modified
by erosion, impact cratering, volcanism, and
sedimentation.
Most terrestrial planets have fairly uniform
crusts.
Earth, however, has two distinct types: continental
crust and oceanic crust.
These two types have different chemical compositions
and physical properties, and were formed by
different geological processes.
== Types of crust ==
Planetary geologists divide crust into three
categories, based on how and when they formed.
=== Primary crust / primordial crust ===
This is a planet's "original" crust.
It forms from solidification of a magma ocean.
Toward the end of planetary accretion, the
terrestrial planets likely had surfaces that
were magma oceans.
As these cooled, they solidified into crust.
This crust was likely destroyed by large impacts
and re-formed many times as the Era of Heavy
Bombardment drew to a close.The nature of
primary crust is still debated: its chemical,
mineralogic, and physical properties are unknown,
as are the igneous mechanisms that formed
them.
This is because it is difficult to study:
none of Earth's primary crust has survived
to today.
Earth's high rates of erosion and crustal
recycling from plate tectonics has destroyed
all rocks older than about 4 billion years,
including whatever primary crust Earth once
had.
However, geologists can glean information
about primary crust by studying it on other
terrestrial planets.
Mercury's highlands might represent primary
crust, though this is debated.
The anorthosite highlands of the Moon are
primary crust, formed as plagioclase crystallized
out of the Moon's initial magma ocean and
floated to the top; however, it is unlikely
that Earth followed a similar pattern, as
the Moon was a water-less system and Earth
had water.
The Martian meteorite ALH84001 might represent
primary crust of Mars; however, again, this
is debated.
Like Earth, Venus lacks primary crust, as
the entire planet has been repeatedly resurfaced
and modified.
=== Secondary crust ===
Secondary crust is formed by partial melting
of silicate materials in the mantle, and so
is usually basaltic in composition.This is
the most common type of crust in the Solar
System.
Most of the surfaces of Mercury, Venus, Earth,
and Mars comprise secondary crust, as do the
lunar maria.
On Earth, we see secondary crust forming primarily
at mid-ocean spreading centers, where the
adiabatic rise of mantle causes partial melting.
=== Tertiary crust ===
Tertiary crust is more chemically-modified
than either primary or secondary.
It can form in several ways:
Igneous processes: partial-melting of secondary
crust, coupled with differentiation or dehydration
Erosion and sedimentation: sediments derived
from primary, secondary, or tertiary crustThe
only known example of tertiary crust is the
continental crust of the Earth.
It is unknown whether other terrestrial planets
can be said to have tertiary crust, though
the evidence so far suggests that they do
not.
This is likely because plate tectonics is
needed to create tertiary crust, and Earth
is the only planet in our Solar System with
plate tectonics.
== Earth's crust ==
=== Structure ===
The crust is a thin shell on the outside of
the Earth, accounting for less than 1% of
Earth's volume.
It is the top component of lithosphere: a
division of Earth's layers that includes the
crust and the upper part of the mantle.
The lithosphere is broken into tectonic plates
that move, allowing heat to escape from the
interior of the Earth into space.
The crust lies on top of the mantle, a configuration
that is stable because the upper mantle is
made of peridotite and so is significantly
denser than the crust.
The boundary between the crust and mantle
is conventionally placed at the Mohorovičić
discontinuity, a boundary defined by a contrast
in seismic velocity.
The crust of the Earth is of two distinctive
types:
Oceanic: 5 km (3 mi) to 10 km (6 mi) thick
and composed primarily of denser, more mafic
rocks, such as basalt, diabase, and gabbro.
Continental: 30 km (20 mi) to 50 km (30 mi)
thick and mostly composed of less dense, more
felsic rocks, such as granite.Because both
continental and oceanic crust are less dense
than the mantle below, both types of crust
"float" on the mantle.
This is isostasy, and it's also one of the
reasons continental crust is higher than oceanic:
continental is less dense and so "floats"
higher.
As a result, water pools in above the oceanic
crust, forming the oceans.
The temperature of the crust increases with
depth, reaching values typically in the range
from about 200 °C (392 °F) to 400 °C (752
°F) at the boundary with the underlying mantle.
The temperature increases by as much as 30
°C (54 °F) for every kilometer locally in
the upper part of the crust, but the geothermal
gradient is smaller in deeper crust.
=== Composition ===
The continental crust has an average composition
similar to that of andesite.
The most abundant minerals in Earth's continental
crust are feldspars, which make up about 41%
of the crust by weight, followed by quartz
at 12%, and pyroxenes at 11%.
Continental crust is enriched in incompatible
elements compared to the basaltic ocean crust
and much enriched compared to the underlying
mantle.
Although the continental crust comprises only
about 0.6 weight percent of the silicate on
Earth, it contains 20% to 70% of the incompatible
elements.
All the other constituents except water occur
only in very small quantities and total less
than 1%.
Estimates of average density for the upper
crust range between 2.69 and 2.74 g/cm3 and
for lower crust between 3.0 and 3.25 g/cm3.
=== Formation and evolution ===
Earth formed approximately 4.6 billion years
ago from a disk of dust and gas orbiting the
newly formed Sun.
It formed via accretion, where planetesimals
and other smaller rocky bodies collided and
stuck, gradually growing into a planet.
This process generated an enormous amount
of heat, which caused early Earth to melt
completely.
As planetary accretion slowed, Earth began
to cool, forming its first crust, called a
primary or primordial crust.
This crust was likely repeatedly destroyed
by large impacts, then reformed from the magma
ocean left by the impact.
None of Earth's primary crust has survived
to today; all was destroyed by erosion, impacts,
and plate tectonics over the past several
billion years.
Since then, Earth has been forming secondary
and tertiary crust.
Secondary crust forms at mid-ocean spreading
centers, where partial-melting of the underlying
mantle yields basaltic magmas and new ocean
crust forms.
This "ridge push" is one of the driving forces
of plate tectonics, and it is constantly creating
new ocean crust.
That means that old crust must be destroyed
somewhere, so, opposite a spreading center,
there is usually a subduction zone: a trench
where an ocean plate is being shoved back
into the mantle.
This constant process of creating new ocean
crust and destroying old ocean crust means
that the oldest ocean crust on Earth today
is only about 200 million years old.
In contrast, the bulk of the continental crust
is much older.
The oldest continental crustal rocks on Earth
have ages in the range from about 3.7 to 4.28
billion years and have been found in the Narryer
Gneiss Terrane in Western Australia, in the
Acasta Gneiss in the Northwest Territories
on the Canadian Shield, and on other cratonic
regions such as those on the Fennoscandian
Shield.
Some zircon with age as great as 4.3 billion
years has been found in the Narryer Gneiss
Terrane.
The average age of the current Earth's continental
crust has been estimated to be about 2.0 billion
years.
Most crustal rocks formed before 2.5 billion
years ago are located in cratons.
Such old continental crust and the underlying
mantle asthenosphere are less dense than elsewhere
in Earth and so are not readily destroyed
by subduction.
Formation of new continental crust is linked
to periods of intense orogeny; these periods
coincide with the formation of the supercontinents
such as Rodinia, Pangaea and Gondwana.
The crust forms in part by aggregation of
island arcs including granite and metamorphic
fold belts, and it is preserved in part by
depletion of the underlying mantle to form
buoyant lithospheric mantle.
== Moon's crust ==
A theoretical protoplanet named "Theia" is
thought to have collided with the forming
Earth, and part of the material ejected into
space by the collision accreted to form the
Moon.
As the Moon formed, the outer part of it is
thought to have been molten, a “lunar magma
ocean.”
Plagioclase feldspar crystallized in large
amounts from this magma ocean and floated
toward the surface.
The cumulate rocks form much of the crust.
The upper part of the crust probably averages
about 88% plagioclase (near the lower limit
of 90% defined for anorthosite): the lower
part of the crust may contain a higher percentage
of ferromagnesian minerals such as the pyroxenes
and olivine, but even that lower part probably
averages about 78% plagioclase.
The underlying mantle is denser and olivine-rich.
The thickness of the crust ranges between
about 20 and 120 km.
Crust on the far side of the Moon averages
about 12 km thicker than that on the near
side.
Estimates of average thickness fall in the
range from about 50 to 60 km.
Most of this plagioclase-rich crust formed
shortly after formation of the moon, between
about 4.5 and 4.3 billion years ago.
Perhaps 10% or less of the crust consists
of igneous rock added after the formation
of the initial plagioclase-rich material.
The best-characterized and most voluminous
of these later additions are the mare basalts
formed between about 3.9 and 3.2 billion years
ago.
Minor volcanism continued after 3.2 billion
years, perhaps as recently as 1 billion years
ago.
There is no evidence of plate tectonics.
Study of the Moon has established that a crust
can form on a rocky planetary body significantly
smaller than Earth.
Although the radius of the Moon is only about
a quarter that of Earth, the lunar crust has
a significantly greater average thickness.
This thick crust formed almost immediately
after formation of the Moon.
Magmatism continued after the period of intense
meteorite impacts ended about 3.9 billion
years ago, but igneous rocks younger than
3.9 billion years make up only a minor part
of the crust.
== See also ==
Eduction
