The
structure of Earth
The interior structure of the Earth is layered in
spherical shells, like an onion. These layers can be defined by either their chemical or their rheological properties. Earth has an outer silicate
solid crust, a highly viscous mantle, a
liquid outer core that is much less viscous than the mantle,
and a solid inner core. Scientific understanding of Earth's
internal structure is based on observations of topography
and bathymetry, observations of rock in outcrop, samples brought to the surface from greater
depths by volcanic activity, analysis of the seismic waves
that pass through Earth, measurements of the gravity
field of Earth, and experiments with crystalline solids at pressures and
temperatures characteristic of Earth's deep interior.
Assumptions,The force exerted by Earth's gravity can be used to calculate
its mass, and by estimating the
volume of the Earth, its average density can be calculated. Astronomers can also
calculate Earth's mass from its orbit and effects on nearby planetary bodies.
Structure
Earth's
gravity according to the preliminary reference earth model (PREM).[1] Comparison to
approximations using constant and linear density for Earth's interior.
Schematic
view of the interior of Earth. 1. continental crust – 2. oceanic crust – 3.
upper mantle – 4. lower mantle – 5. outer core – 6. inner core – A: Mohorovičić discontinuity –
B: Gutenberg Discontinuity –
C: Lehmann–Bullen discontinuity.
The structure of Earth can be defined in two ways: by
mechanical properties such as rheology, or chemically. Mechanically, it can be
divided into lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core. The interior of Earth is divided into 5
important layers. Chemically, Earth can be divided into the crust, upper
mantle, lower mantle, outer core, and inner core. The geologic component layers
of Earth[2] are at the
following depths below the surface:
|
Depth
|
Layer
|
|
|
Kilometres
|
Miles
|
|
|
0–60
|
0–37
|
Lithosphere
(locally varies between 5 and 200 km)
|
|
0–35
|
0–22
|
…
Crust (locally varies between 5 and 70 km)
|
|
35–60
|
22–37
|
…
Uppermost part of mantle
|
|
35–2,890
|
22–1,790
|
Mantle
|
|
100–200
|
62–125
|
…
Asthenosphere
|
|
35–660
|
22–410
|
…
Upper mesosphere (upper mantle)
|
|
660–2,890
|
410–1,790
|
…
Lower mesosphere (lower mantle)
|
|
2,890–5,150
|
1,790–3,160
|
Outer
core
|
|
5,150–6,360
|
3,160–3,954
|
Inner
core
|
The layering of Earth has been inferred indirectly using
the time of travel of refracted and reflected seismic waves created by
earthquakes. The core does not allow shear waves to pass through it, while the
speed of travel (seismic velocity) is different in other layers.
The changes in seismic velocity between different layers causes refraction
owing to Snell's law, like light bending as it passes through a
prism. Likewise, reflections are caused by a large increase in seismic velocity
and are similar to light reflecting from a mirror.
Core
The average density of Earth is 5,515 kg/m3.
Since the average density of surface material is only around 3,000 kg/m3,
we must conclude that denser materials exist within Earth's core. Seismic
measurements show that the core is divided into two parts, a "solid" inner core with a radius of ~1,220 km[3] and a liquid outer core extending beyond it to a radius of
~3,400 km. The densities are between 9.9 and 12.2 kg/m3 in
the outer core and 12.6–13.0 kg/m3 in the inner core.[4]
The inner core was discovered in 1936 by Inge Lehmann and is generally believed to be composed
primarily of iron and some nickel. It is not necessarily a solid, but, because
it is able to deflect seismic waves, it must behave as a solid in some fashion.
Experimental evidence has at times been critical of crystal models of the core.[5] Other experimental studies show a
discrepancy under high pressure: diamond anvil (static) studies at core
pressures yield melting temperatures that are approximately 2000K below those
from shock laser (dynamic) studies.[6][7] The laser studies create plasma,[8] and the results are suggestive that
constraining inner core conditions will depend on whether the inner core is a
solid or is a plasma with the density of a solid. This is an area of active
research.
In early stages of Earth's formation about four and a half
billion (4.5×109) years ago, melting would have caused denser
substances to sink toward the center in a process called planetary differentiation
(see also the iron catastrophe), while less-dense
materials would have migrated to the crust.
The core is thus believed to largely be composed of iron (80%), along with nickel and one or more light
elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or
tend to bind to lighter elements and thus remain in the crust (see felsic materials). Some have argued that
the inner core may be in the form of a single iron crystal.[9][10]
Under laboratory conditions a sample of iron nickel alloy
was subjected to the corelike pressures by gripping it in a vise between 2
diamond tips, and then heating to approximately 4000 K. The sample was observed
with x-rays, and strongly supported the theory that Earth's inner core was made
of giant crystals running north to south.[11][12]
The liquid outer core surrounds the inner core and is
believed to be composed of iron mixed with nickel and trace amounts of lighter
elements.
Recent speculation suggests that the innermost part of
the core is enriched in gold, platinum and other siderophile elements.[13]
The matter that comprises Earth is connected in
fundamental ways to matter of certain chondrite meteorites, and to matter of outer portion
of the Sun.[14][15] There is good reason to believe that Earth
is, in the main, like a chondrite meteorite. Beginning as early as 1940,
scientists, including Francis Birch,
built geophysics upon the premise that Earth is like ordinary chondrites, the
most common type of meteorite observed impacting Earth, while totally ignoring
another, albeit less abundant type, called enstatite chondrites. The principal difference between
the two meteorite types is that enstatite chondrites formed under circumstances
of extremely limited available oxygen, leading to certain normally oxyphile
elements existing either partially or wholly in the alloy portion that
corresponds to the core of Earth.
Dynamo theory suggests that convection in the outer core,
combined with the Coriolis effect, gives rise to Earth's magnetic field.
The solid inner core is too hot to hold a permanent magnetic field (see Curie temperature) but probably acts to
stabilize the magnetic field generated by the liquid outer core. The average
magnetic field strength in Earth's outer core is estimated to be 25 Gauss, 50
times stronger than the magnetic field at the surface.[16][17]
Recent evidence has suggested that the inner core of
Earth may rotate slightly faster than the rest of the planet[18] however more
recent studies in 2011[which?]
found this hypothesis to be inconclusive. Options remain for the core which may
be oscillatory in nature or a chaotic system.In August 2005 a team of geophysicists announced in the journal Science
that, according to their estimates, Earth's inner core rotates approximately
0.3 to 0.5 degrees per year relative to the rotation of the surface.[19][20]
The current scientific explanation for Earth's temperature gradient is
a combination of heat left over from the planet's initial formation, decay of
radioactive elements, and freezing of the inner core.
Mantle
Earth's mantle extends to a depth of
2,890 km, making it the thickest layer of Earth. The pressure, at the bottom of the mantle, is ~140 GPa (1.4 Matm). The mantle is composed
of silicate rocks that are rich in iron and magnesium
relative to the overlying crust. Although solid, the high temperatures within
the mantle cause the silicate material to be sufficiently ductile that it can flow on very long timescales. Convection of the mantle is expressed at the surface
through the motions of tectonic plates. The melting point and viscosity of a substance depends on the pressure it is
under. As there is intense and increasing pressure as one travels deeper into
the mantle, the lower part of the mantle flows less easily than does the upper
mantle (chemical changes within the mantle may also be important). The viscosity
of the mantle ranges between 1021 and 1024 Pa·s, depending on depth.[21] In comparison, the viscosity of water is
approximately 10−3 Pa·s and that of pitch is
107 Pa·s.
Crust
The crust ranges from 5–70 km (~3–44 miles) in depth
and is the outermost layer. The thin parts are the oceanic crust, which underlie the ocean basins
(5–10 km) and are composed of dense (mafic) iron magnesium silicate igneous rocks,
like basalt. The thicker crust is continental crust, which is less dense and
composed of (felsic) sodium potassium aluminium silicate rocks, like granite.
The rocks of the crust fall into two major categories – sial and sima
(Suess,1831–1914). It is estimated that sima starts about 11 km below the Conrad discontinuity (a
second order discontinuity). The uppermost mantle together with the crust
constitutes the lithosphere. The crust-mantle boundary occurs as two
physically different events. First, there is a discontinuity in the seismic velocity, which is known as the Mohorovičić discontinuity or
Moho. The cause of the Moho is thought to be a change in rock composition from
rocks containing plagioclase feldspar (above) to rocks that
contain no feldspars (below). Second, in oceanic crust, there is a chemical discontinuity between ultramafic cumulates and tectonized harzburgites, which has been observed from deep parts of
the oceanic crust that have been obducted onto the continental crust and preserved as ophiolite sequences.
Many rocks now making up Earth's crust formed less than
100 million (1×108) years ago; however, the oldest known mineral
grains are 4.4 billion (4.4×109) years old, indicating that Earth
has had a solid crust for at least that long.[22]
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