The
mantle is a part of an
astronomical object. The interior of the
Earth, similar to the other terrestrial planets, is
chemically divided into layers. The mantle is a
highly
viscous layer between the
crust and the
outer
core.
Earth's mantle is about thick rocky
shell that constitutes about 84 percent of Earth's volume. It is
predominantly
solid and takes over Earth's
iron-rich hot core, which occupies about 15 percent of Earth's
volume. Past episodes of melting and
volcanism at the shallower levels of the mantle
have produced a thin crust of crystallized melt products near the
surface, upon which we live. The gases evolved during the melting
of Earth's mantle have a large effect on the composition and
abundance of
Earth's atmosphere.
Information about structure and composition of the mantle either
result from geophysical investigation or from direct geoscientific
analyses on Earth mantle derived
xenoliths.
Structure
The mantle is divided into sections based upon results from
seismology. These layers (and their
depths) are the following: the upper mantle (
base of the
crust–410 km), the transition zone (410–660 km), the
lower mantle (660–2891 km), and in the bottom of the latter
region there is the anomalous
D" layer with a variable
thickness (on average ~200 km thick)
.
The top of the mantle is defined by a sudden increase in seismic
velocity, which was first noted by
Andrija Mohorovičić in 1909;
this boundary is now referred to as the "Mohorovičić discontinuity"
or "
Moho". The
uppermost mantle plus overlying crust are relatively rigid and form
the
lithosphere, an irregular layer with
a maximum thickness of perhaps 200 km. Below the lithosphere
the upper mantle becomes notably more plastic in its
rheology. In some regions below the lithosphere,
the seismic velocity is reduced; this so-called low velocity zone
(LVZ) extends down to a depth of several hundred km.
Inge Lehmann discovered a seismic discontinuity
at about 220 km depth; although this discontinuity has been
found in other studies, it is not known whether the discontinuity
is ubiquitous. The transition zone is an area of great complexity;
it physically separates the upper and lower mantle. Very little is
known about the lower mantle apart from that it appears to be
relatively seismically homogeneous. The
D" layer
at the
Core–mantle
boundary separates the mantle from the core.
Characteristics
The mantle differs substantially from the crust in its mechanical
characteristics and its chemical composition. The distinction
between crust and mantle is based on chemistry, rock types,
rheology and
seismic
characteristics. The crust is, in fact, a product of mantle
melting. Partial melting of mantle material is believed to cause
incompatible elements to separate from the mantle rock, with less
dense material floating upward through pore spaces, cracks, or
fissures, to cool and freeze at the surface. Typical mantle rocks
have a higher magnesium to iron ratio, and a smaller proportion of
silicon and
aluminium than the crust. This behavior is also
predicted by experiments that partly melt rocks thought to be
representative of Earth's mantle.
Mantle rocks shallower than about 410 km depth consist mostly
of
olivine,
pyroxenes,
spinel-structure
minerals, and
garnet; typical rock types are
thought to be
peridotite,
dunite (olivine-rich peridotite), and
eclogite. Between about 400 km and 650 km
depth, olivine is not stable and is replaced by high pressure
polymorphs with
approximately the same composition: one polymorph is
wadsleyite (also called
beta-spinel
type), and the other is
ringwoodite (a
mineral with the
gamma-spinel
structure). Below about 650 km, all of the minerals of the
upper mantle begin to become unstable. The most abundant minerals
present have structures (but not compositions) like that of the
mineral
perovskite followed by
the magnesium/iron oxide
ferropericlase. The changes in mineralogy at
about 400 and 650 km yield distinctive signatures in seismic
records of the Earth's interior, and like the moho, are readily
detected using seismic waves. These changes in mineralogy may
influence
mantle convection, as
they result in density changes and they may absorb or release
latent heat as well as depress or elevate the depth of the
polymorphic phase transitions for regions of different
temperatures. The changes in mineralogy with depth have been
investigated by laboratory experiments that duplicate high mantle
pressures, such as those using the
diamond
anvil.
Composition of Earth's mantle in weight
percent
| Element |
Amount |
|
Compound |
Amount |
| O |
44.8 |
|
|
| Si |
21.5 |
SiO2 |
46 |
| Mg |
22.8 |
MgO |
37.8 |
| Fe |
5.8 |
FeO |
7.5 |
| Al |
2.2 |
Al2O3 |
4.2 |
| Ca |
2.3 |
CaO |
3.2 |
| Na |
0.3 |
Na2O |
0.4 |
| K |
0.03 |
K2O |
0.04 |
| Sum |
99.7 |
Sum |
99.1 |
The inner core is solid, the outer core is liquid, and the mantle
solid/plastic. This is because of the relative melting points of
the different layers (nickel-iron core, silicate crust and mantle)
and the increase in temperature and pressure as one moves deeper
into the Earth. At the surface both nickel-iron alloys and
silicates are sufficiently cool to be solid. In the upper mantle,
the silicates are generally solid (localised regions with small
amounts of melt exist); however, as the upper mantle is both hot
and under relatively little pressure, the rock in the upper mantle
has a relatively low
viscosity, i.e. it is
relatively fluid. In contrast, the lower mantle is under tremendous
pressure and therefore has a higher viscosity than the upper
mantle. The metallic nickel-iron outer core is liquid despite the
enormous pressure as it has a melting point that is lower than the
mantle silicates. The inner core is solid due to the overwhelming
pressure found at the center of the planet.
Temperature
In the mantle, temperatures range between at the upper boundary
with the crust to over at the boundary with the
core. Although the higher
temperatures far exceed the
melting
points of the mantle rocks at the surface (about 1200 °C for
representative
peridotite), the mantle is
almost exclusively solid. The enormous
lithostatic pressure exerted on the
mantle prevents
melting, because the
temperature at which melting begins (the
solidus) increases with pressure.
Movement
Due to the temperature difference between the Earth's surface and
outer core, and the ability of the crystalline rocks at high
pressure and temperature to undergo slow, creeping, viscous-like
deformation over millions of years, there is a
convective material circulation in the mantle.
Hot material
upwells, while cooler (and
heavier) material sinks downward. Downward motion of material often
occurs at
convergent plate
boundaries called
subduction
zones, while upwelling of material can take the form of
plume. Locations on the
surface that lie over plumes will often
increase in elevation (due to the
buoyancy of the hotter, less-dense plume beneath) and exhibit
hot spot volcanism.
The
convection of the Earth's mantle is a
chaotic process (in the sense of fluid
dynamics), which is thought to be an integral part of the motion of
plates. Plate motion should not be confused with the older term
continental drift which applies
purely to the movement of the crustal components of the continents.
The movements of the lithosphere and the underlying mantle are
coupled since descending lithosphere is an essential component of
convection in the mantle. The observed continental drift is a
complicated relationship between the forces causing oceanic
lithosphere to sink and the movements within Earth's mantle.
Although there is a tendency to larger viscosity at greater depth,
this relation is far from linear, and shows layers with
dramatically decreased viscosity, in particular in the upper mantle
and at the boundary with the core. The mantle within about
200 km above the
core-mantle
boundary appears to have distinctly different seismic
properties than the mantle at slightly shallower depths; this
unusual mantle region just above the core is called
D″ ("D double-prime"), a nomenclature introduced
over 50 years ago by the geophysicist
Keith Bullen.
D″ may
consist of material from subducted slabs that descended and came to
rest at the
core-mantle
boundary and/or from a new mineral polymorph discovered in
perovskite called
post-perovskite.
Earthquakes at shallow depths are a result of stick-slip faulting,
however, below about 50 km the hot, high pressure conditions
ought to inhibit further seismicity. The mantle is also considered
to be viscous, and so incapable of brittle faulting. However, in
subduction zones, earthquakes are observed down to 670 km. A
number of mechanisms have been proposed to explain this phenomenon,
including dehydration, thermal runaway, and phase change.
the geothermal gradient can be lowered where cool material from the
surface sinks downward, increasing the strength of the surrounding
mantle, and allowing earthquakes to occur down to a depth of
400 km and 670 km.
The
pressure at the bottom of the mantle is
~136 G
Pa (1.4 million
atm). There exists increasing pressure
as one travels deeper into the mantle, since the material beneath
has to support the weight of all the material above it. The entire
mantle, however, is still thought to deform like a fluid on long
timescales, with permanent plastic deformation accommodated by the
movement of point, line, and/or planar defects through the solid
crystals comprising the mantle. Estimates for the viscosity of the
upper mantle range between 10
19 and 10
24
Pa·s, depending on depth, temperature,
composition, state of stress, and numerous other factors. Thus, the
upper mantle can only flow very slowly. However, when large forces
are applied to the uppermost mantle it can become weaker, and this
effect is thought to be important in allowing the formation of
tectonic plate boundaries.
Exploration
Exploration of the mantle is generally conducted at the seabed
rather than on land due to the relative thinness of the oceanic
crust as compared to the significantly thicker continental
crust.
The first attempt at mantle exploration, known as
Project Mohole, was abandoned in 1966 after
repeated failures and cost over-runs. The deepest penetration was
approximately .In 2005 the third-deepest oceanic borehole hole
reached below the sea floor from the ocean drilling vessel
JOIDES Resolution.
On 5 March
2007, a team of scientists on board the RRS James Cook embarked on a voyage
to an area of the Atlantic seafloor where the mantle lies exposed
without any crust covering, mid-way between the Cape Verde
Islands
and the Caribbean Sea
. The exposed site lies approximately three
kilometres beneath the ocean surface and covers thousands of square
kilometres.
A relatively difficult attempt to retrieve samples from the Earth's
mantle was scheduled for later in 2007. As part of the
Chikyu Hakken mission, was to use the Japanese
vessel 'Chikyu' to drill up to below the seabed. This is nearly
three times as deep as preceding oceanic drillings.
A novel method of exploring the uppermost hundreds km of the Earth
was recently analysed, consisting of a small, dense,
heat-generating probe which melts its way down through the crust
and mantle while its position and progress are tracked by acoustic
signals generated in the rocks. The probe consists of an outer
sphere of
tungsten about 1 m in diameter
inside which is a
60Co radioactive heat source.
It was calculated that such a probe will reach the oceanic
Moho in less than 6
months and attain minimum depths of well over 100 km in a few
decades beneath both oceanic and continental lithosphere.
Exploration can also be aided through computer simulations of the
evolution of the mantle. In 2009, a
supercomputer application provided new insight
into the distribution of mineral deposits, especially isotopes of
iron, from when the mantle developed 4.5 billion years ago.
References
- The location of the base of the crust varies from approximately
10 to 70 kilometers. Oceanic crust is generally less than 10
kilometers thick. "Standard" continental crust is around 35
kilometers thick, and the large crustal root under the
Tibetan
Plateau is approximately 70 kilometers thick.
- Earth cutaway (image). Retrieved 2007-12-25.
- Anderson, Don L. (2007) New Theory of the Earth. Cambridge
University Press. ISBN 978-0-521-84959-3, 0-521-84959-4
- mantle@Everything2.com. Retrieved 2007-12-26.
- Mantle Viscosity and the Thickness of the
Convective Downwellings retrieved on November 7, 2007
- Ojovan M.I., Gibb F.G.F., Poluektov P.P., Emets E.P. 2005.
Probing of the interior layers of the Earth with
self-sinking capsules. Atomic Energy, 99, 556–562
- Ojovan M.I., Gibb F.G.F. "Exploring the Earth’s Crust and
Mantle Using Self-Descending, Radiation-Heated, Probes and Acoustic
Emission Monitoring". Chapter 7. In: Nuclear Waste Research:
Siting, Technology and Treatment, ISBN 978-1-60456-184-5,
Editor: Arnold P. Lattefer, Nova Science Publishers, Inc.
2008
- University of California - Davis (2009-06-15). Super-computer
Provides First Glimpse Of Earth's Early Magma Interior.
ScienceDaily.
Retrieved on 2009-06-16 from
http://www.sciencedaily.com/releases/2009/06/090615153118.htm.
External links