Mantle (geology)
Earth's
mantle is the thick shell of rock surrounding the
Earth's outer core, and lies directly beneath the Earth's thin
crust. The term is also applied to the rocky shell surrounding the cores of other
planets. Earth's mantle lies roughly between 30 and 2,900 km below the surface, and occupies about 70% of Earth's volume.
The boundary between the crust and the mantle is the
Mohorovičić discontinuity, named for its discoverer, and is usually called the
Moho. The Seismic Moho is a boundary at which there is a sudden change in the speed of
seismic waves, which can be detected by sensitive instruments at Earth's surface. At one time some people thought that the Moho was the structure along which the Earth's rigid crust moved relative to the mantle. Current research considers the motion of the crust associated with plate tectonics as the surface manifestation of a much deeper mantle circulation. The uppermost mantle just below the crust is composed of relatively cold and therefore strong material. This strong layer of mantle and the crust forms the
lithosphere, and cools mainly by conduction.
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, and seismic characteristics. The crust is, in fact, primarily a product of mantle melting. Typical mantle rocks have a higher portion of
iron and
magnesium, a higher magnesium to iron ratio, and a smaller portion of
silicon and
aluminium than the crust.
Mantle rock above about 400 km depth consists mostly of
olivine,
pyroxenes,
spinel, and
garnet: typical rock types are
peridotite,
dunite (olivine-rich peridotite), and
eclogite. Between about 400 km and 650 km depth, olivine is not stable and is replaced by
polymorphs with approximately the same composition: one polymorph is wadsleyite, and the other is ringwoodite (a mineral with the
spinel structure). Below about 650 km, none of the minerals of the upper mantle is stable; the most abundant minerals present have structures (but not compositions) like that of the mineral,
perovskite. The changes in mineralogy at about 400 and 650 km yield distinctive signatures in seismic records of the Earth's interior. The changes in mineralogy with depth have been investigated by laboratory experiments that duplicate high mantle pressures, such as those using the
diamond anvil. These changes in mineralogy may influence mantle convection, as they result in density changes and they may absorb or release heat during phase transitions in convecting mantle.
In the mantle, temperatures range between 100°C at the upper boundary to over 4,000°C at the boundary with the
core. Although these temperatures far exceed the
melting points of the mantle rocks at the surface, particularly in deeper ranges, they are almost exclusively solid. The enormous
lithostatic pressure exerted on the mantle prevents them from
melting.
The subregion of the mantle extending about 250 km (155 mi) below the lithosphere is called the
asthenosphere, this cools mainly by convection. In some regions of the earth, this subregion of the mantle is partly associated with a region of the mantle that passes seismic waves more slowly. This region is called the
low-velocity zone. The cause of this low velocity zone is still debated. Currently theories include the influence of temperature and pressure or the existence of a small amount of partial melt.
Due to the temperature difference between the Earth's surface and outer core there is a
convective material circulation in the mantle. Hot material ascends as a
plutonic diapir from the border with the outer core, while cooler (and heavier) material sinks downward. This is often in the form of large-scale lithospheric downwellings at plate boundaries called subduction zones. During the ascent the material of the mantle cools down
adiabatically. The temperature of the material falls with the pressure relief connected with the ascent, and its heat distributes itself over a larger volume. Near the lithosphere the pressure relief can lead to partial melting of the diapir, leading to
volcanism and plutonism.
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 the dominant driving force for 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. The convection of the mantle is not yet clarified in detail.
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" or "D prime prime"). D may consist of material from subducted slabs that descended and came to rest at the
core-mantle boundary.
Due to the relatively low viscosity in the upper mantle one could reason that there should be no
earthquakes below approximately 300 km depth. However, in subduction zones, the geothermal gradient can be lowered, 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 M
atm). There exists increasing pressure as one travels deeper into the mantle. 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 planar defects through the solid crystals comprising the mantle. The viscosity of the upper mantle ranges between 10
19 and 10
24 Pa·s, depending on depth . 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.
Why is the inner core solid, the outer core liquid, and the mantle solid/plastic? The answer depends both on the relative melting points of the different layers (nickel-iron core, silicate crust and mantle) and on 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. 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.
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 second attempt to retrieve samples from the Earth's mantle is scheduled for 2007 . As part of the
Chikyu Hakken mission, it will use the Japanese vessel 'Chikyu' to drill up to 7000m below the seabed. This is nearly three times as deep as preceding oceanic drillings, which are preferred over land drillings because the crust at the seabed is thinner. The first attempt, known as
Project Mohole, was abandoned in 1966 after repeated failures and ever rising costs. The deepest they managed to penetrate was about 180m.
#
Mantle Viscosity and the Thickness of the Convective Downwellings retrieved on December 17, 2005#
Physorg.com article of 2005/12/15 retrieved on December 17, 2005#
Information on the Mohole Project*
The Biggest Dig : Japan builds a ship to drill to the earth's mantle - Scientific American Magazine (September 2005)
