Mantle (geology)
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Earth's mantle comprises approximately 70% of Earth's volume. It is a rocky shell overlying Earth's iron-rich core, which occupies about 30% of Earth's volume. The 2,900 km thick mantle has partly melted at shallower levels and produced a very 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.
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[edit] Structure
The boundary between the crust and the mantle is the Mohorovičić discontinuity, named for its discoverer, and is usually called the "Moho" as shorthand. The Moho is detected as a boundary at which there is a sudden change in the speed of seismic waves, whose reflections can be detected by sensitive instruments called seismometers at the Earth's surface. The moho varies in depth from as little as 5 km in settings to as much as 80 km in mountainous regions like Tibet. 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, with the minuscule crust playing no significant role in the large scale and long term dynamical evolution of the planet.
The uppermost mantle just below the crust is relatively cold by virtue of being near the cool surface, is chemically different than most of the mantle due to the extraction of melt that produced the crust, and is therefore thought to be anomalously strong (i.e., resistant to deformation). This strong, cool, and chemically modified layer of mantle and the crust forms the lithosphere, which is also variable in thickness but typically extends to about 100 km depth.
The region below the lithosphere extending up to 250 km (155 mi) depth is called the asthenosphere, and in some regions is detected by seismic travel relatively slowly. The low seismic velocity regions are called low-velocity zones or LVZ. The cause of this low velocity zone is still debated, although it has long been noted that Earth's temperature should pass very close to the temperature at which the onset of melting occurs in the depth range of the LVZ.
[edit] 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 portion 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 rock shallower than about 400 km depth consists mostly of olivine, pyroxenes, spinel, 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. 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.
| 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 |
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..
[edit] Temperature
In the mantle, temperatures range between 500°C-900°C (932°F-1,652°F) at the upper boundary with the crust to over 4,000°C (7,200°F) 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.
[edit] 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 ascends as a plutonic diapir, perhaps from the border with the outer core (see mantle plume), 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 both adiabatically and by conduction into surrounding cooler mantle. The temperature of the material falls with the pressure relief connected with the ascent, and its heat distributes itself over a larger volume. Because the temperature at which melting initiates decreases more rapidly with height than does a rising hot plume, partial melting may occur just beneath the lithosphere and causing 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 oceani hosphere 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 [1]. 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"), a nomenclature introduced over 50 years ago by the geophysicist 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.
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 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 GPa (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 1019 and 1024 Pa·s, depending on depth [1], 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.
[edit] Exploration
A more difficult attempt to retrieve samples from the Earth's mantle is scheduled for 2007 [2]. As part of the Chikyu Hakken mission, it will use the Japanese vessel 'Chikyu' to drill up to 7000m (23,000 ft) 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 cost over-runs. The deepest they managed to penetrate was about 180m (590 ft). In 2005 the third-deepest oceanic borehole hole reached 1416 meters (4,644 feet) below the sea floor from the ocean drilling vessel JOIDES Resolution.
On March 5, 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. The anomaly is located mid-way between the Cape Verdes Islands and the Caribbean in the Atlantic Ocean. It lies approximately three kilometres beneath the ocean surface and covers thousands of square kilometres.[3][4]
[edit] References
- ^ a b 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
- ^ http://www.msnbc.msn.com/id/17407745/
- ^ http://www.sciencedaily.com/releases/2007/03/070301103112.htm
- Don L. Anderson, Theory of the Earth, Blackwell (1989), a textbook dealing with the Earth's interior, is now available on the web at
http://resolver.caltech.edu/CaltechBOOK:1989.001
[edit] External links
- The Biggest Dig : Japan builds a ship to drill to the earth's mantle - Scientific American Magazine (September 2005)
- Information on the Mohole Project
| Structure of the Earth |
|---|
| Crust | Lithosphere | Asthenosphere | |
| Mesosphere | Mantle | Outer core | Inner core |
| Plate tectonics |
