Mantle (geology)

Earth cutaway from core to exosphere. Partially to scale.

Mapping the interior of the Earth with earthquake waves.

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 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. It is chiefly the difference of chemistry on which the distinction between crust and mantle is based. The crust is, in fact, primarily a product of mantle melting. Mantle rock consists of olivines, different pyroxenes and other mafic minerals. Typified by peridotite, dunite, and eclogite, mantle rocks also possess a higher portion of iron and magnesium and a smaller portion of silicon and aluminium than the crust. 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 drive 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 thereby partially decoupled, since due to the rigidity of the lithosphere, a tectonic plate can only move as a whole. Continental drift is therefore only a diffuse image of the movements at the upper limit of the Earth's mantle. The convection of the mantle is not yet clarified in detail. There are different theories, according to which the Earth's mantle is divided into different floors of separate convection.

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].

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 GPa (1.4 Matm). 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 10¹⁹ and 10²⁴ Pa·s, depending on depth ^[1]. 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 ^[2]. 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.

References

1. ^ Mantle Viscosity and the Thickness of the Convective Downwellings retrieved on December 17, 2005
2. ^ Physorg.com article of 2005/12/15 retrieved on December 17, 2005
3. Information on the Mohole Project

External links

• The Biggest Dig : Japan builds a ship to drill to the earth's mantle - Scientific American Magazine (September 2005)