Upper mantle

The upper mantle of

The density profile through Earth is determined by the velocity of seismic waves. Density increases progressively in each layer, largely due to compression of the rock at increased depths. Abrupt changes in density occur where the material composition changes.^[1]

The upper mantle begins just beneath the crust and ends at the top of the lower mantle. The upper mantle causes the tectonic plates to move.

Crust and mantle are distinguished by composition, while the lithosphere and asthenosphere are defined by a change in mechanical properties.^[2]

The top of the mantle is defined by a sudden increase in the speed of seismic waves, which Andrija Mohorovičić first noted in 1909; this boundary is now referred to as the Mohorovičić discontinuity or "Moho."^[3]

The Moho defines the base of the crust and varies from 10 km (6.2 mi) to 70 km (43 mi) below the surface of the Earth. Oceanic crust is thinner than continental crust and is generally less than 10 km (6.2 mi) thick. Continental crust is about 35 km (22 mi) thick, but the large crustal root under the Tibetan Plateau is approximately 70 km (43 mi) thick.^[4]

The thickness of the upper mantle is about 640 km (400 mi). The entire mantle is about 2,900 km (1,800 mi) thick, which means the upper mantle is only about 20% of the total mantle thickness.^[4]

The boundary between the upper and lower mantle is a 670 km (420 mi) discontinuity.

There is another major phase transition predicted at 520 km (320 mi) for the transition of olivine (β to γ) and garnet in the pyrolite mantle.^[9] This one has only sporadically been observed in seismological data.^[10]

Other non-global phase transitions have been suggested at a range of depths.^[6][11]

Temperature and pressure

Temperatures range from approximately 500 K (227 °C; 440 °F) at the upper boundary with the crust to approximately 4,200 K (3,930 °C; 7,100 °F) at the core-mantle boundary.^[12] The highest temperature of the upper mantle is 1,200 K (930 °C; 1,700 °F).^[13] Although the high temperature far exceeds the melting points of the mantle rocks at the surface, the mantle is almost exclusively solid.^[14]

The enormous

The highest pressure of the upper mantle is 24.0 GPa (237,000 atm)[13] compared to the bottom of the mantle, which is 136 GPa (1,340,000 atm).^[12][16]

Estimates for the viscosity of the upper mantle range between 10¹⁹ and 10²⁴

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.^[17]

Movement

Because of 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.^[3]

Hot material

The seismic data is not sufficient to determine the composition of the mantle. Observations of rocks exposed on the surface and other evidence reveal that the upper mantle is mafic minerals olivine and pyroxene, and it has a density of about 3.33 g/cm³ (0.120 lb/cu in)^[1]

Upper mantle material that has come up onto the surface comprises about 55% olivine and 35% pyroxene, and 5 to 10% of

Experiments on olivines and pyroxenes show that these minerals change the structure as pressure increases at greater depth, which explains why the density curves are not perfectly smooth. When there is a conversion to a more dense mineral structure, the seismic velocity rises abruptly and creates a discontinuity.[1]

At the top of the transition zone, olivine undergoes isochemical phase transitions to wadsleyite and ringwoodite. Unlike nominally anhydrous olivine, these high-pressure olivine polymorphs have a large capacity to store water in their crystal structure. This has led to the hypothesis that the transition zone may host a large quantity of water.^[18]

In Earth's interior, olivine occurs in the upper mantle at depths less than 410 kilometres (250 mi), and ringwoodite is inferred within the

At the base of the transition zone,

Exploration of the mantle is generally conducted at the seabed rather than on land because of the oceanic crust's relative thinness 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 overruns. The deepest penetration was approximately 180 m (590 ft). In 2005 an oceanic borehole reached 1,416 metres (4,646 ft) below the seafloor from the ocean drilling vessel JOIDES Resolution.

On 5 March 2007, a team of scientists on board the

The Chikyu Hakken mission attempted to use the Japanese vessel Chikyū to drill up to 7,000 m (23,000 ft) below the seabed. On 27 April 2012, Chikyū drilled to a depth of 7,740 metres (25,390 ft) below sea level, setting a new world record for deep-sea drilling. This record has since been surpassed by the ill-fated Deepwater Horizon mobile offshore drilling unit, operating on the Tiber prospect in the Mississippi Canyon Field, United States Gulf of Mexico, when it achieved a world record for total length for a vertical drilling string of 10,062 m (33,011 ft).^[24] The previous record was held by the U.S. vessel Glomar Challenger, which in 1978 drilled to 7,049.5 meters (23,130 feet) below sea level in the Mariana Trench.^[25] On 6 September 2012, Scientific deep-sea drilling vessel Chikyū set a new world record by drilling down and obtaining rock samples from deeper than 2,111 metres (6,926 ft) below the seafloor off the Shimokita Peninsula of Japan in the northwest Pacific Ocean.

A novel method of exploring the uppermost few hundred kilometers of the Earth was proposed in 2005, consisting of a small, dense, heat-generating probe that melts its way down through the crust and mantle while its position and progress are tracked by acoustic signals generated in the rocks.^[26] The probe consists of an outer sphere of tungsten about 1 metre (3 ft 3 in) in diameter with a cobalt-60 interior acting as a radioactive heat source. This should take half a year to reach the oceanic Moho.^[27]

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.^[28]

In 2023 JOIDES Resolution recovered cores of what appeared to be rock from the upper mantle after drilling only a few hundred meters into the Atlantis Massif. The borehole reached a maximum depth of 1,268 meters and recovered 886 meters of rock samples consisting of primarily peridotite. There is debate over the extent to which the samples represent the upper mantle with some arguing the effects of seawater on the samples situates them as examples of deep lower crust. However, the samples offer a much closer analogue to mantle rock than magmatic xenoliths as the sampled rock never melted into magma or recrystallized.^[29]

References