Earth mantle

Location of the mantle in a schematic cut of the ground layers (scale on the right)

The mantle is an intermediate layer of terrestrial planets or some other rocky planetary bodies, such as the one between the core, the innermost layer, and the crust, the outermost. It is mainly composed of silicates. For a mantle to form, the planetary body must be large enough to have gone through the process of planetary differentiation in terms of density. The terrestrial planets (Earth, Venus, Mars, and Mercury), Earth's Moon, two of Jupiter's satellites (Io and Europa), and the asteroid Vesta each have a rocky mantle.

Earth cover

The Earth's mantle is the layer of the Earth between the crust and the core (accounting for approximately 84% of the planet's volume). The Earth's mantle extends from about 33 km deep (or about 8 km in oceanic areas) to 2,900 km (transition to the core). Mantle differentiation began about 3.8 billion years ago, when gravimetric segregation of the components of protoplanet Earth produced the current stratification. The pressure in the lower part of the mantle is around 140 GPa (about 1,400,000 atmospheres). It is divided into two parts: internal, solid, elastic mantle; and outer mantle, fluid, viscous.

Features

The mantle differs mainly from the crust by its chemical characteristics and its mechanical behavior, which implies the existence of a clear sudden alteration (a discontinuity) in the physical properties of the materials, which it is known as the Mohorovičić discontinuity, or simply Moho after Andrija Mohorovičić, the geophysicist who discovered it. This discontinuity marks the border between the crust and the mantle.

For a time it was thought that the Moho represented the border between the rigid structure of the crust and the more plastic zone of the mantle, being the zone where the movement between the plates of the rigid lithosphere would take place. and the plastic asthenosphere. However, recent studies have shown that this border is located much lower, in the upper mantle, at a depth of the order of 70 km below the oceanic crust and 150 km below the continental crust. Thus, the mantle immediately below the crust is made up of relatively cold materials (approx. 600 °C), rigid and fused with the crust, despite being separated from it by the Moho. This demonstrates that the Moho is actually a compositional discontinuity and not a dynamic separation zone.

Composition

The main mechanical alteration in the Moho is evident in the speed of the seismic waves, which increases substantially, given the greater density of the mantle materials (since the propagation speed of a vibration is proportional to the density of the material). This greater density results, in addition to the effect of the increase in pressure, of the differences in their chemical composition, which is due to the increase in the relative proportion of these minerals, the mantle rocks —peridotite, dunite and eclogite— compared with the crustal rocks are characterized by a much higher percentage of iron and magnesium, to the detriment of silicon and aluminum.

The table below gives an approximate composition of the mantle materials as a percentage of their total mass (weight %). Note that the composition of the mantle may not be uniform, and a gradual increase in the Fe/Mg ratio with depth is to be expected; it is estimated to vary from 0.25 in the upper mantle to 0.6 in the lower mantle.

Physical characteristics

In addition to compositional differences, the mantle also has very different physical characteristics from those of the crust and core. In the following points, a characterization of the main physical parameters of the mantle is made.

State of the material

The material of which the mantle is composed can appear in a solid state or as a viscous paste, as a result of the high pressures. However, contrary to what one might imagine, the tendency in high-pressure areas is for rocks to remain solid, since they thus occupy less physical space than the liquids resulting from fusion. Besides that, the constitution of the materials of each layer of the mantle determines the local physical state. Thus, the interior of the Earth, including the inner core, tends to be solid because, despite the very high temperatures, it is subjected to such high pressures that the atoms, when compacted, force the forces of repulsion between the atoms. are overcome by external pressure. As a result, despite the temperature, the substance remains solid.

Determination of the characteristics of the interior of the Earth through seismic waves.

Temperature

The temperatures of the mantle vary between 600 °C (873 K) in the zone of contact with the crust, up to 3500 °C (3773 K) in the zone of contact with the core, approximately. This increase in temperature reflects both the greater difficulty of the deep layers in losing heat by conduction to the surface and the greater endogenous capacity to produce heat in depth (due to increased radioactive decay and friction with moving fluid materials). in the outer core).

Viscosity

The viscosity in the upper mantle (the asthenosphere) varies between 10²¹ and 10²⁴ Pa*s, depending on the depth. The upper layer moves very slowly, behaving simultaneously as a solid and as a highly viscous liquid. This explains the very slow movement of the tectonic plates and the isostatic movements of subsidence and enhancement (rebound) of the tectonic plates when their weight is altered (for example, with the formation of ice masses and their subsequent thaw).

Density

The density in this region increases linearly from 3.4 to 4.6 (in the upper mantle) and from 4.6 to 5.5 (in the lower mantle). In the upper mantle, the presence of the asthenosphere marks zones of partial melting. Apparently, no significant phase change occurs in the lower mantle, although there are small gradients in seismic wave propagation velocity at 1,230 km and 1,540 km depth. Thus, it is believed that the increase in the speed of seismic waves must occur mainly as a result of the compaction of a material of uniform composition.

Several models have been proposed that suggest the lower mantle contains more iron than the upper mantle. In this case, the Fe/Mg ratio would vary from 0.25 in the upper mantle to 0.6 in the lower mantle. The increase in the average atomic mass would increase the density up to the observed values, without the need to resort to complex molecular structures.

These models have generated many discussions, because if the lower mantle is denser than the upper one, it would be difficult for convective movements to exist. On the other hand, if there is generalized convection in the mantle, it would be difficult to maintain the heterogeneity of the chemical composition for long intervals of time. However, these apparent inconsistencies can be ironed out if we consider the existence of independent convection cells in the mantle.

Subdivisions of the mantle

Although there are no marked differences or obvious discontinuities within the mantle, but there are gradients that reflect the increase in pressure and temperature, it is common to divide the mantle into two layers:

• the upper mantle (from the discontinuity of Mohorovičić at 665 km deep is the end and measures 630 km).
• the lower mantle (from 3548 km deep to the end and measures 2883 km).

Upper mantle

The upper mantle (or outer mantle) begins at the Mohorovičić discontinuity, which is an average depth of 6 km below the oceanic crust and an average depth of 35.5 km below the continental crust, although it can reach as far as the latter depths greater than 400 km in subduction zones.

Measured seismic wave velocities in this layer are typically 8.0 to 8.2 km/s, which are higher than those recorded in the lower crust (6.5 to 7.8 km/s). Geophysical data show that between 50 and 200 km (or more in subduction zones) depth there is a decrease in the speed of P waves (longitudinal) and a strong attenuation of S waves (transversal), hence this region is known as low speed zone.

Evidence based on geophysical, geological and petrological data, and comparison with extraterrestrial bodies, indicate that the composition of the upper mantle is peridotitic. Peridotites are a family of ultrabasic rocks, mostly composed of magnesium olivine (approx. 80%) and pyroxene (approx. 20%). Although rare on the surface, peridotites crop out on some oceanic islands, in layers uplifted by orogenesis, and on rare kimberlites.

Peridotite melting experiences show that their partial melting can give rise to oceanic basalts under the pressure and temperature conditions existing in the upper mantle. This process probably occurs in the low-velocity zone, which explains the reduction in seismic velocities due to the partial melting of the materials.

Studies carried out in ophiolites and in the oceanic lithosphere show that the formation of the oceanic crust (with its average thickness of just 5 km) is carried out from the most superficial portion of the upper mantle. The degree of partial melting must reach 25%, which depletes this area of components with a low melting temperature. There is indirect evidence that the mantle becomes less depleted in silicate with increasing depth.

The garnet-lherzolite type peridotites (60% olivine, 30% ortho and clinopyroxenes, and 10% spinels, garnets and plagioclase), probably represent the peridotites of the primitive mantle, which upon partial melting, originate basaltic magmas, leaving as residues harzburgites (80% olivine, 20% orthopyroxenes) and dunites (olivine). Taking into account the pressure and temperature relationships, the conclusion is that at shallower depths the mineralogy is dominated by the plagioclase-lherzolite complex (which is frequently found in ophiolites) and that, with increasing pressure, the complex will dominate. spinel-lherzolite (which sometimes forms nodules in alkaline basalts). At higher pressures, the most stable mineralogy is that of the garnet-lherzolite complex (which forms nodules in kimberlites).

Lower mantle

The lower mantle begins around 665 km deep and extends to the Gutenberg discontinuity, located 3548 km deep, at the transition to the core. The lower mantle is separated from the asthenosphere by the Repetti discontinuity, thus being an essentially solid zone with very low plasticity.

The density in this region increases linearly from 4.6 to 5.5. Apparently, no significant phase change occurs in the lower mantle, although there are small gradients in seismic wave propagation velocity at 1,230 km and 1,540 km depth. Thus, it is believed that the increase in the speed of seismic waves must occur mainly as a result of the compaction of a material of uniform composition. Several models have been proposed that suggest that the lower mantle contains more iron than the upper mantle.

Temperature ranges from 1,000 °C to 3,000 °C, increasing with depth and with heat produced by radioactive decay and conduction from the outer core (where frictional heat production experienced by flows generating geomagnetism is large).

Mantle convection and hot spots

Computer model in which the materials are studied in the mantle.

Due to temperature differences between the Earth's crust and the outer core, there is a thermal possibility of the formation of a convective current that encompasses the entire mantle. However, this capacity is diminished by the very low plasticity of the materials of the lower mantle and by the gradual increase in density (due to the difference in composition and pressure).

However, this does not prevent isolated plutonic diapirs from rising towards the surface and fragments of cooler and denser crust from sinking into subduction zones, forming extensive zones of re-melting of crustal materials. Low plasticity forces these movements extremely slowly, making them last for hundreds of thousands, or even millions, of years.

In areas where diapirs persist and approach the surface, leading to melting of materials as pressure decreases with ascent, hot spots (hot spots) that later translate, on the surface, into intrusive formations, persistent volcanism or a widening of the oceanic crust. In subduction zones, the rise of molten materials and the effect of the introduction of enormous amounts of water into the mantle lead to the emergence of island arcs (such as the Antilles or Japan) and volcanic chains (such as the Andes Mountains).).

Convection in the Earth's mantle is a chaotic process of fluid dynamics that seems to determine the movement of tectonic plates and, in this way, the drift of the continents. In this context, it should be borne in mind that the drift of the continents is only part of the displacement process of the tectonic plates, since the rigidity of these and the phenomena of generation of new crust that occur along the rifts and destruction at along the subduction regions, give this a very complex character.

On the other hand, the movement of the lithosphere is necessarily unrelated to that of the asthenosphere, which causes the plates to move with different relative velocities over the mantle. Hence, hot-spots can give rise to island chains (such as the Hawaiian and Azores archipelagos, where each island or volcano marks the relative position of the hot-spot in relation to lithospheric plate at a certain time).

Given the complexity of mantle convection phenomena, there are great uncertainties in their modeling, even admitting that there are different convective cells in different layers of the mantle, creating a system with multiple layers between the core and the crust.

Although there is a general trend of viscosity increase with depth, this relationship is not linear and it appears that there are layers with much higher viscosity than expected in the upper mantle and near the transition zone to the external nucleus.

Due to the low viscosity of the asthenosphere, it would be expected that there would be no earthquakes with hypocenters located more than 300 km deep. This is generally true, since earthquakes that occur in oceanic areas rarely have their hypocenter below 25 km, and earthquakes in continental areas have their focus at a depth of 30-35 km. However, in subduction zones, the geothermal gradient can be substantially reduced, which increases the stiffness of the surrounding mantle material. Hence, earthquakes with focal depths of 400 km to 670 km have already been recorded in these regions, although they are very rare cases.

The pressure in the lower layers of the mantle reaches ~140 GPa (1.4 Matm). Despite these gigantic pressures, which increase with depth, it is thought that it is still possible for the entire mantle to deform as a highly viscous fluid over long periods of time. The viscosity of the upper mantle varies between 10²¹ and 10²⁴ Pas, depending on the depth. Hence any movement in the mantle must necessarily be hyper-slow.

This high-viscosity situation contrasts sharply with the fluidity of the outer core, even though it is under greater pressure. Such a contrast is the iron composition of the nucleus, whose melting point is much lower than that of the iron compounds existing in the mantle. Thus, the iron compounds of the lower mantle, despite being subjected to a lower pressure, are in a solid state (although, if we take long time scales as a reference, they act like a fluid of extreme viscosity), while the core The outer part, made of almost pure iron, is in a liquid state. The inner core is in a solid state given the extreme pressures to which it is subjected.

The implications of this difference between the mantle and the outer core (and between it and the inner core) are determinant for life on Earth, since this is where the terrestrial magnetic field is born, which functions as an electromagnetic shield that protects the life on the earth's surface from ionizing radiation from outer space and from solar winds.

Exploration

The knowledge we have of the mantle is essentially based on indirect geophysical studies, especially on the study of the propagation of seismic waves, and on the study of rock samples from great depths that are brought to the surface by the orogeny or by volcanism (ophiolites, kimberlites and xenolites). Hence the interest in obtaining direct samples of the mantle, which was attempted, in vain, with the oceanic drilling project called the Mohole project, which sought to drill to reach the Mohorovičić discontinuity. The greatest depth reached in this project, abandoned due to its enormous cost in 1966, was 180 m below the sea floor. In 2005 the third deepest survey reached 1416m below the seabed from the JOIDES Resolution drillship. A new attempt was made in 2007. This time the Japanese ship Chikyu was used to drill 7000 m into the oceanic crust, about three times the maximum depth reached on the ocean floor, with the aim of objective of obtaining materials from the discontinuity and from the layers of the upper mantle located immediately below.

In March 2007, an expedition made up of a dozen scientists led by Professor Roger C. Searle (Durham University, United Kingdom) explored an underwater region about 4,000 meters in diameter, located at a depth of 4,900 meters in the Atlantic Ocean halfway between the coasts of Africa and South America. The area drew the attention of scientists because it is believed that the earth's mantle is exposed, and there is no detectable crust in this particular place. Using a remotely controlled roving robot, drilling was performed in three different zones of the exposed area of the mantle. The perforations were planned to be 4 cm in diameter and 1 meter deep. The mission lasted about six weeks and was made up of geologists and oceanographers from the National Oceanographic Center (NOC) in the English city of Southampton. They traveled aboard the ship "RRS James Cook", named after the famous British explorer of the 18th century.