Mantle convection

As mantle is called slow process of upheaval ( convection ) of the solid mantle. Mantle convection is a special form of thermal convection.

History

The concept of mantle convection has developed since the beginning of the 20th century idea of ​​magma flows and igneous mass displacements below the earth's crust, first to explain the geology of fold mountains, like the Alps, then another geotectonic large forms, such as deep-sea trenches and regional volcanic columns systems.

Energy sources

Mantle convection is a heat transport mechanism, is transported at the ever hot material between the surface of about 5400 ° C hot earth's core to the top ( ie against the direction of gravity ) to the considerably cooler crust. To compensate cooled material has to be transported to the Earth's core by subduction downward, thereby closing the loop. The duration of a round is probably about 240 million years. Because the mantle cools the Earth's core by natural convection, these heat sources must possess the below the liquid mantle - are - so in the area or within the Earth's core:

• A small portion may date back to the early days of the creation of Earth, which is currently controversial: gravitational compression, impact energy of asteroids and meteorites, the release of potential energy in the Earth's core formation and decay of short-lived radioactive elements.
• The greater part, probably 80 %, due to the decay of long-lived radioactive elements ( 235U, 238U, Th and 40K ) in the mantle or is likely to arise today by the decay of potassium 40 in the Earth's core.
• Unproven assumptions, according to latent heat can be released when liquid material at the surface of the solid inner core of the earth crystallized.

Mantle convection is thus a thermal convection, in which the heating is from below through the Earth's core, which is thereby cooled. A heater by mittransportiertes radioactive material is irrelevant, since it can not produce density differences on the ascending and descending mass flows. The heat produced by radioactive decay in the earth's crust would like a " overhead " slow heating, natural convection and therefore can not be the cause. Overall, the mantle convection transports a heat flow of 3.5 × 1013 W (equivalent to 35 TW).

Mantle Convection, Plate Tectonics and geodynamo

The upheavals run very slowly with vertical and horizontal velocities of a few centimeters per year, as one can infer indirectly from seismology and satellite geodesy. The convecting mantle is despite high temperatures because of the high pressures are not liquid but solid, and behaves zähplastisch or viscous ( viscosity from 1021 to 1023 Pa s ).

The Mantle " Paust " on through to the earth's surface, as the drifting, consisting of hard rock lithospheric plates with their continents and ocean floors are a part of the convecting system. The most obvious superficial effects are

• Certain variations of the geothermal heat, which are explored with studies of geothermal energy,
• And the well-known pattern of continental drift and plate movements.

The latter is caused by the slow-moving tectonic plates - the so-called plate tectonics. The continental crustal masses are embedded in the lithospheric plates and move with this speed of several centimeters per year. You can not say that the mantle drifting plates drives - or, conversely, that " stir " the moving plates of the Earth's upper mantle - because plate tectonics is an integral part of the mantle. The situation is similar with the outer core in which also expire convection which seem to be based on the Earth's mantle. Thus, plate tectonics, mantle convection and the geodynamo which are ultimately intertwined result of the upheavals in the outer core.

The principle

The mantle is based on thermal convection: In a viscous liquid which is heated from below and cooled from the inside and from above, differences in temperature lead to thermal expansion or contraction. As in the different temperatures of liquid a home heating evoke the resulting differences in density in this viscous material buoyancy forces. These supervisors driving forces lead to currents that counteract those viscous forces. In addition, counteracts heat conduction convection as it tries to balance the temperature between hot and cold upflow downflow. The physical quantities buoyancy, viscosity and heat conduction in the so -called Rayleigh number Ra summarized, which is thus a measure of the strength of convection.

Here are immense masses in motion, because the Earth's mantle makes about two thirds of the Earth's mass from. Similarly, it is the way with the magnetic field: matter flows in the Earth's core are slowly, but the masses still cause electric currents of millions of amps.

In theory, one can examine any thermal convection by assumptions concerning the mass and temperature distribution and the associated mathematical equations triggers on the computer. As an example, the figure shows a convecting layer with Ra = 10 ^ 6, constant viscosity heated from below. One can see that the bottom of the viscous layer a hot thermal boundary layer (red), ( Mantle plume ) from which so-called hot plumes rise. From the cold thermal boundary layer at the top ( dark blue) fall cold drops or Plumes down.

Stratified or Ganzmantelkonvektion

In 660 km depth, there is a phase boundary ( 660 - km discontinuity ) that the upper mantle ( 30-410 km depth ) and the so -called mantle transition zone ( 410-660 km depth ) from the lower mantle ( 660-2900 km depth) separates. This limit is a barrier to mantle convection. It is believed that in the early Earth's mantle convection was more violent than today and may disconnect the upper and lower mantle expired while we find ourselves today in a kind of transitional phase to Ganzmantelkonvektion: Ascending and descending currents are slowed down by the phase boundary and accumulate there partially, but they penetrate but then usually. This is supported by findings of Xenokristallen which come from at least 660 km depth.

Proof of the flow pattern of mantle convection

In addition to direct observation of the surface-area effects ( plate tectonics ) seismology allows indirectly to identify hot aufströmende and cool sinking Konvektionsäste: Hot areas are characterized by slightly reduced seismic velocities, cool areas by slightly higher seismic velocities. Through the so-called seismic tomography can identify such zones in the mantle (eg, one sees Iceland under a hot, thus ascending region, including Japan, a cold, so sinking region). The density distributions derived from such tomography models can then be used in fluid dynamic equations, and from this the flow fields are calculated directly. The figure shows one such example.

Another way to observe mantle convection indirectly, is in the gravity field or geoid. The density variations described above lead to very small but measurable changes in the earth's gravitational field. Thus, for example, observed in the Western Pacific spacious a slightly stronger gravitational field, which is interpreted by the higher density in the cold convective outflow ( subduction zone ).