Jacket convection

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Heat convection of the solid but flowable earth's mantle

As mantle is slowly running upheavals called (so-called convection ) of the solid mantle . Mantle convection is a special form of convection .

history

The concept of mantle convection has developed since the beginning of the 20th century from the idea of ​​magma flows and magmatic mass displacements below the solid earth's crust, initially to explain the geology of fold mountains such as the Alps, then also of other large geotectonic forms such as deep-sea channels and regional, volcanic fissure systems.

Energy sources

Probable temperature profile between the earth's crust (left) and the earth's core (right)

Mantle convection is a heat transport mechanism in which material that is constantly cooled down on the earth's surface and therefore denser material sinks through gravity to the earth's core , which is around 5400 ° C ( subduction ). To compensate for this, hot and therefore less dense material is blown up from the core-mantle boundary to the earth's crust . At a presumed speed of 5 cm per year, the duration of one orbit is about 240 million years. Because mantle convection cools the earth's core through natural convection , it must have heat sources below the solid but flowable earth's mantle - i.e. in the vicinity or within the earth's core:

  • a small part may still come from the early days of the earth's formation , which is currently controversial: gravitational compression , impact energy from asteroids and meteorites, the release of potential energy when the earth's core is formed, and the decay of short-lived radioactive elements.
  • the greater part, probably 80%, originated from the decay of long-lived radioactive elements ( 235 U, 238 U, 232 Th and 40 K) in the earth's mantle or is probably still created today from the decay of potassium 40 in the earth's core.
  • According to assumptions, latent heat or heat of crystallization is also released when liquid material crystallizes on the surface of the solid inner core of the earth. Since this goes hand in hand with a shrinking of the earth's core, at least the gravitational binding energy contributes to the convective energy transport towards the earth's mantle and thus also to the "drive" of the geodynamics .

Mantle convection is thus a thermal convection in which the heating takes place from below through the earth's core, which is thereby cooled. A heating through carried radioactive material is irrelevant insofar as it cannot produce density differences between the ascending and descending mass flows. On the other hand, the viscosity of the earth's mantle (at the same depth) depends on the temperature, because the more viscous the fluid, the more the conduction competes at the expense of the heat flow . Overall, the jacket convection transports a heat flow of 3.5 × 10 13 W (corresponding to 35 TW ).

Mantle convection, plate tectonics and geodynamo

The upheavals take place very slowly with vertical and horizontal flow speeds of a few centimeters per year, as can be deduced indirectly from seismology and satellite geodesy . In spite of the high temperatures, the convective earth mantle is not liquid, but rather solid and behaves viscously or viscously ( viscosity 10 21 to 10 23 Pa s).

The mantle convection "passes" through to the earth's surface, as the drifting, solid rock lithospheric plates with their continents and ocean floors are part of the convective system. The most obvious superficial effects are

  • certain variations of geothermal energy, which are investigated with studies of geothermal energy ,
  • and the well-known pattern of continental drift and plate movements.

The latter is created by the slowly moving lithospheric plates - the so-called plate tectonics . The continental crust masses are embedded in the lithospheric plates and move with them at speeds of a few centimeters per year. One cannot say that mantle convection drives the drifting plates - or, conversely, that the moving plates “stir” the upper mantle of the earth - because plate tectonics is an integral part of mantle convection. The situation is similar with the outer core of the earth, in which convections also take place, which seem to be oriented towards the earth's mantle. Plate tectonics, convections in the earth's mantle and the geodynamo which ultimately results from the upheavals in the outer core of the earth are interlinked.

The principle

Example of a model for mantle convection: areas with high temperatures flowing upwards are shown in red, areas with low temperatures falling in blue.

Sheath convection is based on thermal convection : in a viscous liquid that is heated from below and inside and cooled from above, temperature differences lead to thermal expansion or contraction. As in the liquid of a house heating system at different temperatures, the resulting differences in density also cause buoyancy forces in this viscous material. These buoyancy forces lead to currents that are counteracted by viscous forces. In addition, heat conduction counteracts convection, as it tries to equalize the temperature between the hot upflow and the cold outflow. The physical quantities buoyancy, viscosity and heat conduction are summarized in the so-called Rayleigh number Ra , which is a measure of the strength of the convection.

Tremendous masses are in motion, because the mantle makes up over two thirds of the total mass of the earth . By the way, it is similar with the magnetic field : the flow of matter in the earth's core is slow, but the large masses still cause electrical currents of many million amperes.

In theory, any thermal convection can be studied by making assumptions about the mass and temperature distribution and solving the associated mathematical equations on the computer. As an example, the figure shows a convective layer with Ra = 10 ^ 6, constant viscosity, heated from below. You can see that the underside of the viscous layer has a hot thermal boundary layer (red) from which so-called hot plumes ( mantle plume ) rise. Cold drops or plumes sink from the cold thermal boundary layer on the top (dark blue).

Stratified or full jacket convection

At a depth of 660 kilometers there is a phase boundary ( 660 km discontinuity ) that separates 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) . This limit is an obstacle to jacket convection. It is assumed that mantle convection was more violent in the early history of the earth than it is today and possibly ran separately in the upper and lower mantle, while today we are in a kind of transition phase to full-mantle convection: Rising and falling currents are slowed down by the phase boundary and accumulate there partially open, but then mostly penetrate them. Finds of xenocrystals which come from at least 660 km depth speak for this .

Mantle convection calculated from seismic tomography. Warm areas are shown in red, cool areas in blue. Arrows indicate the convection flow velocities calculated from this.

Proof of the flow pattern of the jacket convection

In addition to direct observation of the surface effects (plate tectonics), seismology indirectly allows hot upflowing and cool sinking convection branches to be identified: hot areas are characterized by slightly reduced seismic speeds, cool areas by slightly higher seismic speeds. So-called seismic tomography can be used to identify such zones in the Earth's mantle (e.g. a hot, i.e. rising region under Iceland, a cold, i.e. sinking region under Japan). The density distributions obtained from such tomography models can then be used in fluid dynamic equations, and the flow fields can then be calculated directly from them. The illustration shows such an example.

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

Individual evidence

  1. ^ Ricard, Y. (2009). "2. Physics of Mantle Convection". In David Bercovici and Gerald Schubert (ed.). Treatise on Geophysics: Mantle Dynamics. 7. Elsevier Science. ISBN 9780444535801 .
  2. Gerald Schubert; Donald Lawson Turcotte; Peter Olson (2001) Mantle convection in the earth and planets. Cambridge University Press. pp. 16 ff. ISBN 978-0-521-79836-5 .
  3. Thermal convection with a freely moving top boundary (PDF; 717 kB)
  4. ^ Heat flow from the Earth's interior: Analysis of the global data set
  5. John S. Lewis: Consequences of the presence of sulfur in the core of the earth , Earth and Planetary Science Letters, Volume 11, Issues 1–5, May – August 1971, Pages 130–134, ISSN  0012-821X , doi : 10.1016 / 0012-821X (71) 90154-3 . ( http://www.sciencedirect.com/science/article/pii/0012821X71901543 )
  6. http://www.scinexx.de/wissen-aktuell-13445-2011-05-19.html
  7. Torsvik, Trond H .; Smethurst, Mark A .; Burke, Kevin; Steinberger, Bernhard (2006). "Large igneous provinces generated from the margins of the large low-velocity provinces in the deep mantle". Geophysical Journal International. 167 (3): 1447-1460. Bibcode: 2006GeoJI.167.1447T
  8. http://eprints.gla.ac.uk/125/1/Harris,J_1997pdf.pdf

See also

Web links

  • Mantle convection , animations of convection currents, website of the University of Frankfurt.