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How volcanoes are formed

30.06.2006
Volcanoes often gather in groups, the so-called hot spots. The Hawaiian Islands are a striking example. One of the reasons for such hot spots appearance – is formation of the so-called thermochemical plume in the Earth’s mantle.

Researchers from the United Institute of Geology, Geophysics and Mineralogy (Siberian Branch, Russian Academy of Sciences) under the guidance of Academician N.L. Dobretsov have developed such object formation model. The researchers are interested how the plume would behave near the Earth’s surface and whether lava would pour out on the surface. To this end, they have built a mathematical model.

The thermochemical plume is formed at the core and mantle boundary, in the location where chemical additive is present, which lowers melting temperature at the mantle bottom. At this section, the melted rock column starts to move through the mantle and it rises until it reaches the infusible layer of lithosphere. Having set against it, the plume spreads under the infusible layer, forming a mushroom-like head. The head supported from below grows up gradually, the heat coming from the Earth’s interior fuses the lithosphere bottom, the diameter of molten section is also growing. The secondary upflow appears, which in the long run bursts open to the surface as red-hot lava.

All these processes take up rather long time and depend on multiple parameters. Russian scientists tried to take everything into account. According to their calculations, the secondary plume rises up from the depth of 100 to 200 kilometers at the rate of 1.2 – 2.4 centimeters per year, and it can burst out to the surface from the depth of about 30 kilometers. Consequently, this path takes, depending on the depth and the traverse speed, from 2.9 to 14.2 million years. Thus, contemporary eruptions have a long-standing history.

Eruption can take place only under definite conditions and depends, specifically, on heat flow rate and the plume head diameter. If the head diameter is big, then lava may burst out into the surface at a vast territory in several hot spots. According to the Novosibirsk geophysicists’ calculations, if the flow rate is 3?10^11 Wt, eruption will happen, should the head diameter be 770 to 1310 kilometers, but if the flow rate is thrice as little, diameter of the region to be covered by volcanoes soon or not that soon, would make 450 to 770 kilometers.

Determining the growth length and size of the plume head, that rose from deep mantle layers toward the lithosphere, is an important task for geodynamics, and researchers are now actively solving it. Russian geophysicists’ calculations allow to determine the plume head diameter dependence upon time and thermal power of the source, and, consequently, to characterize known upflows and to forecast eruptions several million years in advance.

Sergey Komarov | alfa
Further information:
http://www.informnauka.ru

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