Mantle convection

Earth cross-section showing location of upper (3) and lower (5) mantle

An oceanic plate of the lithosphere is added to by upwelling asthenosphere at a spreading ridge (left) and consumed at a subduction zone (right), causing stratovolcanoes at the convergent boundary with the continental plate.

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Calculated Earth's temperature vs. depth. Dashed curve: Layered mantle convection; Solid curve: Whole mantle convection.

Whole mantle convection

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A superplume generated by cooling processes in the mantle.

Mantle convection is the slow creeping motion of Earth's solid silicate mantle caused by convection currents carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, which rides atop the asthenosphere (the two components of the upper mantle), is divided into a number of plates that are continuously being created and consumed at their opposite plate boundaries. Accretion occurs as mantle is added to the growing edges of a plate, associated with seafloor spreading. This hot added material cools down by conduction and convection of heat. At the consumption edges of the plate, the material has thermally contracted to become dense, and it sinks under its own weight in the process of subduction usually at an ocean trench.

This subducted material sinks to a depth of 660 kilometres (410 mi) in the Earth's interior where it is impeded from sinking further, possibly due to a phase change from spinel to silicate perovskite and magnesiowustite, an endothermic reaction.

The subducted oceanic crust triggers volcanism, although the basic mechanisms are varied. Volcanism may occur due to processes that add buoyancy to partially melted mantle causing an upward flow due to a decrease in density of the partial melt.

Secondary forms of convection that may result in surface volcanism are postulated to occur as a consequence of intraplate extension and mantle plumes.

It is because the mantle can convect that the tectonic plates are able to move around the Earth's surface.

Mantle convection seems to have been much more active during the Hadean period, resulting in gravitational sorting of heavier molten iron, and nickel elements and sulphides in the core, and lighter silicate minerals in the mantle.

Types of convection

There is a current debate within the geophysics community as to whether convection is likely to be 'layered' or 'whole'. This debate is linked to the controversy regarding whether intraplate volcanism is caused by shallow, upper-mantle processes or by plumes from the lower mantle. Geochemists have argued that the lavas erupted in intraplate areas are different in composition from shallow-derived mid ocean ridge basalts (SDMORB). This has been interpreted as their originating from a different region, suggested to be the lower mantle. Others, however, have pointed out that the differences indicate the inclusion of a small component of near-surface material from the lithosphere. Seismologists are also divided, with some arguing that there is no evidence for whole-mantle convection, and others arguing that there is.

Speed of convection

See also: Heat transfer § Convection vs. conduction

Typical mantle convection speed is 20 mm/yr near the crust but can vary quite a bit. The small scale convection in the upper mantle is much faster than the convection near the core. (See whole-mantle discussion above.) A single shallow convection cycle takes on the order of 50 million years, though deeper convection can be closer to 200 million years.

See also

• Geodynamics
• Plate tectonics
• Earth's internal heat budget