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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.〔Kobes, Randy and Kunstatter, Gabor.("Mantle Convection" ). Physics Department, University of Winnipeg. Retrieved 3 January 2010.〕 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 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'.〔 See for example, ; and also: 〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「mantle convection」の詳細全文を読む スポンサード リンク
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