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mantle plumes The mantle of the Earth con-vects with large cells that generally upwell beneath the oceanic ridges and downwell with subduction zones. These convection cells are the main way that the mantle loses heat. In addition to these large cells, a number of linear plumes of hot material upwell from deep within the mantle, perhaps even from the core-mantle boundary. Heat and material in these plumes move at high velocities relative to the main mantle convection cells, and therefore burn their way through the moving mantle and reach the surface, forming thick sequences of generally basaltic lava. These lavas are chemically distinct from mid-ocean ridge and island arc basalts, and they form either as continental flood basalts, oceanic flood basalts (on oceanic plateaus), or shield volcanoes.
Mantle plumes are postulated to be upper mantle hot spots that were relatively stationary with respect to the moving plates, because a number of long linear chains of islands in the oceans were found to be parallel, and all old at one end and younger at the other end. In the 1960s when plate tectonics was first recognized, it was suggested that these hot spot tracks were formed when the plates moved over hot, partially molten spots in the upper mantle that burned their way, like a blow torch, through the lithosphere, and erupted basalts at the surface. As the plates moved, the hot spots remained stationary, so the plates had a series or chain of volcanic centers erupted through them, with the youngest volcano sitting above the active hot spot. The Hawaiian-Emperor island chain is one of the most exemplary of these hot spot tracks. Located in the north-central Pacific to the Pacific northwest near the Aleutian arc, the Hawaiian-Emperor island chain, which is about 70 million years old, shows a sharp bend in the middle of the chain where the volcanoes are 43 million years old, and then are progressively younger toward the island of Hawaii. Magmas beneath Hawaii are still molten, and are assigned an age of zero. The bend in the chain indicates a change in the plate motion direction and is reflected in a similar change in direction of many other hot spot tracks in the Pacific Ocean.
Geochemical data and seismic tomography has shown that the hot spots are produced by plumes of deep mantle material that probably rise from the D" layer at the core-mantle boundary. These plumes may rise as a mechanism to release heat from the core or as a response to greater heat loss than is accommodated by convection. If heat is transferred from the core to D", parts of this layer may become heated, become more buoyant, and rise as thin narrow plumes that rise buoyantly through the mantle. As they approach the base of the lithosphere the plumes expand outward, forming a mushroomlike plume head that may expand to more than 600 miles (1,000 km) in diameter. Flood basalts may rise from these plume heads, and large areas of uplift, doming, and volcanism may be located above many plume heads.
Geologists believe plumes exist beneath the African plate, such as beneath the Afar region, which has experienced uplift, rifting, and flood basalt vol-canism. This region exemplifies a process whereby several (typically three) rifts propagate off a dome formed above a plume head, and several of these link up with rifts that propagated off other plumes formed over a large stationary plate. When several rifts link together, they can form a continental rift system that could become successful and expand into a young ocean basin, similar to the Red Sea. The linking of plume-related rifts has been suggested to be a mecha
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G Infobase Publishing nism to split supercontinents that have come to rest (in a geoid low) above a number of plumes. The heat from these plumes must eventually escape by burning through the lithosphere, forming linked rift systems that eventually rip apart the supercontinent.
Some areas of anomalous young volcanism may also be formed above mantle plume heads. For instance, the Yellowstone area has active volcanism and geother-mal activity and is thought to rest above the Yellowstone hot spot, which has left a track extending northwest back across the flood basalts of the Snake River plain. Other flood basalt provinces probably formed in a similar way. For instance, the 65-million-year-old Deccan flood basalts of India formed when this region was over the Reunion hot spot that is presently in the Indian ocean, and these may be related to a mantle plume.
Mantle plumes may also interact with mid-ocean ridge volcanism. For instance, the island of Iceland is located on the Reykjanes Ridge, part of the mid-Atlantic ridge system, but the height of the island is related to unusually thick oceanic crust produced in this region because a hot spot (plume) has risen directly beneath the ridge. Other examples of mantle plumes located directly beneath ridges are found in the South Atlantic Ocean, where the Walvis and Rio Grande Ridges both point back to an anomalously thick region on the present-day ridge where the plume head is located. As the South Atlantic opened, the thick crust produced at the ridge on the plume head split, half being accreted to the African plate, and half being accreted to the South American plate.
See also convection and the Earth's mantle; energy in the Earth system; hot spot; large igneous provinces, flood basalt; mantle.
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