Mantle Plumes (World of Earth Science)
Convection in Earth's mantle created by the dissipation of internal heat produces up-welling hot columns called mantle plumes and cold, sinking sheets. Numerical modeling suggests the presence of three types of mantle plumes. Regular mantle plumes originate from the core-mantle boundary (a depth of approximately 1,802 mi [2,900 km]) and may be stable for several hundred million years. Such plumes act as fixed reference frames for plate motion. A second type of plume, also originating from the core-mantle boundary, can be bent and move relative to the global circulation in the mantle. Several mantle plumes may also collide to form superplumes. Superplumes rising from the core-mantle boundary may produce additional, secondary plumes that develop above a 416 mi (670 km) boundary layer in Earth's mantle.
Mantle plumes impinge on the base of Earth's lithosphere in all plate tectonic settings and result in surface uplift of up to 875 yd (800 m), lithospheric thinning, extensional stress fields, and a thermal anomaly centered on the plume. Heating the base of the lithosphere by mantle plumes may lead to partial melting and the formation of mafic (i.e., iron and magnesium-rich) magma. Magma may intrude into fractures formed from extension of Earth's upper, brittle crust above mantle plumes to form mafic dike swarms. For example, diabase dikes of the Mackenzie dike swarm in north-western Canada that extend for over 1,243 mi (2,000 km) are thought to result from a single mantle plume source. Dikes typically radiate from a point centered above a mantle plume. Radiating arms of dike swarms from different continents have been used to help reconstruct past continent configurations.
Magma may also be extruded as lava flows on Earth's surface to form flood basalts over areas 621 mi (1,000 km) or more across. For example, the Paraná and Etendeka volcanics represent pre-breakup volcanism on the South American and African margins of the Tristan Plume and volcanics of the Deccan Traps in western India result from melting due to the Reunion Plume. Mantle plumes in the early stages of Earth's history are likely to have been stronger and hotter. Mafic and ultramafic volcanics called komatiites, within Archaean and Paleoproterozoic greenstone belts in Australia, Canada, the Baltic Shield and China, have been attributed to mantle plume sources. Some granitoid plutons in Archaean greenstone belts may also be indirectly related to crustal melting by mantle plumes.
Mantle plumes constitute a driving force in the fragmentation and rifting apart of continents. For example, the separation of South America from Africa and Greater India from Australia and Antarctica during the break-up of the supercontinent Gondwanaland is interpreted as resulting from rifts linking areas above several mantle plumes. Mantle plume-related rifts are typified by triple junctions where rifts, normal faults and dikes define arms at approximately 120° to each other that intersect above the mantle plume. Frequently, continental breakup and formation of oceanic crust occurs along two of the rift arms, whereas the third arm may be less developed, and constitute a failed rift or aulacogen. For example, a plume-related triple junction occurs over the Afar Plume, above which the Red Sea and Golf of Aden Rifts (along which there is active seafloor spreading) and the eastern, Ethiopian branch of the East African Rift (an intra-continental rift system) intersect. Not all plume-related rifts, however, define triple junctions and four or more rift arms may sometimes be present.
Mantle plumes may play an important role in the formation of mineral deposits e.g., nickel, chromium, platinum, palladium, diamonds, rare earth elements, tin, tantalum, niobium, copper, lead, and zinc. Such deposits may be related to alkaline magmatic fluids associated with mantle plumes, as well as being controlled by extensional structures due to plume-related stresses.
Mantle plumes are not unique to Earth. On Venus, where there is no evidence for plate tectonics as is known on Earth, deep rifts called chasmata and prominent radiating fracture systems from central volcanic peaks called novae develop above mantle plumes. Circular to elliptical volcano-tectonic features 37,616 mi (60,600 km) in diameter called coronae may also be sited above mantle plumes, or result from rifts linking several mantle plumes. Prominent concentric faults rimming smaller coronae are thought to form as a result of collapse due to withdrawal of magma produced by melting above a mantle plume.
See also Convergent plate boundary; Divergent plate boundary; Earth, interior structure; Geothermal deep ocean vents; Hawaiian Island formation; Hotspots; Volcanic eruptions