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ScienceWeek
EARTH SCIENCE: ON MANTLE PLUMES
The following points are made by R. Montelli et al (Science 2004 303:338):
1) Hotspots are characterized by higher temperature, topographic swells, and recent volcanism with isotopic signatures distinct from those that characterize mid-ocean ridge or andesitic basalts (1-3). The best known example is the Hawaii-Emperor volcanic chain, which may have formed as the Pacific plate moved over a deep magmatic source (3-5). Narrow thermal up-wellings in the form of plumes are commonly observed in laboratory experiments and numerical simulations, and deep-mantle plumes have been invoked to explain flood basalts, the isotopic signature of ocean island basalts, and the topography of the swells and plateaus that often accompany volcanic hotspots. Although this has led to a coherent (albeit incomplete) theory of much of the geology that characterizes hotspots, undisputed evidence for the existence of lower-mantle plumes in tomographic images of the mantle is lacking.
2) High temperatures reduce the velocity of seismic waves, so that plumes should be evinced as columnar low-velocity anomalies. In the absence of convincing tomographic evidence, it has recently been argued that hotspots could instead be the manifestation of shallow, plate-related stresses that would fracture the lithosphere, causing volcanism to occur along these cracks.
3) The authors present tomographic evidence for the existence of deep-mantle thermal convection plumes. P-wave velocity images show at least six well-resolved plumes that extend into the lowermost mantle: Ascension, Azores, Canary, Easter, Samoa, and Tahiti. Other less well-resolved plumes, including Hawaii, may also reach the lowermost mantle. The authors also see several plumes that are mostly confined to the upper mantle, suggesting that convection may be partially separated into two depth regimes. All of the observed plumes have diameters of several hundred kilometers, indicating that plumes convey a substantial fraction of the internal heat escaping from Earth.
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1. N. H. Sleep, J. Geophys. Res. 95, 6715(1990)
2. G. F. Davies, J. Geophys. Res. 93, 10467 (1988)
3. V. Courtillot, A. Davaille, J. Besse, J. Stock, Earth Planet. Sci. Lett. 205, 295 (2003)
4. J. T. Wilson, Can. J. Phys. 41, 863 (1963)
5. J. T. Wilson, Philos. Trans. R. Soc. London 258A (1965)
Science http://www.sciencemag.org
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ON MANTLE PLUMES AND MOUNTAINS
The following points are made by J.B. Murphy et al (American Scientist 1999 87:146):
1) Theories of mountain building were revolutionized in the 1960s by plate tectonic theory. The horizontal motions of rigid plates of material above a pliable mantle helped explain the creation and destruction of oceans, the generation of mountain belts and sedimentary basins, the distribution of volcanic and earthquake activity, and the locations of ore, oil, and gas deposits. But plate-tectonic theorists have had difficulty accounting for many of the geological details of southwestern North America, including the uplifting of the Rocky Mountains, the extent of the Basin and Range region of Nevada, Utah, and Arizona, and the extensive volcanic deposits of the Columbia Plateau.
2) Established methods of mountain building all depend either directly or indirectly on subduction zones. Three methods are currently recognized:
a) A subduction zone may lead directly to mountains formed by ascending *magma and heat, as is the case in the Andes.
b) The subduction process may also transport *microcontinental fragments to the continental margin, where they accrete as "*terranes", a process that has added significantly to the North American West Coast over the past 400 million years.
c) When an ocean is consumed by colliding continents, as in the example of India and Asia, spectacular mountain building can result.
3) Mantle plumes produce island chains such as the Hawaiian Islands. The plume remains relatively stationary while the oceanic plate moves over it. Plumes are thought to rise all the way from the core-mantle boundary, 2,900 kilometers below the Earth's surface, in relatively narrow columns. On reaching the base of the lithosphere, a plume spreads out, underplating a large area of lithosphere, causing it to heat and be bowed upward.
4) Hot spots, the surface manifestations of mantle plumes, are widely distributed around the Earth, although the exact number is controversial. Hot spots are essentially stationary relative to the faster-moving plates. No modern ocean could be consumed at a subduction zone without a plate margin encountering a hot spot, so the interaction between subduction zones and hot spots must be common throughout geological time.
5) Most of the mountain building activity on the western margin of North America over the past 300 million years represents episodes of *magmatism and deformation associated with microcontinent collisions. The Sonoma, Nevada, and Sevier mountain-building events are examples. The Rocky Mountain (Laramide) orogeny, however, is distinctive because it is characterized by a lack of magmatism coupled with widespread deformation in the continental interior.
6) The authors propose that an additional (fourth) method of mountain building has been largely overlooked and may help explain not only the Laramide Orogeny but also other unusual geological features of the southwestern US. Their model involves the interplay of the horizontal motions of traditional subduction-related mountain-building processes with vertical plumes of hot mantle ascending from thousands of kilometers below the Earth's surface. The authors suggest that "together these mechanisms may offer a convincing explanation for what long has been a geologically puzzling part of the world and may lead to better understanding of mountain building worldwide."
American Scientist http://www.americanscientist.org
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Notes:
magma: In general, any molten mass of rock.
microcontinental fragments: In general, any fragment or remnant of continental crust up to approximately the size of Madagascar (Malagasy).
terranes: (terrains) In general, a terrane is any region of crust with well-defined margins which differs significantly in apparent tectonic evolution from neighboring regions.
magmatism: In general, the development and movement of magma within the Earth.
American Scientist http://www.americanscientist.org
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SUPERPLUMES FROM THE CORE-MANTLE BOUNDARY TO THE LITHOSPHERE: IMPLICATIONS FOR HEAT FLUX
The following points are made by B. Romanowicz and Y. Gung (Science 2002 296:513):
1) Global seismic tomography aims to improve our understanding of mantle dynamics by providing constraints on three-dimensional (3D) temperature and composition with the use of elastic velocities as proxies. Much progress has been made in recent years in resolving increasingly finer details in the 3D distribution of elastic velocities from the inversion of seismic phase and travel time data (1-3). In particular, regions of faster-than-average velocity, associated with subduction around the Pacific rim, have revealed a variety of behaviors of lithospheric slabs in the transition zone, some stagnant around the 670-km discontinuity, whereas others penetrate into the lower mantle to depths in excess of 1500 km (4). These results agree with geodynamic models in which the cold and dense down-going slabs play a driving role in global mantle circulation heated primarily from within (5).
2) The detailed morphology and role of upwellings, as manifested by two prominent zones of lower than average velocity in the lowermost mantle and commonly referred to as "superplumes", is less clear. Their location, under the south-central Pacific and under Africa, correlates with the global distribution of hot spots, as well as two major geoid highs. Recent tomographic S wave velocity models suggest that the superplumes rise high above the core-mantle boundary (CMB) (3), and joint seismic-geodynamic studies imply that they may be active upwellings. However, finer scale resolution is still lacking.
3) In summary: Three-dimensional modeling of upper-mantle anelastic structure reveals that thermal upwellings associated with the two superplumes, imaged by seismic elastic tomography at the base of the mantle, persist through the upper-mantle transition zone and are deflected horizontally beneath the lithosphere. This explains the unique transverse shear wave isotropy in the central Pacific. The authors infer that the two superplumes may play a major and stable role in supplying heat and horizontal flow to the low-viscosity asthenospheric channel, lubricating plate motions and feeding hot spots. The authors suggest that more heat may be carried through the core-mantle boundary than is accounted for by hot spot fluxes alone.
References (abridged):
1. G. Masters, S. Johnson, G. Laske, B. Bolton, Philos. Trans. R. Soc. London Ser. A 354, 1385 (1996).
2. Y. J. Gu, A. M. Dziewonski, W.-J. Su, G. Ekstroem, J. Geophys. Res. 106, 11169 (2001).
3. C. Moegnin and B. Romanowicz, Geophys. J. Int. 143, 709 (2000).
4. Y. Fukao, S. Widiyantoro, M. Obayashi, Rev. Geophys. 39, 291 (2001).
5. D. Bercovici, Y. Ricard, M. A. Richards, Geophys. Monogr. Am. Geophys. Union 121 (American Geophysical Union, Washington, DC, 2000), p. 5.
Science http://www.sciencemag.org
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