P ERSPECTIVES GEOLOGY The apparent controversy can be broken down into two questions. Is there evidence that deep mantle plumes exist? And do all volcanoes not associated with plate bound- Deep Origin of Hotspots— aries require a deep mantle plume? The an- swers seem most likely to be “yes” and “no,” respectively. the Mantle Plume Model Several observations support a deep (that Donald J. DePaolo and Michael Manga is, plume) origin for some hotspots. Mid- 0 9 ocean ridges are able to migrate over 1 0 0 2 0 he workings of the hot interiors of the 1 hotspots without changing the hotspot 2 0 2 rocky planets of the solar system are 2 track, which implies that the hotspot most dramatically expressed by the source is deeper than about 200 km. T 30 size and arrangement of their volcanoes. At Hawaii, the high magma produc- Depth 20 Most volcanoes on Earth are a result of 10 km tion rate in a small area requires an plate tectonics. At mid-ocean ridges, the 0 upwelling velocity of ~50 cm/year spreading of the ocean floor generates up- 300 (6), about 10 times the average veloc- ward flow of hot mantle rock beneath the ity of plates. The Hawaiian upwelling ridge. This flow generates magma as a re- must therefore be distinct from flow sult of adiabatic decompression (1). At 650 associated with plate motions. subduction zones, plates returning to the The upwelling mantle under depths of the mantle carry water down in Hawaii must also be 200 to 300 K hot- hydrous minerals. The water, when re- ter than the surrounding mantle to leased by metamorphism, causes already 1000 achieve the required large melt frac- hot rock material beneath island arcs to tions at depths below the 80-km-thick melt (2). lithosphere (6). Such hot rock materi- But not all volcanoes on Earth are lo- al must come from a thermal bound- cated at mid-ocean ridges or subduction ary layer. The core-mantle boundary zones. “Hotspots”—regions with particu- 1450 is the most likely source, unless there larly high rates of volcanism—are not nec- is another interface within the mantle essarily associated with plate boundaries. between compositionally distinct lay- Hawaii, the premier example, is thousands ers. The chemistry and isotopic com- of kilometers from the nearest plate bound- position of many hotspot lavas, espe- ary yet exudes lava at a higher rate per unit cially the high 3He/4He ratios, indi- 1900 area than at any other place on Earth. The cate that the hotspots sample a part of Hawaiian volcanic anomaly has remained the mantle distinct from that sampled mostly stationary for tens of millions of by mid-ocean ridge basalts (7). years and produced a 6000-km-long chain Numerical simulations of plumes re- of islands and seamounts. This phenome- 2350 produce many of the geophysical ob- non is not explained by plate tectonics. It servations (6), such as the rate of requires a separate mantle process that can magma production and the topogra- account for narrow, long-lived upwellings phy and gravity anomalies produced of unusually hot mantle rock. by plume material as it spreads be- Shortly after the discovery of plate tec- 30 2800 neath the lithosphere. 1 20 tonics in the late 1960s, Morgan (3) pro- 90 Theoretical and laboratory studies 2 00 2 10 posed that hotspots represent narrow (100 10 of fluids also predict that plumes 22 km diameter) upwelling plumes that origi- 0 0 should form in the deep Earth. nate within the lower mantle. Since that Mapping deep plumes. Maps of P wave velocity anom- Because the core is much hotter than time, evidence from geophysics, fluid dy- alies (red, slow; blue, fast) under the Hawaiian Islands the mantle, heat conducted from the namics, petrology, and geochemistry has obtained with finite-frequency tomography, which cor- core warms the base of the mantle, supported if not required the existence of rects for the effects of wavefront healing (16). The red forming a thermal boundary layer. As mantle plumes. For many geoscientists, the and yellow areas are interpreted as regions of anom- this layer thickens, it can become mantle plume model is as well established alously high temperature in the mantle. These images gravitationally unstable. The hot as plate tectonics. suggest that the Hawaiian mantle plume can be traced buoyant boundary layer should then Nonetheless, it is reasonable that we with seismological techniques all the way down to the rise, forming a large mushroom- should want to verify the model by direct core-mantle boundary, at a depth of about 2900 km. shaped plume “head” followed by a observation. The only way we can “see” in- narrow plume “tail” (8). The head is to the deep Earth is with seismology. This stamp that most thought it would. thought to form large flood basalt endeavor has so far not produced the rubber Seismological studies of the Yellowstone provinces once it reaches Earth’s surface, hotspot found no clear evidence for a lower whereas the tail provides a conduit through The authors are in the Department of Earth and mantle source (4), while evidence of a deep which hot mantle continues to flow to the Planetary Science, University of California, Berkeley, plume beneath the Iceland hotspot remains hotspot (9). CA 94720, USA. D. J. DePaolo is also in the Earth Sciences Division, Lawrence Berkeley National equivocal (5). Does the model need re- The persistence of flow through the tail Laboratory, Berkeley, CA 94720, USA. E-mail: depaolo@ thinking, or are the seismological tools still for 100 million years or more (several eps.berkeley.edu not quite up to the task, or perhaps both? times the number of years required for CREDIT: G. UNIVERSITY NOLET/PRINCETON 920 9 MAY 2003 VOL 300 SCIENCE www.sciencemag.org P ERSPECTIVES plume heads to rise through the mantle) plumes or superswells. They require other achieved with standard techniques is too implies that the plume is much less viscous models that are not yet generally agreed poor for this task. However, preliminary re- than the surrounding mantle (10). In a upon. sults (16) suggest that image resolution can planet with plate tectonics such as Earth, The mantle plume model has implica- be improved using alternative data reduc- cooling of the mantle by the subduction of tions beyond accounting for the spatial dis- tion approaches, and that the deep roots of tectonic plates allows large variations of tribution of volcanism. If plumes come mantle plumes can already be resolved viscosity to exist at the base of the mantle, from the base of the mantle, then the erupt- with available data (see the figure). and hence between the plume and its sur- ed lavas from hotspot volcanoes may carry roundings (11). clues about the workings of the deepest References Deep mantle plumes may not be the mantle and even the core. Plumes provide a 1. D. J. McKenzie, J. Petrol. 25, 713 (1984). cause of all hotspots. Courtillot et al. (12) connection between geochemical and iso- 2. Y. Tatsumi, S. Eggins, Subduction Zone Magmatism (Blackwell Science, Boston, 1995). argue that the main features predicted by topic reservoirs (inferred from studies of 3. W. J. Morgan, Geol. Soc. Am. Mem. 132,7 (1972). the plume model are clearly evident in on- lavas) and seismological structures imaged 4. E. D. Humphreys, K. G. Dueker, D. L. Schutt, R. B. Smith, ly seven hotspots—including Hawaii and within the mantle. The origin of hotspots is GSA Today 10,1 (2000). Iceland, but not Yellowstone. Criteria used therefore linked with our ability to inte- 5. R. M. Allen et al., J. Geophys. Res. 107, 2325 to recognize plumes include the presence grate geochemical and seismological ob- (2002). 6. N. M. Ribe, U. R. Christensen, Earth Planet. Sci. Lett. of a hotspot track and an associated flood servations with geodynamic models. 171, 517 (1999). basalt province, a large buoyancy flux (the Moreover, plumes potentially provide a 7. P. E. Van Keken, E. H. Hauri, C. J. Ballentine, Annu. Rev. product of the volume flux through the constraint on the heat flux from the core Earth Planet. Sci. 30, 493 (2002). plume and the density difference between (14) and hence insight into the energy 8. R. W. Griffiths, Phys. Earth Planet. Inter. 43, 261 (1986). the plume and its surroundings), a high budget for the core dynamo that generates 9. M.A. Richards, R.A. Duncan,V. Courtillot, Science 246, 3 4 He/ He ratio, and a monotonic age pro- Earth’s magnetic field (15). 103 (1989). gression in the chain of volcanoes. Many natural phenomena were deduced 10. N. H. Sleep, M. A. Richards, B. H. Hager, J. Geophys. Superswells—regions of the lower mantle correctly from indirect effects before in- Res. 93, 7672 (1988). 11. H.-C. Nataf, Tectonophysics 187, 261 (1991). beneath Africa and the Pacific that are struments were developed with sufficient 12. V. Courtillot,A. Davaille, J. Besse, J. Stock, Earth Planet. characterized by low seismic wave velocity sensitivity to verify their existence directly. Sci. Lett. 205, 295 (2003). (13)—are inferred to be broad, hot up- Direct evidence for mantle plumes will re- 13. B. Romanowicz, Y. C. Gung, Science 296, 513 (2002). wellings. Secondary, weaker plumes are quire seismic imaging at resolution suffi- 14. G. F. Davies, Geophys. J. Int. 115, 132 (1993). 15. B. A. Buffett, Geophys. Res. Lett. 29, 1566 (2002). proposed to form in the mantle from these ciently high to detect narrow conduit-like 16. R. Montelli, G. Nolet, G. Masters, F. A. Dahlen, S.-H.