MANTLE PLUMES AND HOT SPOTS 335 Khramov AN (1987) Paleomagnetology. Berlin: Springer- Opdyke ND and Channell JET (1996) Magnetic Stratig- Verlag. raphy. San Diego: Academic Press. McElhinny MW (1973) Paleomagnetization and Plate Tarling DH (1983) Paleomagnetization. London: Geological Tectonics. Cambridge: Cambridge University Press. Society. McElhinny MW and McFadden PL (2000) Paleomagnetiza- Tauxe L (1998) Paleomagnetic Principles and Practice. tion: Continents and Oceans. San Diego: Academic Press. Dordrecht: Kluwer Academic Publishers. MANTLE PLUMES AND HOT SPOTS D Suetsugu, B Steinberger, and T Kogiso, basalt or large igneous province. Volcanic volumes Japan Marine Science and Technology Center, and age data indicate that these form during short Yokosuka, Japan time-spans with much higher eruption rates than ß 2005, Elsevier Ltd.All Rights Reserved. are found at present-day hotspots. Examples of con- tinental flood basalts (CFBs) include the Deccan Traps (associated with the Reunion hotspot) and the Parana Introduction basalts (associated with the Tristan hotspot). The Hotspots are defined as anomalous volcanism that Deccan Traps have erupted a volume of 1.5 Â 6 3 cannot be attributed to plate tectonics, unlike that 10 km within less than 1 My, whereas the present- associated with island arcs and spreading ridges. day eruption volume at the Reunion hotspot is 3 À1 Mantle plumes, which are upwelling instabilities 0.02 km year . For other tracks, older parts have from deep in Earth’s mantle, are thought to be respon- been subducted, and yet others, particularly shorter sible for hotspots that are relatively stationary, ones, begin with no apparent flood basalt. The length resulting in chains of islands and seamounts on of tracks shows that hotspots may remain active for moving oceanic plates. The volcanic rocks associated more than 100 My. For example, the Tristan hotspot with hotspots have signatures in trace elements and track indicates continuous eruption for 120 My. Nu- isotopes distinct from those observed at mid-oceanic merous shorter tracks exist as well, particularly in the ridges and island arcs. Seismic imaging has revealed south central Pacific, commonly without clear age low-velocity anomalies associated with some deep- progression. This may indicate either that the region rooted hot mantle plumes, but images of their full- is underlain by a broad upwelling or that widespread depth extent are of limited resolution, thus evidence flow from a plume is occurring beneath the litho- for plumes and hotspots is primarily circumstantial. sphere, with locations of volcanism controlled by Commonly, it is not even clear which areas of intra- lithospheric stresses. Geometry and radiometric age plate volcanism are underlain by a mantle plume and data of hotspot tracks indicate that the relative motion should be counted as a hotspot. of hotspots is typically slow compared to plate motions. However, for the Hawaiian hotspot between Surface Expression of Hotspots 80 and 47 million years ago (Ma), inclination of the magnetization of volcanics indicates formation at a The primary surface expression of mantle plumes con- palaeolatitude further north than Hawaii, with hot- sists of hotspot tracks. These are particularly evident spot motion southward of several centimetres per in the oceans as narrow (100 km) chains of islands year. The Hawaiian–Emperor bend may therefore rep- and seamounts, such as the Hawaiian–Emperor chain, resent more than a change in Pacific plate motion. In or as continuous aseismic ridges, such as the Walvis most other cases in which palaeolatitude data are Ridge, up to several kilometres high. These tracks are available, inferred hotspot motion is slow or below thought to form as lithospheric plates move over detection limit. Associated with many tracks is a hot- plumes. The active hotspot is at one end of the chain; spot swell (1000 km wide, with up to 3 km anom- radiometric dating has determined that the ages of the alous elevation). Swells are associated with a geoid volcanics along the chain tend to increase with dis- anomaly. Swell height slowly decreases along the tance from the active hotspot. Interpretation of age track away from the active hotspot, and the swell data is complicated, because volcanics do not neces- also extends a few 100 km ‘upstream’ from the hot- sarily erupt directly above a plume. Late-stage volcan- spot. The geoid-to-topography ratio remains approxi- ism may occur several million years (My) after passage mately constant along swells, and this value indicates over a plume. Many hotspot tracks begin with a flood isostatic compensation at depths 100 km. From the 336 MANTLE PLUMES AND HOT SPOTS product of swell cross-sectional area and plate speed volcanics produced at mid-ocean ridges. Magma pro- relative to the hotspot, plume anomalous mass flux duction at mid-plate hotspots thus appears less efficient (volume flux times density anomaly of plume relative than at mid-ocean ridges. However, during formation to ambient mantle - this is the quantity that is directly of large igneous provinces, magma production rate and estimated from observations), volume flux (volume heat flow were higher than at present. flowing through plume conduit per time unit), and Hotspots cluster in two antipodal regions around heat flux, (volume flux density times heat capacity the Pacific and Africa. They are mostly absent from times temperature anomaly of plume relative to ambi- regions where subduction has occurred in the past ent mantle), can be estimated. For Hawaii, the hotspot 100 My, and tend be in highs of the ‘residual’ geoid track and swell are both evident (Figure 1). The anom- (actual geoid minus contribution of subducted slabs). alous mass flux determined for Hawaii is the largest Probably the hotspot plumes are not the primary of any plume. Its estimated heat flux corresponds to cause of these geoid highs. Rather, both may be 16% of global hotspot heat transport; global hot- due to the same cause: a less dense and presumably spot heat transport in turn is estimated to be 5% of hotter lower mantle in these regions. Plate boundar- total global heat flux. ies, in particular mid ocean ridges, may move across Direct measurements of heat flow above hotspot hotspots; hotspots may hence, successively or simul- swells yield only small anomalies (5–10 m W mÀ2) taneously, leave tracks on different plates. Examples or no anomalies at all. This may indicate that the are the Reunion hotspot, with tracks on the Indian swell is caused by buoyant uplift due to hot plume and African plates, and the Tristan hotspot, with material spreading beneath the lithosphere, rather tracks on the South American and African plates. than by thermal erosion and heating of the litho- If a plume is located close to a spreading ridge, erup- sphere. The amount of volcanics produced at hot- tion of plume material may occur not only directly spots is estimated to be <1km3 yearÀ1, &5% of above the plume, but also at the section of the ridge Figure 1 Topographic map of the North Pacific, showing both the swell and the track associated with the Hawaiian plume. MANTLE PLUMES AND HOT SPOTS 337 close to the hotspot. Elongated aseismic ridges may be ridges. There is a distinct difference between hotspots formed by volcanics above channels along which and ridges, concerning the depth extent of these low- plume material flowed to the spreading ridge (for velocity regions : low seismic velocities beneath hot- example, in the Musicians Seamounts near 200 E, spots extend to a depth of 200 km, whereas low 25 N) (Figure 1). Hotspots and their tracks are less seismic velocities beneath ridges are confined to the obvious on continents. The Yellowstone National upper 100 km. This suggests that hotspots are caused Park area in the United States is frequently regarded by active upwellings (mantle plumes) with deeper as a hotspot plume, with the Snake River Plain being sources, compared to ridges, which may be caused its hotspot track. by passive upwelling. The 200-km depths of the slow velocities under hotspots do not necessarily corres- Seismic Images of the Mantle Plumes pond to the actual source depths of mantle plumes, but rather to the depths to which surface waves can Column-like anomalies of low seismic velocity asso- resolve. Seismic array observations have been carried ciated with high temperatures are expected under out in hotspot regions to resolve fine structures such hotspots, if hotspots are the surface expression of as plume conduits. A recent example of an S-velocity mantle plumes. Seismic imaging of hotspots has model beneath the Icelandic hotspot was obtained advanced in the past decade, and seismic images from body and surface wave data recorded by a tem- beneath some hotspots have been obtained. Com- porary seismic array; a low-velocity plume can be monly, these are imaged only at specific depths. For seen beneath the hotspot (Figure 2). A 200-km-thick some hotspots, no low-velocity anomalies have been low-velocity zone extends laterally beneath Iceland; a found. vertical column of low velocities under central Iceland extends to a depth of at least 400 km. Similar array Upper Mantle observations carried out at other hotspots (e.g., Global mapping of the upper mantle by long-period Hawaii, Yellowstone, and Massif Central) detected (50–300 s) surface waves has revealed low seismic low-velocity anomalies extending to sublithospheric velocities associated with hotspots and spreading depths in the upper mantle. Figure 2 The S-velocity profile beneath the Icelandic hotspot.Reproduced with permission from Allen RM, Nolet G, Morgan WJ, et al. (2002) Imaging the mantle beneath Iceland using integrated seismological techniques. Journal of Geophysical Research—Solid Earth 107(B12): 2325, doi:10.1019/2001 JB000595. 338 MANTLE PLUMES AND HOT SPOTS The Transition Zone the discontinuities, to be 20–30 km thinner than it is in the surrounding area (an area 500 km in diam- The mantle transition zone is bounded by seismic eter), suggesting temperatures that are 150–250 K discontinuities at depths around 410 and 660 km hotter (Figure 3).