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Ridge-Hotspot Interactions What Mid-Ocean Ridges Tell Us About Deep Earth Processes Ceanography Society

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Ridge-Hotspot Interactions What Mid-Ocean Ridges Tell Us About Deep Earth Processes Ceanography Society This article has This been published in or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The approval portionthe ofwith any permitted articleonly photocopy by is of machine, reposting, this means or collective or other redistirbution S P E C I A L I ss U E F E AT U R E Oceanography , Volume 20, Number 1, a quarterly journal of The 20, Number 1, a quarterly , Volume RIDGE-HOTSPOT InTERACTIOns What Mid-Ocean Ridges Tell Us O About Deep Earth Processes Society. ceanography C BY JÉRÔME DYME N T, J I A N L I N , A nd Ed WAR D T. B A K E R The 2007 by opyright Earth is a thermal engine that dissipates its internal heat primarily through convec- tion. The buoyant rise of hot material transports heat to the surface from the deep O ceanography Society. Society. ceanography interior while colder material sinks at subduction zones. Mid-ocean ridges and hot- O ceanography Society. Send all correspondence to: [email protected] or Th e [email protected] Send Society. ceanography to: correspondence all spots are major expressions of heat dissipation at Earth’s surface, as evidenced by their abundant volcanic activity. Ridges and hotspots, however, could differ significantly in their origins. Ridges are linear features that wind more than 60,000 km around the A ll rights reserved. globe, constituting the major diverging boundaries of Earth’s tectonic plates. Hot- spots, on the other hand, are localized regions of abnormally robust magmatism and distinctive geochemical anomalies (Figure 1). P The causes of hotspots and their depths of origin are the focus of an intense article use for research. and this copy in teaching to granted is ermission debate in the scientific community. The “plume” model hypothesizes rising of buoy- ant mantle plumes as the primary cause of prominent hotspots such as Iceland and Hawaii (Morgan, 1971). In contrast, the “anti-plume” school argues that many of the observed “hotspot” volcanic and geochemical anomalies are simply due to melts leak- ing through tensional cracks in Earth’s lithospheric plates—in other words, hotspots O ceanography Society, Society, ceanography reflect only where the lithospheric plate is cracked, allowing melts to pass through, and not where the underlying mantle is hotter (see www.mantleplumes.org). A hybrid notion is that only a relatively small number of hotspots, especially those of enormous magmatic volumes, have their origin in buoyant thermal plumes rising from the deep PO mantle (e.g., Courtillot et al., 2003). Regardless of its specific depth of origin, however, Box 1931, when a hotspot is located close enough to a mid-ocean ridge, the two volcanic systems R ockville, R will interact, resulting in unique volcanic, geochemical, and hydrothermal features. In reproduction, systemmatic epublication, this paper, we discuss major features of hotspot-ridge interactions. M D 20849-1931, U S A . 102 Oceanography Vol. 20, No. 1 -180° -150° -120° -90° -60° -30° 0° 30° 60° 90° 120° 150° 180° Iceland 60° Bowie Cobb Yellowstone Azores Bermuda 30° Meteor Canary Hoggar Hawaii Cape Verde Afar Caroline 0° Fernando Galapagos Comores Samoa Marquesas Ascension St. Helena Society Pitcairn TTrrinidadeinidade Reunion Easter San Felix -30° Macdonald E. Australia Juan Fernandez TTrristanistan St. Paul Foundation Amsterdam Gough Discovery Crozet Tasmantid Marion Kerguelen Louisville Shona Heard -60° Bouvet Balleny 150° 180° -150° -120° -90° -60° -30° 0° 30° 60° 90° 120° 150° 90° 90° JM Ic 60° 60° Bo Co Az 30° Gu 30° Af 0° 0° Ga As SH Re ES -30° Tr -30° Go SA Lo Sh Ma Cr Ke -60° Ba Bv -60° 150° 180° -150° -120° -90° -60° -30° 0° 30° 60° 90° 120° 150° 87Sr/86Sr Residual Bathymetry (km) -0.5 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Figure 1. (Top) Map of the world’s major hotspots (orange circles) showing that many of them are integrally connected to the global mid-ocean ridge systems (red lines) (Lin, 1998). (Bottom) Map of residual bathymetry of the ocean basins and 87Sr/86Sr geochemical anomalies from samples collected along the mid-ocean ridges and ocean islands (Ito et al., 2003). A positive residual bathymetry marks anomalously shallow seafloor relative to the theoretical prediction of Stein and Stein (1992). Circles mark rock sample locations and are colored according to 87Sr/86Sr value. Hotspots are shown by stars, and hotspots influencing mid-ocean ridges are labeled: Af = Afar, As = Ascension, Az = Azores, Ba = Balleny, Bo = Bowie, Bv = Bouvet, Co = Cobb, Cr = Crozet, ES = Easter/Sala y Gomez, Ga = Galápagos, Go = Gough, Gu = Guadalupe, Ice = Iceland, JM - Jan Mayen, Ke = Kerguelen, Lo = Louisville, Ma = Marion, Re = Reunion, SA = St. Paul- Amsterdam, Sh = Shona, SH = St. Helena, Tr = Tristan de Cunha. Oceanography March 2007 103 MULTIDISCIPLINARY modeling and laboratory-based physical underwater plateaus, or volcanic islands APPROACHES ARE EssENTIAL experiments play an equally critical role rising from the seafloor (Figure 2). The Ridge-hotspot interactions illustrate in shaping our thinking on the physical elevated topography near a hotspot is the important thermal and geological pro- processes of these systems. Here we illus- direct result of thickening of the oceanic cesses and provide unique windows into trate how commonly used observational crust both by erupting magmas on top the chemical composition and hetero- approaches help to advance understand- of it and intruding magmas near its base. geneities of Earth’s mantle. To best un- ing of ridge-hotspot interaction. The active upwelling of hotter mantle derstand these processes, it is essential plumes can also lead to the development to adopt multidisciplinary approaches Bathymetry of long-wavelength seafloor topographic and to analyze and interpret observa- The influence of hotspots on mid-ocean swells, as observed in some hotspot-ridge tional constraints within the framework ridges can be seen most clearly in un- systems (e.g., Sleep, 1990; Canales et al., of conceptual models of ridge-hotspot usual bathymetry, including shallower- 2002). It has also been observed that a) d) interactions. Meanwhile, computational than-normal ridge-axis seafloor depth, ridge segments most influenced by hot- 70˚ IcelanIcelandd 70˚ 30˚ 30˚ Ridge lantic 65˚ 65˚ 35˚ 35˚ Mid-At Figure 2. Maps of predicted seafloor bathymetry (Smith a) d) and Sandwell, 1997) and corresponding tectonic interpre- Azores 70˚ IcelanIcelandd 70˚ 60˚ 60˚ tations of seven ridge-hotspot systems in oblique Merca- 30˚ 30˚ tor projection so that all maps are at the same scale and40˚ 40˚ kjanes Ridge Ridge the spreading direction is horizontal. A large arrow in each ReykjanesRey Ridge bathymetric map indicates north. (a) Iceland hotspot lantic 33 33 325˚ 33 3 3 33 325˚ 32 3 20 1 5 1 0˚ 5˚ 0˚ 0 5 65˚ 5 ˚ 65˚ ˚ ˚ ˚ 55˚ 55˚ and the Reykjanes Ridge. (b) Galápagos hotspot and the ˚ 35˚ 35˚ Mid-At 325˚ 330˚ 335˚ 340˚ 325˚ G330alápagos335˚ Spreading34 Center. (c) Reunion hotspot and the 0 e) ˚ ˚ Central Indian Ridge. (d) Azores hotspot and the M-30id-˚ -30˚ b) Atlantic Ridge. (e) Foundation seamount chain and the Azores 60˚ 60˚ Pacific-Antarctic Ridge. (f) St. Paul-Amsterdam hotspot and ntarctic Foundation Seamounts 40˚ 40˚ 265˚ 265˚ A the Southeast Indian Ridge. (g) Marion hotspot and the kjanes Ridge Ridge Center 5˚ ˚ Galapagos Southwest Indian Ridge.-3 ReykjanesRey Ridge -35 230 230˚ ng ˚ 33 33 325˚ 33 3 3 33 325˚ 32 3 Pacific Antarctic 20 1 5 1 0˚ 5˚ 0˚ 0 5 ˚ 5 ˚ ˚ ˚ ˚ ˚ 24 2 250˚ 250 ading 230˚ 235˚ 500 55 55˚ 230 235˚ 2 2 45 40 45 -3 0 ˚ ˚ 0 ˚ ˚ ˚ ˚ ˚ -3 325˚ 330˚ 335˚ 340˚ 325˚ 0˚ 330 335˚ 34 km 270˚ 270˚ 0 e) ˚ ˚ -30˚ -30˚ f) ˚ b) ˚ 90 90 500 km St Paul Foundation Seamounts ntarctic 265˚ 265˚ A 275˚ 275˚ Galapagos SpreadiSpre AmsterdaAmsterdamm Ridge Southeast Center ˚ Galapagos -35 -35˚ 0˚ 230 5˚ 0˚ 230˚ 5 ˚ Indian ng ˚ Pacific Antarctic 85˚ Ridge 85˚ ading 230˚ 235˚ 24 2 250˚ 250 500 230 235˚ 2 2 45 40 45 -3 0 c) ˚ ˚ 0 ˚ ˚ ˚ ˚ -2 -3 ˚ -3 ˚ -25˚ ˚ - ˚ -35˚ ˚ ˚ -3 5˚ 0 5 30˚ 0˚ idge km 80˚ 270˚ ˚ 75 ˚ 70 270˚ 80 75 70 R f) ReunioReunionn g) Southwest IndiIndian ˚ ˚ -2 -2 90 90 5˚ 5˚ 500 km 35˚ 35˚ St Paul 275˚ 275˚ an Ridge Galapagos SpreadiSpre AmsterdaAmsterdamm ˚ Southeast 5˚ 55 5 Ridge 0˚ 5˚ Marion 0˚ 5 Central Indian Ridge ˚ Indian -25 -2 -25 -20 ˚ ˚ ˚ ˚ 0˚ ˚ ˚ ˚ 40 40˚ 0 0 ˚ 6 65˚ 70 6 65˚ 70 85˚ Ridge 85˚ c) -2 -3 ˚ -3 ˚ -25˚ ˚ - ˚ -35˚ ˚ 5˚ 0 5 30˚ idge 80˚ ˚ 75 ˚ 70 80 75 70 R 45˚ 45˚ ReunioReunionn g) Southwest IndiIndian -2 -50 -45 -40˚ - - -3 -2 -50˚ -4 5˚ 35 5˚ 40 5 5 ˚ ˚ ˚ ˚ ˚ ˚ 35˚ 35˚ an Ridge ˚ 5˚ hotspot location 55 5 Marion Ridge Central Indian Ridge -25 -2 -25 -20 ˚ ˚ ˚ ˚ 0˚ ˚ ˚ ˚ 40 40˚ 0 0 ˚ recent hotspot plateaus / islands 6 65˚ 70 6 65˚ 70 hotspot plateaus / island chains 45˚ 45˚ 104 Oceanography Vol. 20, No. 1 -6000 -5000 -4000 -3000 -2000 -1000 0 1000 2000 m -50 -40˚ - -45 -50˚ -4 - -3 35 40 volcanic continental margins 5 5 ˚ ˚ ˚ ˚ ˚ ˚ other plateaus / seamounts high axial bathymetry hotspot location ridge : axial high recent hotspot plateaus / islands ridge : axial valley hotspot plateaus / island chains ridge : atypical -6000 -5000 -4000 -3000 -2000 -1000 0 1000 2000 m volcanic continental margins fossil ridge other plateaus / seamounts transform fault / fracture zone other segment discontinuities high axial bathymetry volcanic ridges ridge : axial high other tectonic elements ridge : axial valley ridge : atypical fossil ridge transform fault / fracture zone other segment discontinuities volcanic ridges other tectonic elements a) d) 70˚ IcelanIcelandd 70˚ 30˚ 30˚ Ridge lantic 65˚ 65˚ 35˚ 35˚ Mid-At Azores 60˚ 60˚ 40˚ 40˚ kjanes Ridge ReykjanesRey Ridge 33 33 325˚ 33 3 3 33 325˚ 32 3 20 1 5 1 0˚ 5˚ 0˚ 0 5 ˚ 5 ˚ ˚ ˚ 55˚ 55˚ ˚ 325˚ 330˚ 335˚ 340˚ 325˚ 330 335˚ 34 0 e) ˚ ˚ -30˚ -30˚ b) Foundation Seamounts ntarctic spots commonly elongate (“propagate”) and Forsyth, this issue).
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