Hotspots, Mantle Plumes, Flood Basalts, and True Polar Wander

Home , Iceland hotspot, Kerguelen hotspot, Louisville hotspot, Mantle plume, New England hotspot, Tristan hotspot

HOTSPOTS, MANTLE PLUMES, FLOOD BASALTS, AND TRUE POLAR WANDER

R. A. Duncan M. A. Richards Collegeof Oceanography Departmentof Geology OregonState University, Corvallis Universityof California,Berkeley

Abstract. Persistent,long-lived, stationarysites of lithospherethe plume head flattens and melts by excessivemantle melting are called hotspots. Hotspots decompression,producing enormous quantifies of magma leave volcanictrails on lithosphericplates passing across which erupt in a short period. These are flood basalt them. The global constellationof fixed hotspotsthus events that have occurred on continents and in ocean formsa convenientframe of referencefor plate motions, basinsand that signal the beginningof major hotspot throughthe orientationsand age distributionsof volcanic tracks. The plume-supportedhotspot reference frame is trailsleft by thesemelting anomalies. Hotspots appear to fixed in the steadystate convective flow of the mantleand be maintainedby whole-mantleconvection, in the form of is independentof the core-generated(axial dipole) upwardflow throughnarrow plumes. Evidencesuggests paleomagneticreference frame. Comparisonof plate that plumesare deflectedlittle by horizontalflow of the motions measured in the two frames reveals small but uppermantle. Mantle plumesare largelythermal features systematicdifferences that indicatewhole-mantle motion and arise from a thermalboundary layer, most likely the relativeto theEarth's spin axis. This is termedtrue polar mantle layer just above the core-mantle boundary. wanderand has amounted tosome 12 ø sinceearly Tertiary Experimentsand theoryshow that gravitationalinstability time. The directionand magnitudeof true polar wander drivesflow, beginningwith the formationof diapirs. Such havevaried sporadically through the Mesozoic,probably a diapir will grow as it rises, fed by flow throughthe in responseto majorchanges in platemotions (particularly trailing conduitand entrainmentof surroundingmantle. subductionzone location) that change the planet's The structurethus develops a large, sphericalplume head moments of inertia. and a long, narrow tail. On arrival at the base of the

1. INTRODUCTION their geometry and age distributions these volcanic provincesare thoughtto resultfrom focusedupper mantle The plate tectonicsmodel has been largely successful in zonesof meltingcalled hotspots [Wilson, 1963, 1965] that explainingthe distributionof basalticvolcanism on our remainstationary as theEarth's outer shell of lithospheric planet. Mid-ocean ridge basalts are produced by plates move acrossthem. The proposedlocations of decompressionmelting of passively upwelling upper presenthotspot volcanism are shownin Figure1. mantle(asthenosphere) at the edgesof separatingplates, Morgan [1971, 1972] proposedthat hotspotsare while subductionof oceanic lithospherecauses upper maintainedby unusuallywarm materialrising from the mantlehydration and melting,yielding volcanic arc basalts lower mantle through narrow conduitsthat he called along collisionalplate boundaries. Such plate margin mantleplumes. He furtherspeculated that these plumes environmentsaccount for the vast majorityof all magma comprisedthe upwardflow of a long-lived,stable pattern production.A volumetricallyminor but significantclass of whole-mantleconvection (Figure 2). Onceestablished, of basalticvolcanism occurs within plates or crossesplate theseconduits for heat and materialtransport from the boundaries,and it is characterizedby linear chainsof lowermantle do not, Morganproposed, change position volcanoes(hotspot lineaments) that grow older in the relativeto one another. Plumesprobably initiate in the directionsof plate motion. Classic examplesof this seismicallydefined thermal layer at theboundary between volcanicphenomenon are the HawaiianIslands (and other the core and mantle. Convection in the lower mantle is parallel island chains in the Pacific Ocean), the likely to be very sluggish[Gurnis and Davies, 1986] so Yellowstone-SnakeRiver Plain province,and the volcanic plumes rising through this region should not be platformand ridge system centered on Iceland. Becauseof "deflected,"or drift with respectto one another. Convec-

Copyright1991 by the AmericanGeophysical Union. Reviewsof Geophysics,29, 1 / February1991 pages31-50 8755-1209/91/90RG-02372 $05.00 Papernumber 90RG02372 .31. 32 ß Duncan and Richards: HOTSPOT VOLCANISM 29, 1 / REVIEWSOF GEOPHYSICS

Erebus

Figure 1. Worldwidesystem of hotspots.Criteria defining are concentratedin zonesof high geoid residuals(contours in hotspotsare excess volcanism and a spatialprogression in the age metersare from Croughand Jurdy [ 1980]). The geoidanomalies of volcanismconsistent with localplate motion. Magmaproduc- alsocorrelate with regions of slowW in thelower mantle tionrates vary both from one hotspot to anotherand along indi- [Morelli and Dziewonski, 1987], suggestingthat hotspotsare vidualhotspot tracks. Hotspots are not randomly distributed, but connectedwith mantle-wideupward convection.

tionrates in the uppermantle are muchlarger, of theorder of platevelocities (10-100 mm/yr),so the horizontal upper flood basalts mantle flow might bend plumeson shortertime scales [Whiteheadand Luther, 1975]. If mantleplumes do indeedform a long-lived,stable

patternof convectionof heatand material from the lower ho ' • plate to the uppermantle, then the geographicorientations and tra'i•ts ot ductton' age distributionsof volcanicchains related to hotspots offer a very simpleand directrecord of the directionand velocityof lithosphericplate motionsover the last 100 m.y.(million years) or more. Ratesand directions of plate separationderived from the alternatingnormal and reversely magnetizedstripes of seafloor parallel to spreadingridges measurepast large-scalehorizontal Figure 2. Cartoonof whole-mantleconvection. Piecesof the movementsof piecesof theEarth's surface relative to their rigid outershell of the Earth(lithosphere) separate at spreading ridgesand collideat subductionzones, coupled with horizontal neighbors.This is calleda relativemotion reference frame flow of the uppermantle. The deepmantle circulates through becausethe spreadingridges from which motion is thermalplumes of gravitationallyunstable (warmer, less dense) measuredare themselvesmoving over the mantle,albeit materialthat arisefrom nearthe core-mantleboundary. Flood moreslowly than plates. A thirdreference frame for plate basaltsform as the leading"head" of a new plumemelts at the motionsis providedby theEarth's magnetic field, which is baseof thelithosphere. Hotspot trails form as plates move across assumedto have the shape(on average)of a dipolefield long-livedplume "tails." Convectionin the coreproduces the alignedalong the spinaxis. Rocksthat are magnetizedin Earth'smagnetic field. 29, 1/REVIEWSOF GEOPHYSICS Duncan and Richards: HOTSPOT VOLCANISM ß 33 this field acquirea magneticinclination (angle of the rock Morgan [1972] first notedthe congruenceof the major magneticvector from horizontal)unique to their latitude. island and seamountchains on the largestand fastest- Thus any translatitudinal(north-south) motion of litho- movingPacific plate (Hawaiian-Emperor,Tuamotu-Line, sphericplates can be determinedfrom the paleomagnetic and Austral-Gilbert-Marshall island and seamount referenceframe; east-west motions are not foundby this lineaments).The geometryof thesevolcanic traces can be method. Comparisonof plate motionsdeduced from the reasonablywell matched by rigid plate motion over hotspotand paleomagneticreference frames indicatesa hotspotsfixed at the presentlocations of Hawaii, Easter smallshift of the wholemantle with respectto the Earth's Island,and MacdonaldSeamount, respectively. Subse- spinaxis since early Tertiary time (55-65 Ma). quentstudies, incorporating the distributionof the ageof In this paper we review the evidencefor interhotspot volcanismas well as the geometryof the traces[Clague motion and examine the questionof whether hotspot and Jarrard, 1973; Jarrard and Clague, 1977; Duncan lineamentsconstitute a usefulframe of referencefor plate and McDougall, 1976; McDougall and Duncan, 1980; motions. This issue also pertainsto the geometryand Duncan and Clague, 1985; Lonsdale, 1988], have dynamicsof convectiveflow in the mantle,which in turn confirmedthe fixedpositions of Pacifichotspots over the constrainestimates of the viscositystructure of the mantle. past65 m.y. Morgan [1981] andDuncan [1981] showed Many hotspotsappear to have begun with massive that a similarcase can be made for rigid motionof the outpouringsof basalticmagmas over broadregions, both Africanplate over a fixed set of hotspotsunderlying the on continents and in ocean basins. These volcanic central and South Atlantic and western Indian ocean basins provincesare calledflood basaltsand probablymark the since120 Ma (Figure3). arrivaland initial meltingof the"heads" of mantleplumes Minster et al. [1974] and Minster and Jordan [1978] at the baseof the lithosphere.Subsequent volcanism over foundno detectableinterhotspot movement for the past the plume conduits(or "mils"), establishedby the initial 5-10 m.y., on the basisof a best fitting global set of plume upwelling,are the familiar tracksthat connectflood relativeplate motionstied to Pacific hotspots. Early basaltsto presentlyactive hotspots. comparisonsof Atlantic with Indian and Pacific Ocean hotspots,on the other hand,reported relative motionsof hotspotsup to 10-20 mm/yrduring Tertiary time [Burkeet 2. ARE HOTSPOTS STATIONARY? al., 1973; Molnar and Atwater, 1973; Molnar and Francheteau,1975]. However,improved relative plate An importantquestion concerning hotspot volcanism is motioninformation and betterage resolutionof the older whether or not mantle plumes are, in fact, fixed with ends of hotspottraces led Morgan [1981, 1983] and respectto oneanother and thus define an irregularbut rigid Duncan [1981] to conclude that movement between North referenceframe. Some amountof interplume(hotspot) Atlantic and Indian Ocean hotspotsmust be less than 5 motion might be expectedbecause of viscouscoupling mm/yr. Molnar and Stock[1987] haverecently revived betweenthe base of the movinglithospheric plates and the this debateby proposingthat the Hawaiianhotspot has uppermantle owing to horizontalflow of the uppermantle movedsouthward 10-20 mm/yr(km/m.y.) with respect to awayfrom spreadingcenters (Figure 2). If theseperturba- hotspottraces in the Atlanticand Indianoceans. By tions are small or constantover long periods, then startingwith the Pacificplate hotspottraces, however, all interplumedrift may be significantlyless than plate plate circuitsto platesbordering the Indian or Atlantic velocities,and hotspotswith associatedvolcanic traces oceansmust include Antarctica. Molnar and Stock[1987, constitute a convenient and direct reference frame for p. 587] specifically"neglect deformation between East and reconstructingplate motions, independent of the West Antarctica." This contradictspaleomagnetic paleomagneticreference frame. evidencethat East and West Antarcticahave experienced The magnitudeof interplumemotion can be assessedby differentialrotations since the Early Cretaceous [Watts and comparingthe geometryand age distributionof volcanism Bramall, 1981; Mitchell et al., 1986; Martin, 1986] along hotspot tracks with past plate movements accommodatedby a proposedplate boundary through the reconstructed from relative motion data. If the motion of Ross Sea-Weddell Sea region during Cenozoic time one plate over hotspotsunderlying it is determinedfrom [Molnar et al., 1975; Gordon and Cox, 1980; Stock and theorientation and age of volcanismalong trails of islands, Molnar, 1982;Jurdy, 1979;Duncan, 1981]. Identification seamounts,and linear ridges,reconstructions of the past of a smalloceanic plate in the BellinghausenSea [Stock positionsof neighboringplates with respectto those andMolnar, 1987]now requires significantly less motion hotspotscan be calculatedfrom seafloor spreading data. If betweenPacific and other hotspots (5-10 mm/yr(J. Stock, hotspotsunderlying all plates are stationary,then the personalcommunication, 1990). calculatedmotions of the neighboringplates should follow A far lessambiguous test of hotspotfixity is to compare hotspottracks observed on them. Any deviationsof these Atlanticand Indian Ocean hotspot positions through time predictedplate movementsfrom actual hotspottracks (becauseall interveningplate boundaries are divergent and would then indicate the magnitudeand direction of relativemotions are recorded accurately on the seafloor). interplumedrift. Since the Morgan [1981, 1983] and Duncan [1981] 34 ß Duncan and Richards: HOTSPOT VOLCANISM 29, 1 /REVIEWS OF GEOPHYSICS

African Plate

outh Fernando , Cameroon • Line Plate i

i Parana , 0 i Etendeka Martin 125 Vas • St Helena

•WalvisRidge i -30

Rio Grande Rise Rise 30 o ,, Tristan

Mid-Atlantic Marion St•o•t-I ... IndianRidge ,v•f'-'" AntarcticPlate -60 -60 -30 0 30 60

Figure 3. Modeled hotspottracks for the southernAtlantic Ridge, and MascarenePlateau lineaments). Relativeplate Oceanbasin. (The ages,in millionsof years,of radiometrically motionsare added to obtain predictedhotspot tracks on dated locationsare indicated,while solid dots along tracksare neighboringplates, assuming fixed hotspots.The generally 10-m.y. incrementsof predicted volcano age.) Rotation excellentmatch between predicted and observed tracks supports parametersare chosen by trial anderror to fit simultaneouslythe the ideathat hotspots move very slowly(2-5 mm/yr),if at all. geometryand age progression of volcanismalong hotspot tracks The Parana-Etendekaflood basaltsattending the birth of the on the Africanplate (Walvis Ridge,Cameroon line, Madagascar Tristanhotspot are stippled.

analyses,there have been several revisions in relativeplate of the ShonaRidge-Meteor Rise track [Hartnadyand le motions,and the volcanic historiesof many prominent Roex, 1985] is in accordwith a stationaryhotspot. Hence hotspottraces have been resolved. Hencea moredefini- theselinear volcanic tracks across the entireAfrican plate tive examinationof interhotspotmotions can now be made, are compatiblewith fixed hotspots,during the past 120 anda revisedset of platerotations appears in Table 1. m.y. Figure3 illustratesthe rotation parameters for motionof South and North American plate motions over the the African plate over stationaryhotspots since 120 Ma hotspotnetwork are calculatedby addingthe latestrelative (the time of continentalbreakup in the SouthAtlantic), on platemotions [Cande et al., 1988;Klitgord and Schouten, thebasis of thegeometry and radiometric ages for the Wal- 1985] to the motionof the African plate over hotspots vis Ridge and CameroonLine [O'Connor and Duncan, (Figures3 and 4). Hotspottraces predicted from these 1990; O'Connor, 1990]. As notedby earlierstudies, the plate rotationscan thenbe comparedwith volcaniclinea- hotspottraces can be matchedby rigid platemotion over a mentsassociated with Atlantic hotspotsto checkfurther setof fixedhotspots distributed over 100 ø (11,000km) of the fixed hotspothypothesis. The most prominentand the Earth's surface. Now that the age of volcanismalong long-livedhotspot traces are the Rio GrandeRise (South the Walvis Ridge is known in detail (from reliable Atlantic, Figure 3) and the New England Seamounts 4øAr-39Ar incrementalheating experiments on dredged (North Atlantic,Figure 4). The geometryand few dated and coredvolcanic samples), rates of plate motioncan be positionsalong the Rio GrandeRise [0' Connorand Dun- assignedfor time intervalswith moreprecision, especially can, 1990] are matchedby the calculatedrotations. The for the period30-80 Ma. Similarlydated samples from Tristanda Cunha hotspotgenerated the Rio GrandeRise the Cameroon Line [O'Connor, 1990], the Marion- simultaneouslywith the easternhalf of the WalvisRidge, Madagascartrack 0t. A. Duncan,unpublished data, 1990), whenthe SouthAtlantic spreading ridge lay centeredover andthe R6union-Mascarenetrack [Duncanand Hargraves, the hotspot.From about70 to 50 Ma the spreadingridge 1990] corroboratethese angular velocities. The geometry migratedwestward in a seriesof jumpsto its presentposi- 29, 1/REVIEWSOF GEOPHYSICS Duncan and Richards:HOTSPOT VOLCANISM ß 35

TABLE 1. Plate Rotationsin the Hotspot ReferenceFrame, 120 Ma to Present

TotalRotations, de• TotalRotations, de• Period,Ma Latitude(+N) Longitude(+E) Angle (+CCW)* Period,Ma Latitude(+N) Longitude(+E) Angle(+CCW)*

African Plate Eurasian Plate (continued) 11-0 (A5) • 60.0 --30.0 --2.0 58-0 7.6 -46.2 --12.4 21-0 (A6) 51.0 --45.0 --4.5 66-0 9.5 -44.6 --16.0 36-0 (AI3) 40.0 -45.0 -8.0 80-0 11.0 --56.2 --18.0 42-0 (AI8) 42.0 --42.5 --9.0 100-0 7.0 107.0 17.8 48-0 (A21) 42.0 --40.0 -9.6 120-0 27.5 88.9 19.7 58-0 (A26) 40.0 --40.0 --10.7 Indian Plate 66-0 (A29) •JJ.U • _Ar•---'•'I'U.U r• =/•"l',J, A • 80-0 31.0 --50.0 --20.0 11-0 37.7 26.7 -6.4 21-0 31.0 27.6 -12.7 100-0 25.0 --47.0 --25.5 36-0 24.2 26.4 -21.8 120-0 (M0) 26.0 --42.5 -30.0 42-0 26.4 27.2 -25.6 South American Plate 48-0 26.8 24.2 -29.6 11-0 58.7 --45.3 2.1 58-0 26.9 13.5 -36.4 21-0 68.8 --13.5 3.3 66-0 20.4 10.3 -48.6 36-0 72.3 13.5 6.4 80-0 19.3 2.8 -62.5 42-0 70.7 3.1 7.6 100-0 14.3 6.6 -70.7 48-0 70.6 --6.0 10.3 120-0 14.0 3.7 -77.0 58-0 74.4 0.1 13.0 Antarctic Plate 66-0 78.0 64.5 13.4 11-0 57.1 --117.1 --3.0 80-0 66.2 63.8 18.0 21-0 74.5 --110.2 -3.7 100-0 62.1 31.2 22.9 36-0 72.7 --63.5 -4.7 120-0 56.1 6.7 27.1 42-0 85.9 --3.4 -4.8 North American Plate 48-0 83.2 136.4 -6.4 11-0 27.0 128.2 1.4 58-0 77.5 --113.5 -10.2 21-0 30.4 113.2 3.3 66-0 78.8 --16.9 --10.1 36-0 39.8 108.9 6.7 80-0 76.6 --152.4 --14.1 42-0 46.1 111.2 7.8 100-0 89.1 --52.7 -15.5 48-0 53.1 113.1 9.4 120-0 80.4 --169.4 -22.4 58-0 59.1 112.3 12.6 Australian Plate 66-0 50.3 118.6 15.9 11-0 25.9 41.3 -7.8 80-0 49.3 101.8 20.6 21-0 20.1 40.3 --15.6 100-0 58.4 80.2 29.9 36-0 17.7 34.1 -24.6 120-0 62.3 61.1 37.7 42-0 20.3 33.6 -28.4 Eurasian Plate 48-0 24.6 33.1 -28.0 11-0 38.2 --47.2 -2.7 58-0 32.9 23.1 -27.5 21-0 30.5 -55.7 -5.3 66-0 29.2 23.7 --29.5 36-0 20.0 -60.4 -8.4 80-0 35.4 28.1 --28.2 42-0 18.2 -55.9 -9.3 100-0 49.5 26.8 --32.1 48-0 14.3 -52.0 --10.0 120-0 72.7 28.1 --39.5

* CCW means counterclockwise. '{'A5etc. are the seafloor magnetic anomalies used in relativeplate motion reconstxuctions. tion 400 km westof the Tristanhotspot; this process even- areestimated from subsidence calculations to bein therange tually terminatedthe growthof the Rio GrandeRise at its of 80-70 Ma [Tucholkeand Smoot,1990]. Crough[1981] easternend. The Tristanhotspot was born at about125 Ma andMorgan [1983] proposeda hotspotposition near Great with massiveeruptions of flood basalts,now preservedat Meteor Seamount in the eastern central Atlantic basin that the western end of the Rio Grande Rise (the Parana was overridden(and henceseparated from its track on the basalts)and the easternend of the Walvis Ridge (the Eten- NorthAmerican plate) by theMid-Atlantic Ridge soon after dekabasalts), a point we will returnto in the next section. theformation of theComer Seamounts. Figure 4 compares The New England Seamountsform a 1000-km-long, the traceof the New Englandhotspot predicted from fixed northwest-southeasttrending chain of submarinevolcanoes hotspotswith the positionand age of volcanicstructures in in the westerncentral Atlantic basin. Radiometricdating thecentral Atlantic basin. The close match in geometryand (40At_39 Ar ages)has shown that the ageof volcanism age of volcanismbetween predicted and observedtraces is decreasessmoothly from 103 to 82 Ma (southeastward) strongevidence that the New Englandhotspot has remained along the lineament[Duncan, 1984]. This volcanictrace fixed withrespect to hotspotsin the SouthAtlantic since the continueseastward to the Comer Seamounts,whose ages beginningof Cretaceoustime. 36 ß Duncan and Richards: HOTSPOT VOLCANISM 29, 1 /REVIEWS OF GEOPHYSICS

Voring 60 ,' Plateau

6O Hatton Bank

Figure 4. Modeled hotspot tracks for the northern Atlantic Seamounts EuropeanPlate orth Ocean basin (see Figure 3 for Azores details). The text discussesthe Seamounts Col'lieF volcanismattending the birth of PlateBermuda the Iceland hotspot. 3O Ridge African Plate

-90 -60 -39 0 30

CalculatedNorth Americanplate motionin the vicinity hotspots (New England, Tristan, Marion) show no of the Icelandhotspot also agrees well with the orientation detectable relative motion since 120 Ma. of the Iceland-GreenlandRidge, but not with the timing Turning next to the Indian Ocean, Indian plate and (60 Ma) of flood basalt volcanismalong the eastcoast of Antarcticplate motions over hotspotsare foundby adding Greenland,an observationnoted by Morgan [1983] and the latestappropriate relative plate motions[Molnar et al., Vink [1984]. Our modeling predictsthat at 60 Ma the 1987;Royer and Sandwell,1989] to the Africanplate over Iceland hotspotlay beneathcentral Greenland, some 600 hotspotsrotations. The Royer and Sandwell [1989] km from the plate edge where the North Atlantic Tertiary rotationsincorporate a wealthof improvedrelative motion Province flood basalts were erupted. This situation re- data for the Indian Ocean basin [Patriat, 1983]. The quireseither that (1) the Icelandhotspot has drifted south- resultingIndian plate motionscan thenbe comparedwith eastat an averagerate of 10 mm/yr relative to SouthAt- prominentvolcanic traces emanating from theR6union and lantic hotspots,or (2) duringits initial eruptionsat 60 Ma Kerguelenhotspots (Figure 5). Recently,legs 115 and 121 the hotspotwas very large (-1000 km diameter,Figure 4) of the Ocean Drilling Programfocused on obtaining and could feed flood basaltsto a regionincluding the Hat- volcanic samplesfrom these lineaments:the Mascarene- ton Bank, VOting Plateau,and the Disko Basaltsof west- Chagos-MaldivesRidge (R6unionhotspot trace) and the ern Greenland [White and McKenzie, 1989]. Vink [1984] NinetyeastRidge (Kerguelenhotspot trace). The age presenteda modelin whicha small-diameterhotspot under distributionof the volcanismalong thesetrends is now Greenland fed spreadingridge eruptionsat the VOting well resolvedby 4øAr-39Ar dating [Duncanand Plateau, easternGreenland, and the FaeroesPlateau by Hargraves,1990; Duncan, 1978, 1991], so any departures horizontal asthenosphericflow until Greenland drifted from fixed hotspotsshould be easilyidentified. away from the hotspotand ridge-centeredvolcanism was Figure 5 showspredicted hotspot traces (which assume established. In view of the large region of flood basalt fixed hotspots)superimposed on actualvolcanic structures eruptionslikely during a plume initiation event (see next foundon thefloor of the IndianOcean basin. A verygood section)we feel that the notion of a point sourcefor the match betweenpredicted and observedtraces is obtained Iceland hotspotis not applicableat 60 Ma. Following the by locatingthe R6unionhotspot beneath a largeseamount flood basaltevent the hotspotprobably decreased to "nor- 150 km to the west of the volcanicallyactive island of mal" size, and the predictedyounger volcanic trail (50 Ma R6union,and the Kerguelenhotspot beneath a groupof to present) follows the Iceland-Greenlandand western prominentseamounts on the northwestmargin of the Iceland-Faeroesridges. We can concludethat hotspots KerguelenPlateau. Both hotspotsbegan with flood basalt throughoutthe Atlantic basin,from Marion and R6unionto events (the Deccan Traps and the Rajmahal Traps, the centralAtlantic (and possiblyIceland), have remained respectively;see also Figure 12), thenleft parallel,north to fixed for at least the past 60 m.y., and the longer-lived southgrowing volcanic ridges on the trailingedge of the 29, 1/REVIEWSOF GEOPHYSICS Duncanand Richards: HOTSPOT VOLCANISM ß 37

30 ! Indian Plate 117

African Figure 5. Modeled hotspottracks for the Indian Oceanbasin (see Figure 3 for details). Ages shownare radiometricage determina- Chagos tions for basalts from the two prominent Ridge volcanic lineaments: the R6union and :carene Kerguelen hotspot tracks. Flood basalt ,•, provinces(stippled) mark the initiation of eachof thesehotspots. -30 ' 38 BrokenRidge

Australian Plate Kerguelen '. CD 27 Marion •q• Kerguelen X.,•x-•Plateau

AntarcticPlate •.....'.':•....:...... ,...••114• -60 30 60 90 120 rapidlymoving Indian plate and wereoverridden by the centraland southeastIndian spreading ridge at 36-38 Ma. From that time forward the R6union hotspothas formed the Mascarene Plateau and the islands of Mauritius and R6unionon theAfrican plate, while the Kerguelenhotspot hasformed a largepart of the northernKerguelen Plateau Australian 32 on the slowlymoving Antarctic plate. Plate

Finally, seamountchains in the Tasman Sea and 20' continental basaltic volcanism in eastern Australia have -30 beenlinked to hotspotactivity [Vogt and Conoily,1971]. Lord Howe Recentradiometric dating of the seamounts[McDougall East and Duncan, 1988] has confirmed a north to south, monotonicage progression in the volcanismof the same rate as observed in chains of central volcanoes in eastern 36 Tasmanti••,•• ' Australiafor the past35 m.y. [Wellmanand McDougall, 1974]. This region lies in the easternmostpart of the ! Pacific Indianplate. Thereis evidencefrom seismicity and geoid anomaliesthat an east-westplate boundaryoccurs within Plate -60 the central Indian basin which has allowed a small amount of north-southcompression between the Indianplate and the Australianplate over the past 10 m.y. [Royer,1988; DeMets et al., 1988; Stein et al., 1990]. This proposed relativeplate motion has been incorporated in thepredicted motionfor the Australianplate, which closely matches the orientationand age progressionof the Tasmanseamount 120 150 180 chain(Figure 6). Hotspotactivity in the vicinityof the AntarcticBalleny Islands has left a volcanictrail on the Australianplate that showsup particularlywell on geoid Figure6. Modeledhotspot tracks for theeastern Australian plate anomalymaps. A singledated seamount fits well with the (see Figure 3 for details). Ages shownare radiometricage predictedage of thetrace southeast of Tasmania. determinations[McDougall and Duncan, 1988; R. A. Duncan, In summary,geometrical and age comparisonsof unpublisheddata for southeastTasmania, 1990]. Solid squares volcanicchains with computer-generatedtraces calculated are locations of volcanic centers. 38 ß Duncan and Richards: HOTSPOT VOLCANISM 29, 1 /REVIEWS OF GEOPHYSICS

by linking African plate motion over hotspotsmotion to destroyed[Engebretson et al., 1984, 1986; Duncan and neighboringplates demonstratesthat there is no per- Hargraves, 1984; Rea and Duncan, 1986; Pollitz, 1988]. ceptibleinterhotspot motion over periodsas long as 120 Continental tectonic and volcanic events in western North m.y. Moreover,the examinedhotspots range from Iceland Americaare particularlywell correlatedwith changesin in the North Atlanticthrough the SouthAtlantic and Indian themagnitude and direction of plateconvergence predicted oceansto the Tasman sea, a region exceedinghalf the from the hotspotreference frame [Engebretsonet al., Earth'ssurface and includingsix majorplates. According 1984; Wells et al., 1984; Verplanckand Duncan, 1987]. to Molttar and Stock [1987], 95% uncertaintyellipses for This methodeliminates the need to constructglobal relative plate motion additionshave typical semimajor circuitsof platemotions that crossonly spreadingridges axes of 200-300 kin, so the maximum allowable relative [Jurdy,1984] and reducesthe large uncertaintiesaccumu- motionamong these hotspots is about2-5 mm/yr. The lated in combiningsequences of rotationpoles [Molnar real motionis thusat leastan orderof magnitudeless than and Stock,1985]. typicallithospheric plate velocities. There is no evidence Now, differential motion between the mantle and the for any systematicdeparture between modeled and spinaxis can in principlebe determinedby comparing observedhotspot traces from one plate to the next, which platemotions recorded by the hotspotand paleomagnetic might be expectedif hotspotsare greatly affected by referenceframes. Such motion has been termed true polar horizontalflow in the upper mantle. Over this half of the wander(distinguished from apparentpolar wanderof the globe, then, hotspotsare fixed to the extent that they magnetic(spin) axis inferred from time sequencesof constitutea stable reference frame for plate motions, paleomagneticfield directionsfor given plates). It is recordedsimply as volcanictrails. possible that redistributionof mass within the Earth Furtherextension of this analysisinto the Pacificregion throughprocesses such as mantle convectionand plate is limited by subductionplate boundaries,where relative motions may change the planet's momentsof inertia motion historiesare largely destroyed,or the ice-covered sufficientlyto causea shiftof theentire body relative to its Antarcticplate, whereplate boundariesare poorly known. spinaxis [Goldreichand Toorare,1969]. This is, in fact, However,we know that Pacificbasin hotspots have been an old idea, suggestedas an alternativeto continentaldrift stationary since 65 Ida [Duncan and Clague, 1985]. as an explanationfor paleoclimaticevidence for trans- Molnar and Stock's [1987] conclusionthat as much as 20 latitudinalmotion of the Earth's surface[Irving, 1964]. mm/yr interhotspotmotion has occurred can thushave one True polar wander,however, was largely ignoredafter it of two explanations:either Pacific hotspotshave moved was demonstratedthat separatecontinents had distinct (southward)en massewith respectto fixed hotspotsin the apparentpolar wander paths and that continentaldrift Atlantic and Indian oceans,or all hotspotsworldwide are (platemotions) must have occurred [e.g., Runcorn, 1965]. fixed and thereis someas yet poorly known plate bound- Couldboth motions occur simultaneously? ary within the Antarctic plate that has accommodated In the absenceof true polar wander,mantle plumes do relative motion during Cenozoic time. The second not movewith respectto the geomagnetic(or spin)axis. possibilityappears to be more likely, and the required Henceevery volcano generated along a givenhotspot track relative motion between East and West Antarctica can be wouldrecord the magneticinclination, usually expressed calculatedfrom the fixed hotspotreference frame [Duncan, asthe paleolatitude, of thesite of presenthotspot activity. 1981]. If measuredpaleolatitudes are constantalong the hotspot track,then the hotspothas not movedwith respectto the spin axis. On the contrary,a change in hotspot 3. TRUE POLAR WANDER: DOES THE MANTLE paleolatitudewith time would indicate motion of the ROLL? mantle(because plumes are stationarywithin the mantle) relativeto the spinaxis, which is truepolar wander. (Note If we now acceptthe conclusionof section1 thatmantle that the low viscosityof the fluid outer core allows the plumesare stationary,to within about 5 mm/yr or less, geomagneticfield to remaincoupled with thespin axis.) over periodsas long as 120 m.y., thenthe volcanictraces Hargravesand Duncan [1973] first noted the systematic left by hotspotsrecord the historiesof plate motionsover northwardmotion of hotspotsin the Atlanticregion and the lower mantle. This mantle reference frame is inde- simultaneoussouthward motion of hotspotsin the Pacific pendentof the paleomagneticreference frame and has region,relative to the geomagneticpole, over the past--50 certain distinct advantages. It does not dependon the m.y. Thisobservation was explained by a 12ø clockwise assumptionof the geocentricaxial dipolefield (for which rotationof themantle about an equatorialaxis emerging in thereis goodevidence except during polarity reversals of the westernIndian Ocean. Subsequentstudies [Morgan, the field), and it resolveseast-west plate motionwhich is 1981;Gordon and Cape,1981; Harrison and Lindh, 1982; not detectedin paleomagneticstudies. Severalgroups Livermore et al., 1983; Andrews, 1985; Gordon and have used the mantle reference frame to determine the Livermore, 1987; Courtillot and Besse, 1987] have history of plate convergenceacross subduction zones, confirmedthis surprisingconclusion and have determined where the record of relative plate motion is largely a historyof truepolar wander since 200 Ma. 29, 1/REVIEWSOF GEOPHYSICS Duncan and Richards:HOTSPOT VOLCANISM ß 39 Figure7 [from Courtillotand Besse, 1987] summarizes polarwander prior to Jurassictime, but its magnitudeand thelatest comparison of platemotions measured in thetwo directionare less certain becauseof the paucity of reference frames. In this illustration, apparentpolar identifiablehotspot tracks older than 120 Ma. wanderpaths (paleomagneticdata) for all plates were Intriguingly,the dramaticchange in directionof true transferredto the African apparentpolar wanderpath by polarwander at 40 Ma occurredwithin a periodof global removing relative plate motions. This would be the platemotion reorganization to significantlymore east-west apparentmotion of the geomagneticpole with respectto a collisionalplate boundaries [Rona and Richardson,1978]. (long-lived)observer on the African plate. The apparent The abrupt change in direction of the Pacific plate, motion of the hotspotreference frame with respectto reflectedin the Hawaiian-Emperorbend (43 Ma), and the Africa can alsobe describedby a polar wanderpath. (In "hard"collision of India andAfrica/Arabia against Eurasia this case the rotation of the mantle relative to Africa, as occurred at this time. Courtillot and Besse [1987] inferredfrom the many hotspottracks, is appliedto the concludedthat thesechanges in subductionzone location spinaxis.) Grossfeatures of the two pathsare similar,but and activityperturbed the torquebalance of the coupled significantdifferences are seenby subtractingone curve uppermantle-lithosphere system, which then alteredthe from the other,to derivetrue polar wander. directionof truepolar wander. The earlier60-m.y. period when no true polar wanderoccurred is correlatedwith a

180 time of decliningmagnetic field reversalfrequency and low lithosphericvelocities which, Courtillot and Besse [1987] speculated,indicate a reduced state of mantle convectionand decreasinginstability in the outer core, •ots•ot• probably linked by a lower heat flow acrossthe core- mantleboundary.

4. DO PLUMES SWAY IN THE MANTLE WIND?

Having surveyedthe surface,volcanic expression of hotspots,let us now turn to the deeperconvective regime of mantleplumes. The fixity of hotspotswith respectto plate motionsis difficult to explainwith simplemantle convectionmodels. In fact, no numericalor laboratory convection experiment has ever demonstrated such behavior,that is, plumesthat are independentof upper boundarylayer motions. Apparently, the sourceregion for 0 mantleplumes is recoupledfrom mantle flow associated with plate motions,presumably as a result of chemical Figure7. Comparisonof thesynthetic paleomagnetic apparent and/orrheological stratification in the mantle. However, polar wanderpath (triangles,which are paleomagneticpole the lack of any self-consistentmodel is (and shouldbe) positionsfor all platesrotated to the Africanplate, in present troubling. coordinates)with the syntheticpath deduced from the hotspot Oneapproach to thisproblem is to considerthe effect of referenceframe (circles). True polar wander(squares) is calculatedby subtractingone from the other;this represents horizontalshear in the uppermantle beneath a moving globalmotion of themantle with respect to theEarth's spin axis. plate(the "mantle wind") upon a cylindricalplume rising Pointson eachcurve are 20-m.y. increments, from 200 Ma to the throughthe mantleto form a hotspot. Shearflow bends present. Confidenceintervals of 95% (not shown)are of the the plumeover horizontally,and someremarkable results orderof 4 ø [fromCourtillot and Besse, 1987]. have been obtainedfrom laboratoryfluid dynamical simulationsof this flow interaction.Horizontally tilted Approximately12 ø of the true polar wanderhas plume conduitswere first studied by Skilbeckand occurredin the past40 m.y., that is, clockwiserotation of Whitehead[1978] and Whitehead [1982], who discovered the mantle about a pole on the equator,emerging near thata tiltedconduit of low-densitymaterial can break up 60øE(western Indian Ocean). Between about 110 Ma and into a seriesof individualrising blobs. This gravitational 40 Ma themantle rolled nearly 20 ø aboutapproximately instability occurs for conduitsdeflected to greaterthan the same pole but in the oppositedirection, forming a about60 ø fromthe vertical (Figure 8). Whitehead[1982] hairpinturn in the truepolar wander path at about40 Ma suggestedthat this behavior may occur within a shallow (Figure 7). The period from 170 Ma to 110 Ma was shearzone beneath an oceanicplate, perhaps accounting remarkablebecause there was virtually no true polar for individualisland volcanic centers along such hotspot wander;hotspots did notmove with respectto thespin axis tracksas the Hawaiian-Emperor chain (Figure 1 la). Olson duringthis time. There is evidencefor significanttrue and Singer[1985] and Olsonand Nam [1986] suggested 40 ß Duncan and Richards: HOTSPOT VOLCANISM 29, 1 /REVIEWS OF GEOPHYSICS

The aboverelation can be used to calculatea plume trajectoryin a fluid layer, if the backgroundflow in the layer (e.g., due to plate motion)is known. One simply calculatesthe trajectory of a Stokes conduit element releasedfrom the plume origin from a combinationof its rise velociiyv and advectiondue to motionin the surroundingfluid. The combinationof the simplelinear shear flow in the experimentaltank and the constantStokes rise velocity of the plume conduitresults in the parabolic trajectoriesshown in Figure9. Althoughthe experimentsshown have a particularly simpleflow geometry,it is possibleto use (1) to model plumedeflection under more realistic conditions. Figure 10 (top) illustratesagain a simpleshear flow, but with a large increasein viscosityat the level of the dashedline. The horizontalflow velocity is much larger in the low- viscosityregion immediatelybeneath the plate, but the

Figure 8. Photographof a weak plume injectedinto a shear flow. The tilted conduithas developeda numberof instabilities •½...... , .. . .•½• .....

that are separatingfrom theplume. '::..;?':'.':"•.':::.::,:""•':'"• •::::ii•;ii:•.::•¾.;•:.,:'.... .'-•¾...... ::•:.:"-'...... • '*-'......

that a chainof risingblobs of mantle(diapirs) as largeas -'-i•: ..::•:•:•.: .•: •:; ;:--',•:?'•-....:•-: 400 km in diametermay result from the sametype of ...... • ...... •...... plume instabilityin the deep mantle. Olsonand Singer [1985] alsopointed out that the relativefixity of hotspots was not necessarilyinconsistent with deflected,unstable

plumes,at leastfor steadystate mantle return flow...... j...... ½½...•., ..•,.....½•.•.....• ....•::::•:::• ...... ;• -• ..• ..• In light of thesephenomena it is naturalto ask under (!::•::':•:'•.,•..•.%.•;1;•?•:.:?::.:::...•.•.*`..::%7•.::C.•i5;•*•*•;•;•;•;•;•i•" %.•.•.•";.;:•:•:•:i:•...... -m.....•...:'.X'•;;• ...... '•:•...... '*...... •:•:'•:•:•:'•:'::•:• ...... ' "::•::"• what conditionsplumes may or may not suffersignificant ..:';:•';"•.:':-;;:?:.•.;...:.-;.'•,:.....,. ::..'.::: '-•;;;;.•.;::•;;:'..'::.....:.;:;-:;;;•::::;.;:;:':....;.' '.;•??:.•:.'....'...... :::.,::,...:.::....:.:..:;,,..:.:.::..:...... :...;;.:..:.;:;....:..•.:.::.,:•::....:.:.:..•;:.:;::.•::;•::....•:.....:.:.....::..:.;:.:•;•:...,.::.:..::...;:..• .:.:.:.:.:...... ::•.•:•.:..:.::: .,::;:;;;;-•;;;:..'.•,;;; horizontal deflection. Deflection of buoyant plume ,:½...::.... ;...:•::•...... :..:.::ß...... ;...... ;:;• ...... conduitsby mantleshear flow beneathmoving plates was studiedin detailby Richardsand Griffiths[1988]. Figures 9a-9c show a series of increasinglystronger mantle plumes deflected in a simple shear flow in laboratory experiments.In eachof thephotographs a cylindrical tank of glycerol is fired with a lid which rotatesand createsa

shearflow in the tank. Motion is from left to fight in the ...-:•;• side-viewphotographs. A buoyantplume (80% glycerol and 20% water with red dye) is injectedat a constantrate .,:::-;.;;-.:,.---;;--:,{;:.:.:7.½?•;:; ;..•:4.•..•::...:..::•½• .. througha smallhole in the bottomof the tank to the left of ..... :...... ,...... •:;½:::::;::.A.,•.-...... ;• ...... ,...... the dark verticalband formedby the rotationalaxis of the ?•?...... •.:•;...... -..:}-.•; ...... :...... •,,Z:•;:::•.;•-.-.;::,;:•:;::;,::;•;•;,;•:---..::'.:•:-•,•,,.•::.; ;':"::.5..:;•;•?..•:`.;?.:•;•..•..•.:*•.•..`.?:•.•;:•;?::•:•.:5•;:;:•%..;:.7..::::•:•:..:•.•....•.`•......

tank lid. The movinglid ("plate") generatesa cylindrical ;:?.?:::•;:::•;•?--4:½-;,-;;;?;--::,.,---:,-,•?;';;:::.•::•;:;•:.;•?:½:::•?:.•.;•.?.?----'•'•••;;....:...}•;::"•'•----- .....•.:...... ½.....•..•._..•:...... •....••..••,..,.,:.:,.-.:•..., ......

shearflow, with velocityincSeasing linearly from zero at .::;;•...... 4 ½•½:; ...... the bottomof the tank. The plumetrajectories are seento be almostperfect parabolas, and Richardsand Griffiths

[1988] were able to model these trajectoriesaccurately '.; usinga simplerelation: I.. •

v = k$pga 2/q (1) ....

where v is an effectiveStokes rise velocity for a given plumeconduit element, •5p is thedensity contrast between the plume and surroundingfluid, g is the gravitational acceleration,a is theplume radius, and fl is theviscosity of Figure 9. Photographsof laboratorytank experiments showing the surroundingfluid; k is a constantdetermined empiri- plumedeflection by horizontalshear flow exertedby rotating cally from the experiments(k = 0.54 + 0.02). surfaceplate [Richardsand Griffiths,1989]. 29,1/REVIEWS OFGEOPHYSICS Duncanand Richards: HOTSPOT VOLCANISM ß41

u o N-S andE-W spatialscales in Figure1 lb arenormalized by thefactor Uh/2v, which is the maximumhorizontal deflectionof the plume. With this normalizationthe approximateradius of curvatureof thebend in thesurface I

"ridge" uo •_ •"trench"

• 75 ! ! .•_.•p/•xh ! '• Emperor Seamounts zC I x

Figure10. (Top)Diagram of plumedeflection in a plate-driven •HawaiianRidge shearflow with verticalviscosity stratification and (bottom) diagramof plume deflection in a simplereturn flow model.

steadystate plume profile is unaffected.The higher horizontalflow velocityis just offsetby the effectof the lowerviscosity on the Stokesrise velocityof the plume conduit(see (1)). For steadystate flow thelow-viscosity Figure 11a. Thevolcanic trail of theHawaiian hotspot on the zonedoes not prevent plume deflection. (However, it must Pacificplate (present direction indicated by arrow).Ages along be bornein mind that the part of the plume in the low- the lineamentare in millionsof years;an abruptchange in plate viscositylayer will adjustmuch more rapidly to changes in directionoccurred at 43 Ma, and a lesserone occurredat 75 Ma. motion). In the exampleof Figure10 (bottom)a return flow is addedto accountfor trench-ridgemass balance in accordwith the constantpressure model of Turcotteand Schubert[1982]. A more complicatedplume trajectory results,with maximumhorizontal deflection at aboutone fourthof thelayer depth beneath the plate. (The bottom no-slipboundary condition in Figure 10 (bottom)is probablyunrealistic. A freeslip condition would result in a finitevelocity at thebottom of thelayer, with no vertical velocitygradient there.) The Stokes velocity characterization,which was developedfor steadystate plume deflection, also accurately describesthe transientadjustment of plumesto changesin the backgroundshear flow [Richardsand Griffiths,1988]. Thisleads to a veryspecific application of (1) above,namely, the adjustmentof the Hawaiian plume to the changein Pacificplate motionwhich occurredabout 43 million years ago, resultingin the -1 , I , I , I , I , Hawaiian-Emperorbend (Figure 1 la). Figure1 lb showsa -4 -3 -2 -1 0 1 mapview of the theoreticalsurface trace of a deflected (2v/Uh) X mantleplume, subject to a 60ø changein platemotion (comparableto the changein Pacificplate motion at 43 Figure11b. A theoreticalplume track relative to a movingplate Ma). The "track"star•s at the top left with steadyplate whichundergoes an instantaneouschange in directionthrough motion(speed U) at N30øW,and then adjusts to due 60ø withno changein speedis alsoshown. The plume rises westerlyplate motion(also speedU) with the newest througha layer of constantshear. Spatialcoordinates are "volcano"at the far right. In thismodel, there is simple nondimensionalized so that unit distance is the maximum shearflow beneaththe platein a fluid layerof depthh. horizontal deflection. This becomesthe radius of curvaturefor The Stokesrise velocityof the plumeconduit is v. The the bend. 42 ß Duncan and Richards: HOTSPOT VOLCANISM 29, 1 /REVIEWS OF GEOPHYSICS

trace is seen to be about 1. In other words,Figure lib now locatedat the present-dayJuan de Fuca spreading shows the intuitive result .,let the radius of curvature of the center [Desonie and Duncan, 1990]. In the Tasman Basin plume trace in responseto the changein plate motionis east of the Australiancontinental margin the Tasmanfid approximatelyequal to the maximumhorizontal deflection Seamountshave been shownto representAustralia-Indian of the plume. This theoreticalresult was shown to be quite platemotion over three fixed hotspotssince at least24 Ma, accurate in laboratory experimentssimulating a plate formingthree parallel seamount/volcano chains (Figure 6 motionchange [Griffiths and Richards, 1989]. [McDougalland Duncan, 1988]). Sincethese hotspots are The Hawaiian-Emperorbend itself has a radius of at leastan orderof magnitudeweaker than Hawaii (e.g., in curvatureof no morethan about 100-200 km (Figure1 la). terms of volcanic production rates or associated (See for example, Jackson et al. [1972] for detailed bathymetricswells), it is even more remarkablethat these bathymetry.) Of course,some apparentcurvature must seamountchains are consistentwith the hotspotreference resultfrom the finite width of the traceitself and,perhaps, frame. a noninstantaneouschange in plate motion. From the Although it is plausible that plumes remain vertical above theoretical considerations it is clear that the beneathfast plates,we have not justifiedthe fixity of the observed sharpnessof the Hawaiian-Emperor bend plume sources. Thermal plumesmust originatefrom a constitutes a f'trst-order observation concerning the thermal boundary layer, and either the core-mantle dynamicsof the Hawaiian plume. The Hawaiian plume boundary(for mantle-wideconvection) or a hypothetical cannotbe horizontallydeflected more than about200 km boundarylayer in the mantle transitionzone (for chemi- in the regionof the uppermantle that is stronglycoupled to cally layered convection)is the only plausiblesource plate motions. This was first pointedout by Whitehead region. Motions at the core-mantleboundary may be [1982], who suggestedthat the Hawaiianplume mightbe effectivelydecoupled from plate-relatedconvective flow stronglydeflected within a shallowshear zone (-200 km by an increasein mantleviscosity with depthby abouta deep)beneath the Pacific plate, giving rise to individual factorof 100-1000 (see,for example,Gurnis and Davies islandvolcanic centers via conduitinstability. However, [1986]). A boundarylayer at 670 km depthpresents a an additionalcondition for suchan instabilityis that the greaterdifficulty since trench-ridge return flow is confined plume diameter be much less than the layer depth; to the uppermantle, regardless of the viscositystructure. otherwisethe diapirs would not have sufficientroom in Global modelsfor deep mantleflow derivedfrom plate which to form. Alternatively,the plume may be vertical motions[Hager and O'Connell,1981] and from attempts through the entire upper mantle, and the formation of to modelsesimically imaged mantle heterogeneity and the individual volcanic centers might be controlled by geoid[Forte and Peltier, 1987;Richards and Hager, 1988] lithosphericstrength and heterogeneity. yield midmantlevelocities somewhat smaller than plate Using (1), it is instructiveto calculatethe sizeof plume velocitiesbut generally greater than about 5 mm/yr. necessaryto remainvertical in the faceof the mantlewind Thereforeit is not clearhow a midmantleboundary layer beneaththe Pacificplate. An appropriatecriterion would source for hotspot plumes could provide an absolute be that the vertical Stokesrise velocity v associatedwith referenceframe with respectto plate motions[Richards, the Hawaii plume is not much less than the overriding 1991]. platevelocity of about100 mm/yr. Setting v• > 10mm/yr andusing reasonable values for uppermantle viscosity q = 10zz P andplume density contrast 5p = 0.05g/cm 3, this 5. ARE FLOOD BASALTS THE RESULT OF PLUME requiresa plume diameter d = 2a > 70 km. This is INITIATION? consistentwith Morgan's [1972] original estimate for plume width basedon the gravity anomaly over Hawaii The origin of continentalflood basalt eruptionshas and is certainlya reasonablevalue. remaineda major geologicalmystery which has not been More remarkable perhaps than the spectacular solvedby any aspectof the theory of plate tectonics. Hawaiian-Emperorbend is the fact that much weaker Recently,much attention has been focused on thegeologi- hotspottracks also appear to represent"absolute" plate cal, geochemical,and geodynamicalnature of these motions quite reliably. Although there are no other extraordinaryevents, and this interestresults in largepart similarly distinctbends, our ability to formulatea fixed from the clear association of flood basalt events with the hotspotreference frame shows that other hotspotsin the beginningof hotspottracks as well as the coincidenceof Pacific, Atlantic, and Indian oceansare not undergoing the spectacularDeccan flood basalt eruptionswith the significanttransient adjustment to overlyingplate motions. Cretaceous-Tertiarymass extinctionevent [Duncanand In additionto the well-knownmajor hotspots mentioned in Pyle, 1988; Courtillot et al., 1988]. The associationof previoussections, several seamount chains which are tiny floodbasalts and hotspots [Morgan, 1972, 1981]is shown by comparisonalso appearto recordplate motion faith- in Figure12, and spatial and temporal relationships such as fully. The Cobb-Eickelbergseamounts in the northeast thatbetween the R6union hotspot and the Deccan eruptions Pacific have been shown by radiometricdating to be are now firmly documentedby precise K-Ar and explainableby a fixed hotspotactive since at least 10 Me, 40Ar_39 Ar dating[Duncan and Hargraves, 1990]. These 29,l/REVIEWS OFGEOPHYSICS Duncanand Richards: HOTSPOT VOLCANISM ß43

i i i

Yellowstone

i i i ! i ! ,, i

i i ,, i !,Ontong $aval i

i i i i i •. ,,

I I Louisville[ ...... I

I I Figure12. Worldwidedistribution of flood basalt provinces nental margins, others have formed in continentalinteriors or in eruptedin thelast 250 m.y. and associated hotspots (where oceanbasins. CRB are Columbia River basalts,and NATB are knownor conjectured). Note that while many occur along confi- North ArianticTertiary basalts.

hotspot/floodbasalt correlations also include Iceland/North guishable in termsof the geologicalrecord. However, Atlantic Tertiary basalts,Tristan da Cunha/Parana-their implicationsconcerning the relationshipbetween Etendekabasalts, Marion/Karoobasalts, Yellowstone/ continentalbreakup and plumes,the origin of plumes ColumbiaRiver basalts, and Kerguelen/Rajmahal traps. In themselves,and the processesof meltingin flood basalt addition,some major oceanicbasalt plateaus(e.g., events are different in important ways. White and Ontong-Java,Caribbean) may mark the initial activity of McKenzie [1989] have recenfiy developeda detailed hotspots(Louisville, Galapagos). Of all themajor flood formulationof the morepassive and uniformitarian rifting basaltprovinces only the Siberiantraps lack a clear model,providing an elegantexplanation for thepresence associationwith a knownhotspot track, perhaps because of of huge basaltaccumulations along rifted continental ourpoor knowledge of theArctic Ocean basin. marginsbut placinglittle emphasison howplumes begin and reach the base of the lithosphere. Richards et al. PassiveRifling or PlumeInitiation? [1989]have emphasized the meritsof theplume initiation Many of the continentalflood basalteruptions are modelin explainingthe extraordinaryeruption rates and associatedwith the early stages of continentalbreakup and suddenonset of flood basalt volcanism,also suggesting rifting(No• Atlantic,Parana, Karoo, Deccan), and this that rifting is not a necessaryprerequisite to the initial observationhas given rise to a numberof modelsfor their stagesof volcanism. Campbellet al. [1989] further originbased on rifting [e.g., Cox, 1980; Bott, 1982; Wtu'te suggestedthat Archeangreenstone terranes (thick se- et al., 1987;Meissner and Kopnick, 1988]. Morgan [1981] quencesof low-temperaturealtered basaltic rocks) may be proposedtwo relatedhypotheses: (1) that continentalthe remnants of plumeinitiation events. lithospherewas weakened by thelong-term activity of a Bothmodels require the presence of a broadreservoir of hotspot,which warmed the upper manfie (asthenosphere) anomalously hot mantlebeneath the lithosphere. The beneaththe continentand led to massiveeruptions derived riftingmodel developed by Whiteand McKenzie [1989] is fromthis material during extension and rifting; and (2) that very specificabout how melt is generated.A plume thelarge "head" of a newmantle plume results first in a (steadyor otherwise)warms the sublithosphericmantle to massivemelting event, followed by moremodest activity temperaturesasmuch as 200øC in excessof thenormal alongthe ensuing hotspot track because of theremaining mantlepotential temperature of 1280 ø, whichis typically steadyplume. These two modelsare closely related and, found beneathmid-ocean spreading ridges far removed uponsuperficial examination, may seem almost indistin- from hotspots.Large degrees of extension(factors of at 44 ß Duncan and Richards: HOTSPOT VOLCANISM 29, 1 / REVIEWSOF GEOPHYSICS

least 2-4) in the overlying continentallithosphere cause tion of hot plume material beneath the plate due to decompressionmelting of the hot plumematerial, produc- "ordinary"R6union plume activity. (2) The mainsequence ing a melt thicknessof about25 km (as comparedwith 7 of tholeiitic basalt lavas in the Parana event in South km for normal melting at spreadingridges) that eruptsto America preceded significant extension in that area fill the rift. Thusa hugeamount of melt is generatedwhen [Piccirillo et al., 1988a, b], again with smaller, more a continentrifts abovean active mantleplume, as opposed alkalic basalteruptions accompanying the extensionthat to normal mantle. This model has been applied to the eventuallyled to continentalbreakup. (3) The large developmentof the volcanicrift marginsof the North ~195-Ma central area tholeiitic basalt eruptionsof the AtlanticTertiary Province (Figures 4 and 12 [Whiteet al., Karoo province(Lesotho) are associatedwith geochemi- 1987]) and is most successfulin explaining the great cally identicalfeeder dike complexeswhich are pervasive thicknessesof onlappingbasalt (and perhapsunderlying throughouta very broadarea of what is clearly unrifted, intrusive)structures mapped in seismicreflection profiles stablecontinental lithosphere [Marsh, 1987]. "Associated" of the Hatton Bank and V0ring Plateau marginson the rifting about20 m.y. later betweenAfrica and Antarctica easternmargins of the North Atlantic [Eldholmet al., was accompaniedby huge alkalic basalt eruptionsthat 1989]. Cox [1989] has also invoked this model for the shiftedtoward the rifting margin. (4) The GrandeRonde evolutionof volcaniccontinental margins to explain the eruptionsof the Columbia River Basalt Group were developmentof drainagesystems following the Deccan, accompaniedby at most~1% crustalextension [Hooper, Parana, and Karoo flood basalt events. Despite the 1990], with subsequentextension (~20%) associatedwith objectionswe raisebelow, the White andMcKenzie model more silica-richeruptions. (5) The SiberianTraps were providesa usefulquantitative framework for understanding precededby a long "geosynclinal"period, but structures the evolutionof volcanic continentalmargins subsequent disruptedby thesehuge intracontinental eruptions show no to rifting(for example,see Cox [1989]). evidenceof significantextension either prior to or during The plume initiation hypothesiswas originally based flood volcanism [Makarenko, 1976]. Hence the Siberian upon the largely circumstantialevidence (1) that flood Traps volcanismdid not coincidewith major continental basaltsoften representthe earliestactivity of hotspotsand rifting. (6) Even in the North AtlanticTertiary Province, (2) that the truly catastrophicnature of floodbasalt events wherethe rifting modelhas found detailed application, it is requiredsome special cause other than one associated with not obviousthat all of the flood basalt eruptionswere the time scale for plate motions. The first of these accompaniedby largedegrees of extension.For example, observationsis questionable,since it is possiblethat such the famousdike complex of Skye (westernScotland), hotspotsas R6unionand Tristanda Cunhacannot normally which fed some of the tholeiitic flood basalteruptions, penetrate stable continentallithosphere. The second indicatesonly mild extensionoriented perpendicular rather observationis more compelling becausethe alternative thanparallel to thedirection of subsequentrifting. rifting mechanismdescribed above predicts the duration of From this evidence we conclude that most flood basalt floodbasalt events to be of theorder of~10 m.y., clearlyin events result from melting events at the base of the disagreementwith evidencefor ~1 m.y. (or smaller)time lithosphere,whether or not suchthermal events precipitate scalesfor some events such as the Deccan basalts [Duncan subsequentrifting. Thereforewe must appeal to an and Pyle, 1988; Courtillotet al., 1988; Baksi and Kunk, extraordinarypulse of mantleplume materialfar in excess 1988]. Also, as we describe in some detail below, of the flux representedby the subsequenthotspot track. laboratory and numerical models suggestthat plume This givesrise to an interestingand fundamental question: initiation may be expectedto result in a huge volcanic Canlarge degrees of partialmelting (say 10-20%) occurat eventat thebeginning of a hotspottrack. thebase of normalcontinental lithosphere in theabsence of Althoughthis evidenceis suggestive,we find that the significantmechanical thinning (rifting)? Althoughwe observedtectonic settings for flood basaltevents provide will not attempta full discussion,a partial answerto this compelling evidence that these eruptions do not, in questionis found by referringto recentmantle melting general,result from decompressionmelting in responseto models based upon melting experimentson candidate lithosphericextension. In fact, many flood basaltevents mantlerocks and upon observationsof major and trace are neitherpreceded nor accompaniedby largedegrees of elementvariations in oceanridge basaltsand peridotites extension. Many examplesmay be cited: (1) The main (Mg, Fe silicaterocks thought to berepresentative of upper tholeiiticeruptions of the DeccanTraps preceded signifi- mantle material). Both McKenzie and Bickle [1988] and cant extensionalong the west coast of India [Hooper, Klein andLangmuir [1987] conclude that excess tempera- 1990], while much smaller(volumetrically) eruptions of turesof about250øC above ambient are required to obtain more alkalic lavas (typical of volcanismassociated with large melt fractions at depths of about 70-100 km. extensionin other areas)accompanied the extensionand Although this value is somewhatlarger than some rifting eventwhich carriedthe SeychellesBank westward. estimatesof maximumhotspot temperatures [Courtney and Furthermore,the Indian subcontinentwas moving very White,1986], it is at leastconsistent with thosesuggested rapidly over the mantle (-150-180 mm/yr) at the time of by others[Jacques and Green, 1980]. Hence the differ- the Deccaneruptions, thus precluding any largeaccumula- encewith regardto melt productionbetween the plume 29, 1/REVIEWSOF GEOPHYSICS Duncan and Richards: HOTSPOT VOLCANISM ß 45 initiationmodel and the rifting model for melt production significantlylower viscositythan the material through is that we assumea slightlyhigher temperature for at least whichthey rise. Figure 13a showssuch a plume formed the leadingdiapir of a startingmantle plume [Richardset by steadyinjection of dyedwater into higher-viscosityand al., 1989] than that advocatedby White and McKenzie higher-densityglucose syrup. A very large "head"forms [1989]. (It should also be noted that these inferred at the top of the plume, followed by a thin, connected temperaturesresult from experimentson water-freemelts conduit("tail") of buoyantfluid. Hence the possible andthat the presence of volatiles,particularly COo., in the analogyto floodbasalts and hotspottracks as plume heads more enrichedplume sourcematerial may enhancethe and mils is noted[Richards et al., 1989; Campbellet al., degreeof meltingat depthor producemelting at lower 1989]. temperatures[Willie, 1977]. We emphasize that neither the tectonic nor the petrologicalconstraints on the nature of flood basalt eruptionsare conclusive. More field work neexlsto be directedtoward elucidatingthe field relationswe have attemptedto summarize,and thisaspect of the problemhas beenneglected in comparisonto the enormousamount of work on the petrologyand geochemistryof flood basalts exemplified by several recently published reviews [Macdougall, 1988]. We have made no attempt to summarizethis vast literaturehere, and the main purpose of our brief review is to emphasizethe importanceof field observationsconcerning the relations between crustal extension and flood basalt volcanism. In additionto modelsbased on mantleplume activity, Rampinoand Stothers[1988] have suggestedthat flood basalteruptions are triggeredby meteoriteimpact. This remarkablehypothesis results mainly from the association of some flood basalt events with extinction events, in particular,the Deccanbasalts with the Cretaceous-Tertiary (K-T) boundaryevents and, possibly,the Siberianbasalts with the Permo-Triassicextinctions. This impactorigin for floodbasalts is inferredlargely from thecoincidence of the Deccan volcanism with the K-T boundary,but a generalizedmodel for flood basaltvolcanism and associated post impact hotspot track activity has not been formulated.In our opinionthe impacthypothesis for flood basalt and hotspotvolcanism lacks physicalplausibility (andcraters). However, the apparentcoincidence of two of the largestflood basaltevents (Deccan, Siberian) with the two largestextinction events further suggests the potential Figure 13a. Photographof a plume head followed by a trailing importanceof developinga derailedunderstanding of the conduit.The plumeis formedby injectionof water(colored with dynamical origins of flood basalt volcanism. In the red dye) at a constantrate into the bottomof a large tank of remainingdiscussion we brieflydiscuss the fluid dynamics glucose. of the plume initiation model. Having stated our preferencefor the plumeinitiation model [Richardset al., The starkcontrast between plume head size and feeder 1989], the following discussionreflects our bias against conduitwidth is mainlya resultof the viscositycontrast thepassive rifting model. betweenthe plume and the surroundingmaterial. The largerthe viscositycontrast, the thinnerthe conduit. This StartingPlumes canbe understoodeasily. The riseof the plumethrough Given that mantle plumes have for some time been the fluid is governedalmost entirely by the "Stokesian" widely acceptedas the likely causeof hotspottracks, it rise of the plumehead through the surroundingmaterial seems strangethat little work has been directed at the [Olsonand Singer, 1985], while the conduit must supply question,How do plumesstart? The answerappears to be the growingplume according to the volumetricfeed rate "Spectacularly!"at leastin thecases involving flood basalt (an artificeaddressed below). The lower the plume eruptions.Simple laboratory experiments [Whitehead and viscosity,the narrowerthe plumeconduit [Whitehead and Luther, 1975] showthat this behavioris to be expected Luther, 1975]. Returningto the hotspotanalogy, this from starting plumes, especiallyif the plumes have relationshipmay explain the very narrow apparent width of 46 ß Duncan and Richards: HOTSPOT VOLCANISM 29, 1 /REVIEWS OF GEOPHYSICS

mantleplumes (-10-100 km)comparedto the largeareal surfacemust be about50øC hotter (with only a small extentof flood basaltevents. Of course,a largeviscosity fractionof originalplume source material). contrastbetween the plume and the surroundingmantle is The experimentsshown in Figure 13 are artificialin the expectedif the plumebuoyancy is derivedfrom tempera- sensethat the plumesare injected(at a constantrate) and turesexceeding the surroundingmantle by perhapsseveral are not the result of the type of thermal convective hundred degrees Celsius [McKenzie and Bickle, 1988; instabilitylikely to occurat an internalboundary layer Sleep,1990]. such as the core-mantleboundary. Unfortunately,no On the basisof this simplemodel of diapiricinstability laboratoryor numericalconvection experiments have been and rise we have shown, for reasonablechoices of mantle conductedin the regime thought appropriateto the viscosity(1022 P) andplume density contrast (0.1 g/cm3), formation of manfie plumes. Such startingplumes are thata startingplume fed by a conduitsufficient to account expectedto arise in time-dependent,three-dimensional, for volcanismalong the R6unionhotspot track will achieve thermalconvection at very highRayleigh number in fluids a minimumvolume of about5 x 106km 3 [Richardset al, with stronglytemperature-dependent viscosity, but these 1989] (see Griffiths and Campbell [1990] for a more conditionshave not yet been reached in simulations. detailexldiscussion of this type of estimate). Extensive Thereforethis representsa gap in our ability to model melting of such a plume head beneaththe lithosphere startingplumes satisfactorily and to understandfully how would accountfor the largevolume of lavasin the Deccan to relate observationand theory,especially in regardto eruptions(1-2 x 106kma). In thiscontext it is important hotspots and flood basalts. to notethat the resultingmagma would have a higholivine Perhapsthe experimentsthat come closest to exhibiting content[Campbell and Griffiths, 1990] and that basaltic he expectedstarting behavior are those of Olson et al. magmas would result from olivine crystallizationand [1988] in which two-dimensionalplumes were generated removal at a density trap somewhereabove the crust- in a pulseheating mode. Plumedevelopment in onesuch mantleboundary (Moho) at a typicaldepth of 30-40 km. numericalexperiment is shownin Figure 14 [from Olson Such a fractionationprocess is well documentedfor the et al., 1988,Figure 18], wherea layercooled initially from tholeiitesof the ColumbiaRiver basalts[Hooper, 1988], and the plume initiation model suggests,at least in the caseswhere hugeeruptions precede large-scale extension, thatflood basalts may not represent primary magmas. One interestingaspect of thermalplumes is the process of entrainmentof surroundingmantle during their ascent

.. [Griffiths, 1986; Richards and Grij•ths, 1989]. Such .

.: mixing has importantconsequences for the geochemical : ...

ß signaturesof both hotspotand flood basalts,and it is of .. particular interest in light of the often enrichedtrace elementcontents and isotoperatios associatedwith these lavas [gindler and Hart, 1986]. In plumesdriven by compositionalbuoyancy as in Figure 13a, there is little mixing between the two fluids. However, in plumes driven by thermal buoyancy,thermal diffusion causes surroundingmaterial to becomebuoyant and rise along with the plume. This is shown in Figure 13b [from Gri•'ths and Campbell,1990], wherea hot, dyed starting plume of glucosewraps laminae of clear surrounding glucoseinto the head as it rises. If the original plume materialis fertile lower mantle,then the compositionally distinct(depleted in certainelements by repeatedmelting episodes)upper manfie material will be entrainedinto the plumehead, resulting in a complicatedmixing process as melting and volcanic eruptionsoccur. A particularly interestingaspect of Gfiffiths and Campbell'sanalysis is themanner in whichthe mantleacts as a plumebuoyancy flux filter in thermally dissipatingweak plumes (i.e., plume instabilitieswith effectively low Rayleigh num- bers). They concludedthat the strongeststarting plumes maintaintemperature excesses of-250øC in risingthrough Figure 13b. Photographof a startingthermal plume formed by the mantle (with only a small fraction of entrained injectionof hot,dyed glucose into a tankof cooler,clear glucose material),while the weakestplumes that make it to the [fromGriffiths and Campbell, 1990]. 29, 1/REVIEWSOF GEOPHYSICS Duncan and Richards: HOTSPOT VOLCANISM ß 47

I I 1 I I I

i I I I I I I

Figure 14. Isothermsfrom numericalpulse-heating experiments theologyat approximately10-m.y. intervals [from Olsonet al., on plume-lithosphereinteraction with temperature-dependent1988].

above (yielding a stiff lithosphere)is suddenlyheated (Figure12), are floodbasalt provinces, equivalent to the uniformly from below. The fluid has a mildly continentalexamples. We expectthat basalt ages across temperature-dependentviscosity, with maximumcontrast the entire Ontong-Javaplateau are contemporaneous. of a factorof 1000. Plume instabilitiesform and coalesce, Recentlycompleted deep sea drilling (OceanDrilling then rise to the top, followed by narrower,connected Projectleg 130) has recoveredthick, flood-typebasalt plume "conduits." This study showsthat the trailing flowsthat are contemporaneouswith submarine-erupted plumeconduits become thinner compared with thestarting flow sequencesexposed at the islandsof Malaita andSanta plumesas the viscositycontrast is increased,as expected Isabel,at the southernmargin of the plateau[Kroenke et on the basis of the simpler injection experiments. al., 1990]. However,we emphasizethat even theseexperiments are inadequateand artificial. The Earth, in threedimensions, is likely to exhibit strongercontrasts in rheological 6. CONCLUSIONS behaviorand probablydoes not conductpulse-heating experiments. (This is not a criticism of Olson et al.'s More than a peculiarvolcanic phenomenon that has experiments,since they were directedtoward understand- defied plate tectonicclassification, hotspot volcanism ing plume-lithosphereinteraction more than plume appearsto be a manifestationof whole-mantleconvection formation.) Clearly, thereis a needfor betterconvection via plumeflow. While we have focusedon the kinematics experimentsin orderto understandeven this very basic and dynamicsof hotspotsand plumes,the geochemical phenomenonof plumeinitiation. characteristicsof hotspot provinces indicate that plumes There is no doubt that some flood basalt events are delivermaterial from the lower mantle that is primordialor related to continentalfiring, but the cause and effect has beenaccumulating over billionsof yearsof plate relationshipbetween volcanism and rifting is unclear. subruction[e.g., Zindler and Hart, 1986]. Thepattern of Becausenot all continentalrift marginsshow a phaseof hotspotsis stablewithin useful time scales because plumes flood volcanism,even a modelin which rifting plays a originatein the deepmantle where the viscosity structure majorrole in melt generationmust involve extraordinary permits only horizontalvelocities that are muchlower than circumstances.A crucialdistinction between the passive platevelocities; plumes are not significantlydeflected by rift and the activeplume initiation models will be whether uppermantle horizontal flow. Plumeinitiation is governed certainoceanic plateaus, such as theOntong-Java plateau by heatflowing from the core to thelower mantle. Diapir 48 ß Duncanand Richards: HOTSPOT VOLCANISM 29, 1/ REVIEWSOF GEOPHYSICS formationand rise result in vastamounts of meltingfrom Duncan,R. A., Geochronologyof basalts from the Ninetyeast theplume head. This flood basalt phase is followedby an Ridgeand continental dispersion in the eastern Indian Ocean, J. Volcanol.Geotherm. Res., 4, 283-305, 1978. age-progressivehotspot track that is suppliedby melting of Duncan,R. A., Hotspotsin the southernoceans--An absolute the plume tail. Furtherstudies of continentaland oceanic frameof referencefor motionof theGondwana continents, flood basalt provincesshould better constrainthe time Tectonophysics,74, 29-42, 1981. scale,fluid dynamics,and heatflux of theseimportant Duncan,R. A., Ageprogressive volcanism in the New England events.It is clearthat hotspot traces form a rich sourceof seamountsand the openingof the centralAtlantic Ocean, J. constraintson the dynamical,compositional, and thermal Geophys.Res., 89, 9980-9990, 1984. Duncan,R. A.,The age distribution ofvolcanism along aseismic historiesof thedeep mantle which we havehardly begun ridges in the easternIndian Ocean, Proc. Ocean Drill. to utilize. ProgramSci. Results, 121, in press,1991. Duncan,R. A., andD. A. Clague,Pacific plate motion recorded ACKNOWLEDGMENTS.This research was supported by the by linearvolcanic chains, The Ocean Basins and Margins, NationalScience Foundation and the Office of NavalResearch. vol.7A, edited by A. E. M. Nairn,F. G. Stehli,and S. Uyeda, DavidScholl 'was the editor responsible for thispaper. He pp. 89-121, Plenum,New York, 1985. thanks David Engebretsonand Richard Carlson for their Duncan,R. A., andR. B. Hargraves,Plate tectonic evolution of assistancein evaluatingthe technicalcontent and Eric Wood for the Caribbeanregion in the mantlereference frame, The Caribbean-SouthAmerican Plate Boundary and Regional servingas a cross-disciplinaryreferee. Tectonics,edited by W. E. Bonini,R. B. Hargravesand R. Shagan,Mere. Geol. Soc. Am., 162, 81-93, 1984. Duncan,R. A.,and R. B. Hargraves,noAr_•9Ar geochronology REFERENCES ofbasement rocks from the Mascarene Plateau, Chagos Bank andthe Maldive Ridge, Proc. Ocean Drill. ProgramSci. Andrews,J. 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