HOTSPOTS, MANTLE PLUMES, FLOOD
TRUE
POLAR
WANDER
R. A. Duncan
M. A. Richards
Collegeof Oceanography
OregonStateUniversity,
Corvallis
Department
of Geology
University
of California,Berkeley
Abstract.
Persistent,long-lived,stationarysites of
excessivemantle melting are called hotspots. Hotspots
leave volcanictrails on lithosphericplatespassingacross
them. The global constellationof fixed hotspotsthus
formsa convenientframe of referencefor plate motions,
throughthe orientationsand age distributions
of volcanic
trailsleft by thesemeltinganomalies.Hotspotsappearto
be maintainedby whole-mantleconvection,in the form of
upwardflow throughnarrowplumes. Evidencesuggests
that plumesare deflectedlittle by horizontalflow of the
uppermantle. Mantle plumesare largelythermalfeatures
and arisefrom a thermalboundarylayer, mostlikely the
mantle layer just above the core-mantle boundary.
Experimentsand theoryshowthat gravitationalinstability
drivesflow, beginningwith the formationof diapirs. Such
a diapir will grow as it rises, fed by flow throughthe
trailing conduitand entrainmentof surroundingmantle.
The structurethusdevelopsa large, sphericalplumehead
and a long, narrow tail. On arrival at the base of the
1.
BASALTS, AND
INTRODUCTION
lithospherethe plume head flattens and melts by
decompression,
producingenormous
quantifiesof magma
which erupt in a short period. These are flood basalt
events that have occurred on continents and in ocean
basinsand that signal the beginningof major hotspot
tracks. The plume-supported
hotspotreferenceframe is
fixed in the steadystateconvectiveflow of the mantleand
is independentof the core-generated
(axial dipole)
paleomagnetic
referenceframe. Comparisonof plate
motions measured in the two frames reveals small but
systematicdifferencesthat indicatewhole-mantlemotion
relativeto theEarth'sspinaxis. Thisis termedtruepolar
wander
andhasamounted
tosome12ø sinceearlyTertiary
time. The directionand magnitudeof true polar wander
havevariedsporadically
throughthe Mesozoic,probably
in response
to majorchanges
in platemotions(particularly
subductionzone location) that change the planet's
moments of inertia.
their geometry and age distributions these volcanic
The plate tectonicsmodelhasbeenlargelysuccessful
in
explainingthe distributionof basalticvolcanismon our
planet. Mid-ocean ridge basalts are produced by
decompressionmelting of passively upwelling upper
mantle(asthenosphere)
at the edgesof separatingplates,
while subductionof oceaniclithospherecausesupper
mantlehydrationandmelting,yieldingvolcanicarcbasalts
along collisionalplate boundaries. Such plate margin
environments
accountfor the vast majorityof all magma
production.A volumetricallyminor but significantclass
of basalticvolcanismoccurswithinplatesor crosses
plate
boundaries,and it is characterized
by linear chainsof
volcanoes(hotspotlineaments)that grow older in the
directionsof plate motion. Classicexamplesof this
volcanicphenomenon
are the HawaiianIslands(and other
parallel island chains in the Pacific Ocean), the
Yellowstone-Snake
River Plainprovince,andthevolcanic
platformandridgesystemcenteredon Iceland. Becauseof
Copyright1991 by theAmericanGeophysical
Union.
provinces
are thoughtto resultfromfocuseduppermantle
zonesof meltingcalledhotspots[Wilson,1963, 1965]that
remainstationary
as theEarth'soutershellof lithospheric
plates move acrossthem. The proposedlocationsof
presenthotspotvolcanismareshownin Figure1.
Morgan [1971, 1972] proposedthat hotspotsare
maintained
by unusuallywarm materialrisingfrom the
lower mantle through narrow conduitsthat he called
mantleplumes. He furtherspeculated
thattheseplumes
comprised
theupwardflow of a long-lived,stablepattern
of whole-mantle
convection(Figure2). Onceestablished,
theseconduitsfor heat and materialtransportfrom the
lowermantledo not, Morganproposed,
changeposition
relativeto one another. Plumesprobablyinitiate in the
seismically
definedthermallayerat theboundary
between
the core and mantle.
Convection in the lower mantle is
likely to be very sluggish[Gurnis and Davies, 1986] so
plumes rising through this region should not be
"deflected,"or drift with respectto oneanother. Convec-
Reviews
of Geophysics,
29, 1/ February
1991
pages31-50
8755-1209/91/90RG-02372
$05.00
Papernumber90RG02372
.31.
32 ß Duncan and Richards: HOTSPOT VOLCANISM
29, 1 / REVIEWSOF GEOPHYSICS
Erebus
Figure 1. Worldwidesystemof hotspots.Criteriadefining are concentratedin zonesof high geoid residuals(contoursin
hotspots
areexcess
volcanism
anda spatialprogression
in theage metersarefromCroughandJurdy[ 1980]). The geoidanomalies
of volcanismconsistent
with localplatemotion. Magmaproduc-
alsocorrelate
withregions
of slowW in thelowermantle
tionratesvarybothfromonehotspotto anotherandalongindividualhotspottracks.Hotspots
arenotrandomly
distributed,
but
[Morelli and Dziewonski, 1987], suggestingthat hotspotsare
connectedwith mantle-wideupwardconvection.
tionratesin theuppermantlearemuchlarger,of theorder
of platevelocities
(10-100 mm/yr),sothehorizontal
upper
mantle flow might bend plumeson shortertime scales
flood basalts
[Whiteheadand Luther, 1975].
If mantleplumesdo indeedform a long-lived,stable
patternof convection
of heatandmaterialfromthelower ho ' •
plate
ts ot
'
to the uppermantle,thenthe geographic
orientations
and tra'i•
ductton
age distributions
of volcanicchainsrelatedto hotspots
offera very simpleanddirectrecordof the directionand
velocityof lithospheric
plate motionsover the last 100
m.y.(millionyears)or more.Ratesanddirections
of plate
separationderived from the alternatingnormal and
reversely magnetizedstripes of seafloor parallel to
spreadingridges measurepast large-scalehorizontal Figure 2. Cartoonof whole-mantleconvection. Piecesof the
separate
at spreading
movements
of piecesof theEarth'ssurfacerelativeto their rigidoutershellof the Earth(lithosphere)
ridgesandcollideat subduction
zones,coupledwith horizontal
neighbors.
Thisis calleda relativemotionreference
frame flow of the uppermantle. The deepmantlecirculates
through
becausethe spreadingridges from which motion is thermalplumesof gravitationally
unstable(warmer,lessdense)
measuredare themselves
movingover the mantle,albeit materialthat arisefrom nearthe core-mantle
boundary.Flood
moreslowlythanplates. A thirdreference
framefor plate basaltsform as the leading"head"of a new plumemeltsat the
motionsis providedby theEarth'smagnetic
field,whichis baseof thelithosphere.Hotspottrailsformasplatesmoveacross
assumedto have the shape(on average)of a dipolefield long-livedplume"tails." Convection
in the coreproduces
the
alignedalongthespinaxis. Rocksthatare magnetized
in Earth'smagneticfield.
29, 1/REVIEWSOF GEOPHYSICS
Duncan and Richards: HOTSPOT VOLCANISM ß 33
thisfield acquirea magneticinclination(angleof the rock
Morgan[1972]first notedthecongruence
of the major
magneticvectorfrom horizontal)uniqueto their latitude. island and seamountchainson the largestand fastestThus any translatitudinal(north-south)motion of litho- movingPacificplate (Hawaiian-Emperor,
Tuamotu-Line,
island and seamount
sphericplatescan be determinedfrom the paleomagnetic and Austral-Gilbert-Marshall
referenceframe;east-westmotionsare not foundby this lineaments).The geometryof thesevolcanictracescanbe
method. Comparisonof plate motionsdeducedfrom the reasonablywell matched by rigid plate motion over
hotspotand paleomagneticreferenceframes indicatesa hotspotsfixed at the presentlocationsof Hawaii, Easter
smallshiftof the wholemantlewith respectto the Earth's Island,and MacdonaldSeamount,respectively. SubsespinaxissinceearlyTertiarytime(55-65 Ma).
quentstudies,
incorporating
the distribution
of the ageof
In this paperwe review the evidencefor interhotspot volcanism
as well as the geometryof the traces[Clague
motion and examine the questionof whether hotspot and Jarrard, 1973; Jarrard and Clague, 1977; Duncan
lineamentsconstitutea usefulframeof referencefor plate and McDougall, 1976; McDougall and Duncan, 1980;
motions. This issuealso pertainsto the geometryand Duncan and Clague, 1985; Lonsdale, 1988], have
dynamicsof convectiveflow in the mantle,which in turn confirmed
the fixedpositions
of Pacifichotspots
overthe
constrainestimates
of the viscositystructure
of the mantle. past65 m.y. Morgan [1981] andDuncan [1981] showed
Many hotspotsappear to have begun with massive that a similarcasecan be made for rigid motionof the
outpouringsof basalticmagmasover broadregions,both Africanplateovera fixed setof hotspots
underlying
the
on continents
and in ocean basins.
These
volcanic
provincesare calledflood basaltsandprobablymark the
arrivalandinitial meltingof the"heads"of mantleplumes
at the baseof the lithosphere.Subsequent
volcanismover
the plume conduits(or "mils"),established
by the initial
plumeupwelling,are the familiar tracksthatconnectflood
basaltsto presentlyactivehotspots.
central and South Atlantic and western Indian ocean basins
since120Ma (Figure3).
Minster et al. [1974] and Minster and Jordan [1978]
foundno detectable
interhotspot
movementfor the past
5-10 m.y., on the basisof a best fitting global set of
relativeplate motionstied to Pacific hotspots. Early
comparisonsof Atlantic with Indian and Pacific Ocean
hotspots,on the other hand,reportedrelative motionsof
hotspots
up to 10-20 mm/yrduringTertiarytime[Burkeet
2. ARE HOTSPOTS
STATIONARY?
al., 1973; Molnar and Atwater, 1973; Molnar and
Francheteau,1975]. However,improvedrelativeplate
motioninformationand betterage resolutionof the older
ends of hotspottraces led Morgan [1981, 1983] and
An importantquestionconcerning
hotspotvolcanismis
whether or not mantle plumes are, in fact, fixed with
respectto oneanotherandthusdefinean irregularbutrigid Duncan [1981] to conclude that movement between North
referenceframe. Some amountof interplume(hotspot) Atlantic and Indian Oceanhotspotsmust be lessthan 5
motionmight be expectedbecauseof viscouscoupling
betweenthebaseof the movinglithospheric
platesandthe
uppermantleowingto horizontalflow of theuppermantle
awayfromspreading
centers(Figure2). If theseperturbations are small or constantover long periods, then
interplumedrift may be significantlyless than plate
velocities,and hotspotswith associatedvolcanictraces
constitute a convenient and direct reference frame for
reconstructingplate motions, independent of
paleomagnetic
referenceframe.
the
mm/yr. Molnar and Stock[1987] haverecentlyrevived
thisdebateby proposing
that the Hawaiianhotspothas
movedsouthward
10-20 mm/yr(km/m.y.)withrespect
to
hotspottracesin the Atlanticand Indianoceans. By
startingwith the Pacificplatehotspottraces,however,all
plate circuitsto platesborderingthe Indian or Atlantic
oceansmustincludeAntarctica.Molnar and Stock[1987,
p. 587] specifically"neglectdeformation
betweenEastand
West Antarctica." This contradictspaleomagnetic
evidencethatEastandWestAntarcticahaveexperienced
differential
rotations
sincetheEarlyCretaceous
[Wattsand
The magnitude
of interplume
motioncanbe assessed
by
comparingthegeometryandagedistribution
of volcanism Bramall, 1981; Mitchell et al., 1986; Martin, 1986]
along hotspot tracks with past plate movements accommodated
by a proposed
plateboundary
throughthe
reconstructed from relative motion data. If the motion of
Ross Sea-Weddell Sea region during Cenozoic time
one plate over hotspotsunderlyingit is determinedfrom [Molnar et al., 1975; Gordon and Cox, 1980; Stock and
theorientation
andageof volcanism
alongtrailsof islands, Molnar, 1982;Jurdy, 1979;Duncan, 1981]. Identification
seamounts,
and linear ridges,reconstructions
of the past of a smalloceanicplate in the Bellinghausen
Sea [Stock
positionsof neighboringplates with respectto those andMolnar, 1987]nowrequiressignificantly
lessmotion
hotspots
canbe calculated
fromseafloorspreading
data. If between
Pacificandotherhotspots
(5-10 mm/yr(J.Stock,
hotspotsunderlyingall plates are stationary,then the personalcommunication,
1990).
calculated
motionsof theneighboring
platesshouldfollow
A farlessambiguous
testof hotspot
fixityis tocompare
hotspottracksobservedon them. Any deviationsof these AtlanticandIndianOceanhotspot
positions
throughtime
predictedplate movementsfrom actual hotspottracks (because
all intervening
plateboundaries
aredivergent
and
would then indicate the magnitudeand direction of relativemotions
arerecorded
accurately
on theseafloor).
interplumedrift.
Since the Morgan [1981, 1983] and Duncan [1981]
34 ß Duncan and Richards: HOTSPOT VOLCANISM
29, 1 /REVIEWS OF GEOPHYSICS
African Plate
Fernando ,
outh
Cameroon
•
Plate
Line
i
i
,
Parana
0
Etendeka
125
i
Martin
Vas
• St Helena
i
•Walvis
Ridge
-30
Rise
Rio Grande Rise
30
o ,, Tristan
Mid-Atlantic
Marion
St•o•t-I
...
Indian
Ridge
,v•f'-'"
Antarctic
Plate
-60
-60
-30
0
Figure 3. Modeled hotspottracks for the southernAtlantic
Oceanbasin. (The ages,in millionsof years,of radiometrically
dated locationsare indicated,while solid dots along tracksare
10-m.y. incrementsof predicted volcano age.) Rotation
parameters
arechosenby trial anderrorto fit simultaneously
the
geometryandageprogression
of volcanismalonghotspottracks
on theAfricanplate(WalvisRidge,Cameroonline, Madagascar
30
60
Ridge, and MascarenePlateaulineaments). Relativeplate
motionsare added to obtain predictedhotspottracks on
neighboring
plates,assuming
fixed hotspots.The generally
excellent
matchbetweenpredicted
andobserved
trackssupports
the ideathathotspots
movevery slowly(2-5 mm/yr),if at all.
The Parana-Etendeka
flood basaltsattendingthe birth of the
Tristanhotspotarestippled.
analyses,
therehavebeenseveralrevisions
in relativeplate
motions,and the volcanic historiesof many prominent
hotspottraceshavebeenresolved.Hencea moredefinitive examinationof interhotspot
motionscannowbe made,
anda revisedsetof platerotationsappearsin Table 1.
Figure3 illustratestherotationparameters
for motionof
the African plate over stationaryhotspotssince 120 Ma
(the time of continentalbreakupin the SouthAtlantic),on
thebasisof thegeometryandradiometricagesfor theWalvis Ridge and CameroonLine [O'Connor and Duncan,
1990; O'Connor,1990]. As notedby earlierstudies,the
hotspottracescanbe matchedby rigidplatemotionovera
of the ShonaRidge-MeteorRise track [Hartnadyand le
Roex, 1985] is in accordwith a stationaryhotspot.Hence
theselinearvolcanictracksacrossthe entireAfricanplate
are compatiblewith fixed hotspots,duringthe past 120
the Cameroon Line [O'Connor, 1990], the Marion-
simultaneously
with theeasternhalf of the WalvisRidge,
m.y.
South and North American plate motions over the
hotspotnetworkare calculatedby addingthe latestrelative
platemotions[Candeet al., 1988;Klitgordand Schouten,
1985] to the motionof the African plate over hotspots
(Figures3 and 4). Hotspottracespredictedfrom these
platerotationscan thenbe comparedwith volcaniclineamentsassociated
with Atlantic hotspotsto checkfurther
setof fixedhotspots
distributed
over100ø (11,000km)of the fixed hotspothypothesis. The most prominentand
the Earth's surface. Now that the age of volcanismalong long-livedhotspottracesare the Rio GrandeRise (South
the Walvis Ridge is known in detail (from reliable Atlantic, Figure 3) and the New England Seamounts
4øAr-39
Ar incremental
heatingexperiments
on dredged (North Atlantic,Figure4). The geometryand few dated
andcoredvolcanicsamples),ratesof platemotioncan be positionsalongthe Rio GrandeRise [0' ConnorandDunassignedfor time intervalswith moreprecision,especially can, 1990] are matchedby the calculatedrotations. The
for the period30-80 Ma. Similarlydatedsamplesfrom Tristanda Cunhahotspotgeneratedthe Rio GrandeRise
Madagascartrack 0t. A. Duncan,unpublished
data,1990), whenthe SouthAtlanticspreading
ridgelay centeredover
andtheR6union-Mascarene
track[DuncanandHargraves, the hotspot.From about70 to 50 Ma the spreading
ridge
1990] corroboratetheseangularvelocities. The geometry migratedwestwardin a seriesof jumpsto itspresentposi-
29, 1/REVIEWSOF GEOPHYSICS
Duncan and Richards:HOTSPOT VOLCANISM ß 35
TABLE 1. Plate Rotationsin the HotspotReferenceFrame, 120 Ma to Present
TotalRotations,
de•
Period,Ma
11-0
21-0
36-0
42-0
48-0
58-0
66-0
(A5) •
(A6)
(AI3)
(AI8)
(A21)
(A26)
(A29)
80-0
Latitude(+N)
TotalRotations,
de•
Longitude(+E) Angle(+CCW)*
African Plate
60.0
--30.0
51.0
--45.0
40.0
-45.0
42.0
--42.5
42.0
--40.0
40.0
--40.0
JJ.U
---'•'I'U.U
• •
_Ar• r•
31.0
--50.0
Period,Ma
Latitude(+N)
Longitude(+E) Angle(+CCW)*
EurasianPlate (continued)
--2.0
--4.5
-8.0
--9.0
-9.6
--10.7
=/•"l',J
, A•
--20.0
100-0
25.0
--47.0
--25.5
120-0 (M0)
26.0
--42.5
-30.0
11-0
21-0
36-0
42-0
48-0
58-0
66-0
80-0
100-0
120-0
South American Plate
58.7
--45.3
68.8
--13.5
72.3
13.5
70.7
3.1
70.6
--6.0
74.4
0.1
78.0
64.5
66.2
63.8
62.1
31.2
56.1
6.7
2.1
3.3
6.4
7.6
10.3
13.0
13.4
18.0
22.9
27.1
11-0
21-0
36-0
42-0
48-0
58-0
66-0
80-0
100-0
120-0
North American Plate
27.0
128.2
30.4
113.2
39.8
108.9
46.1
111.2
53.1
113.1
59.1
112.3
50.3
118.6
49.3
101.8
58.4
80.2
62.3
61.1
1.4
3.3
6.7
7.8
9.4
12.6
15.9
20.6
29.9
37.7
11-0
21-0
36-0
42-0
48-0
Eurasian Plate
38.2
--47.2
30.5
-55.7
20.0
-60.4
18.2
-55.9
14.3
-52.0
-2.7
-5.3
-8.4
-9.3
--10.0
58-0
66-0
80-0
100-0
120-0
7.6
9.5
11.0
7.0
27.5
-46.2
-44.6
--56.2
107.0
88.9
--12.4
--16.0
--18.0
17.8
19.7
Indian Plate
11-0
21-0
36-0
42-0
48-0
58-0
66-0
80-0
100-0
120-0
11-0
21-0
36-0
42-0
48-0
58-0
66-0
80-0
100-0
120-0
11-0
21-0
36-0
42-0
48-0
58-0
66-0
80-0
100-0
120-0
37.7
31.0
24.2
26.4
26.8
26.9
20.4
19.3
14.3
14.0
Antarctic
57.1
74.5
72.7
85.9
83.2
77.5
78.8
76.6
89.1
80.4
Australian
25.9
20.1
17.7
20.3
24.6
32.9
29.2
35.4
49.5
72.7
26.7
27.6
26.4
27.2
24.2
13.5
10.3
2.8
6.6
3.7
-6.4
-12.7
-21.8
-25.6
-29.6
-36.4
-48.6
-62.5
-70.7
-77.0
Plate
--117.1
--110.2
--63.5
--3.4
136.4
--113.5
--16.9
--152.4
--52.7
--169.4
--3.0
-3.7
-4.7
-4.8
-6.4
-10.2
--10.1
--14.1
-15.5
-22.4
Plate
41.3
40.3
34.1
33.6
33.1
23.1
23.7
28.1
26.8
28.1
-7.8
--15.6
-24.6
-28.4
-28.0
-27.5
--29.5
--28.2
--32.1
--39.5
* CCW means counterclockwise.
'{'A5etc.aretheseafloormagneticanomalies
usedin relativeplatemotionreconstxuctions.
tion 400 km westof the Tristanhotspot;thisprocesseventually terminatedthe growthof the Rio GrandeRise at its
easternend. The Tristanhotspotwasbornat about125 Ma
with massiveeruptionsof flood basalts,now preservedat
the western end of the Rio Grande Rise (the Parana
basalts)andthe easternendof theWalvis Ridge (the Etendekabasalts),a pointwe will returnto in thenextsection.
The New England Seamountsform a 1000-km-long,
northwest-southeast
trendingchainof submarinevolcanoes
in the westerncentralAtlantic basin. Radiometricdating
areestimated
fromsubsidence
calculations
tobein therange
of 80-70 Ma [Tucholke
and Smoot,1990]. Crough[1981]
andMorgan [1983] proposeda hotspotpositionnearGreat
Meteor Seamount in the eastern central Atlantic basin that
was overridden(and henceseparatedfrom its trackon the
NorthAmericanplate)by theMid-AtlanticRidgesoonafter
theformation
of theComerSeamounts.
Figure4 compares
the traceof the New Englandhotspotpredictedfrom fixed
hotspots
with thepositionandageof volcanicstructures
in
thecentralAtlanticbasin.Theclosematchin geometry
and
(40At_39Ar ages)hasshownthatthe ageof volcanismageof volcanismbetweenpredictedand observedtracesis
decreasessmoothlyfrom 103 to 82 Ma (southeastward) strongevidencethattheNew Englandhotspot
hasremained
along the lineament[Duncan, 1984]. This volcanictrace fixedwithrespectto hotspots
in the SouthAtlanticsincethe
continueseastwardto the Comer Seamounts,
whoseages beginningof Cretaceous
time.
36 ß Duncan and Richards: HOTSPOT VOLCANISM
29, 1 /REVIEWS OF GEOPHYSICS
Voring
60 ,'
Plateau
6O
Hatton Bank
Figure 4.
orth
Modeled hotspot
tracks for the northern Atlantic
EuropeanPlate
Seamounts
Oceanbasin (see Figure 3 for
Plate
details). The text discussesthe
volcanismattendingthe birth of
theIcelandhotspot.
Azores
Seamounts
Col'lieF
Bermuda
3O
African Plate
Ridge
-90
-60
-39
0
CalculatedNorth Americanplate motionin the vicinity
of the Icelandhotspotalsoagreeswell with the orientation
of the Iceland-GreenlandRidge, but not with the timing
(60 Ma) of flood basaltvolcanismalong the eastcoastof
Greenland,an observationnoted by Morgan [1983] and
Vink [1984]. Our modelingpredictsthat at 60 Ma the
Icelandhotspotlay beneathcentralGreenland,some600
km from the plate edgewhere the North Atlantic Tertiary
Province flood basalts were erupted. This situationrequireseitherthat (1) the Icelandhotspothasdriftedsoutheastat an averagerate of 10 mm/yr relative to SouthAtlantichotspots,or (2) duringits initial eruptionsat 60 Ma
the hotspotwas very large (-1000 km diameter,Figure4)
and couldfeed flood basaltsto a regionincludingthe Hatton Bank, VOtingPlateau,and the Disko Basaltsof west-
30
hotspots (New England, Tristan, Marion) show no
detectable relative motion since 120 Ma.
Turning next to the Indian Ocean, Indian plate and
Antarcticplatemotionsoverhotspots
are foundby adding
the latestappropriaterelativeplate motions[Molnar et al.,
1987;RoyerandSandwell,1989]to the Africanplateover
hotspotsrotations. The Royer and Sandwell [1989]
rotationsincorporatea wealthof improvedrelativemotion
data for the Indian Ocean basin [Patriat, 1983].
The
resultingIndianplatemotionscan thenbe comparedwith
prominentvolcanictracesemanatingfrom theR6unionand
Kerguelenhotspots
(Figure5). Recently,legs115 and 121
of the Ocean Drilling Programfocusedon obtaining
volcanic samplesfrom these lineaments:the MascareneChagos-Maldives
Ridge (R6unionhotspottrace) and the
ern Greenland [White and McKenzie, 1989]. Vink [1984] NinetyeastRidge (Kerguelenhotspottrace). The age
presented
a modelin whicha small-diameter
hotspotunder distributionof the volcanismalong thesetrendsis now
Greenland fed spreadingridge eruptionsat the VOting well resolvedby 4øAr-39Ar dating [Duncanand
Plateau, easternGreenland,and the FaeroesPlateau by Hargraves,1990;Duncan,1978, 1991], soany departures
horizontal asthenosphericflow until Greenland drifted from fixed hotspots
shouldbe easilyidentified.
away from the hotspotand ridge-centeredvolcanismwas
Figure 5 showspredictedhotspottraces(whichassume
established. In view of the large region of flood basalt fixed hotspots)superimposed
on actualvolcanicstructures
eruptionslikely during a plume initiationevent (see next foundon thefloorof theIndianOceanbasin. A verygood
section)we feel that the notion of a point sourcefor the match betweenpredictedand observedtracesis obtained
Icelandhotspotis not applicableat 60 Ma. Followingthe by locatingtheR6unionhotspotbeneatha largeseamount
flood basaltevent the hotspotprobablydecreasedto "nor- 150 km to the west of the volcanicallyactive island of
mal" size, and the predictedyoungervolcanictrail (50 Ma R6union,and the Kerguelenhotspotbeneatha groupof
to present) follows the Iceland-Greenlandand western prominentseamountson the northwestmargin of the
Iceland-Faeroesridges. We can concludethat hotspots KerguelenPlateau. Both hotspotsbeganwith flood basalt
throughoutthe Atlanticbasin,from Marion andR6unionto events (the Deccan Traps and the Rajmahal Traps,
the centralAtlantic (and possiblyIceland),have remained respectively;
seealsoFigure12), thenleft parallel,northto
fixed for at least the past 60 m.y., and the longer-lived southgrowingvolcanicridgeson the trailingedgeof the
29, 1/REVIEWS
OFGEOPHYSICS
DuncanandRichards:
HOTSPOT
VOLCANISM
ß 37
30
! Indian Plate
117
African
Figure 5. Modeled hotspottracksfor the
Indian Oceanbasin(seeFigure3 for details).
Agesshownare radiometricage determinations for basalts from the two prominent
Chagos
Ridge
volcanic
lineaments:
the
R6union
and
:carene
Kerguelen hotspot tracks. Flood basalt
provinces(stippled)mark the initiation of
eachof thesehotspots.
,•,
-30
'
38
BrokenRidge
Australian
CD
Kerguelen
27
Plate
'.
Marion •q• Kerguelen
X.,•x-•Plateau
Antarctic
Plate •.....'.':•...
.:..........,...••114
•
-60
30
60
90
120
rapidlymovingIndianplateandwereoverridden
by the
centralandsoutheast
Indianspreading
ridgeat 36-38 Ma.
From that time forward the R6unionhotspothas formed
the Mascarene Plateau and the islands of Mauritius and
R6unionon theAfricanplate,while theKerguelenhotspot
hasformeda largepartof the northernKerguelenPlateau
on theslowlymovingAntarcticplate.
Finally, seamountchains in the Tasman Sea and
Australian
32
Plate
20'
continental basaltic volcanism in eastern Australia have
-30
Lord Howe
beenlinkedto hotspotactivity[Vogtand Conoily,1971].
Recentradiometricdatingof the seamounts
[McDougall
Tasman
• '
East
and Duncan, 1988] has confirmed a north to south,
monotonic
ageprogression
in the volcanismof the same
36
rate as observed in chains of central volcanoes in eastern
Australiafor the past35 m.y. [Wellmanand McDougall,
1974]. This regionlies in the easternmost
part of the
Indianplate. Thereis evidence
fromseismicity
andgeoid
anomaliesthatan east-westplateboundaryoccurswithin
!
Pacific
Plate
-60
the central Indian basin which has allowed a small amount
of north-south
compression
betweenthe Indianplateand
the Australianplateover the past10 m.y. [Royer,1988;
DeMets et al., 1988; Stein et al., 1990]. This proposed
relativeplatemotionhasbeenincorporated
in thepredicted
motionfor theAustralianplate,whichcloselymatchesthe
orientationand age progression
of the Tasmanseamount
chain(Figure6). Hotspotactivityin the vicinityof the
AntarcticBallenyIslandshasleft a volcanictrail on the
Australianplatethat showsup particularlywell on geoid
anomalymaps.A singledatedseamount
fitswell with the
predicted
ageof thetracesoutheast
of Tasmania.
In summary,geometricaland age comparisons
of
volcanicchainswith computer-generated
tracescalculated
120
150
180
Figure6. Modeledhotspot
tracksfor theeastern
Australian
plate
(see Figure 3 for details). Ages shownare radiometricage
determinations
[McDougalland Duncan, 1988; R. A. Duncan,
unpublished
datafor southeast
Tasmania,1990]. Solidsquares
are locations of volcanic centers.
38 ß Duncan and Richards: HOTSPOT VOLCANISM
by linking African plate motionover hotspotsmotion to
neighboringplates demonstratesthat there is no perceptibleinterhotspotmotionover periodsas long as 120
m.y. Moreover,the examinedhotspots
rangefrom Iceland
in theNorth Atlanticthroughthe SouthAtlanticandIndian
oceansto the Tasman sea, a region exceedinghalf the
Earth'ssurfaceand includingsix majorplates. According
to Molttar and Stock[1987], 95% uncertaintyellipsesfor
relative plate motion additionshave typical semimajor
axes of 200-300 kin, so the maximum allowable relative
motionamongthesehotspotsis about2-5 mm/yr. The
real motionis thusat leastan orderof magnitudelessthan
typicallithosphericplate velocities. There is no evidence
for any systematicdeparture between modeled and
observedhotspottracesfrom one plate to the next, which
might be expectedif hotspotsare greatly affected by
horizontalflow in the uppermantle. Over this half of the
globe, then, hotspotsare fixed to the extent that they
constitutea stable reference frame for plate motions,
recordedsimplyas volcanictrails.
Furtherextensionof thisanalysisinto the Pacificregion
is limited by subductionplate boundaries,whererelative
motionhistoriesare largely destroyed,or the ice-covered
Antarcticplate, whereplate boundariesare poorlyknown.
However,we know that Pacificbasinhotspotshavebeen
stationarysince 65 Ida [Duncan and Clague, 1985].
29, 1 /REVIEWS OF GEOPHYSICS
destroyed[Engebretsonet al., 1984, 1986; Duncan and
Hargraves, 1984; Rea and Duncan, 1986; Pollitz, 1988].
Continental tectonic and volcanic events in western North
Americaare particularlywell correlatedwith changesin
themagnitude
anddirectionof plateconvergence
predicted
from the hotspotreferenceframe [Engebretsonet al.,
1984; Wells et al., 1984; Verplanckand Duncan, 1987].
This methodeliminatesthe need to constructglobal
circuitsof platemotionsthatcrossonly spreading
ridges
[Jurdy,1984] andreducesthe largeuncertainties
accumulated in combiningsequences
of rotationpoles [Molnar
and Stock,1985].
Now, differential motion between the mantle and the
spinaxis can in principlebe determined
by comparing
platemotionsrecordedby the hotspotandpaleomagnetic
reference
frames.Suchmotionhasbeentermedtruepolar
wander(distinguished
from apparentpolar wanderof the
magnetic(spin) axis inferredfrom time sequences
of
paleomagnetic
field directionsfor given plates). It is
possible that redistributionof mass within the Earth
throughprocessessuch as mantle convectionand plate
motions may change the planet's momentsof inertia
sufficientlyto causea shiftof theentirebodyrelativeto its
spinaxis [Goldreichand Toorare,1969]. This is, in fact,
an old idea,suggested
as an alternativeto continentaldrift
as an explanationfor paleoclimaticevidencefor transMolnar and Stock's [1987] conclusionthat as much as 20 latitudinalmotionof the Earth's surface[Irving, 1964].
mm/yr interhotspot
motionhasoccurredcan thushaveone True polar wander,however,was largely ignoredafter it
of two explanations:eitherPacific hotspotshave moved was demonstratedthat separatecontinentshad distinct
(southward)en massewith respectto fixed hotspotsin the apparentpolar wander paths and that continentaldrift
Atlantic and Indian oceans,or all hotspotsworldwideare (platemotions)musthaveoccurred[e.g.,Runcorn,1965].
fixed and thereis someas yet poorlyknownplate bound- Couldbothmotionsoccursimultaneously?
ary within the Antarctic plate that has accommodated In the absenceof truepolar wander,mantleplumesdo
relative motion during Cenozoic time. The second not movewith respectto the geomagnetic
(or spin)axis.
possibilityappearsto be more likely, and the required Henceeveryvolcanogenerated
alonga givenhotspottrack
relative motion between East and West Antarctica can be
wouldrecordthe magneticinclination,usuallyexpressed
calculatedfrom the fixed hotspotreferenceframe[Duncan, asthepaleolatitude,
of thesiteof present
hotspot
activity.
1981].
If measuredpaleolatitudes
are constantalongthe hotspot
track,thenthe hotspothasnot movedwith respectto the
spin axis.
3. TRUE POLAR WANDER:
DOES THE MANTLE
ROLL?
If we nowacceptthe conclusion
of section1 thatmantle
plumesare stationary,to within about 5 mm/yr or less,
over periodsas long as 120 m.y., thenthe volcanictraces
left by hotspots
recordthe historiesof plate motionsover
the lower mantle.
This mantle reference frame is inde-
pendentof the paleomagneticreferenceframe and has
certain distinctadvantages. It does not dependon the
assumption
of the geocentricaxial dipolefield (for which
thereis goodevidenceexceptduringpolarityreversalsof
the field), and it resolveseast-westplatemotionwhichis
not detectedin paleomagnetic
studies. Severalgroups
have used the mantle reference frame to determine the
On the contrary,a changein hotspot
paleolatitudewith time would indicate motion of the
mantle(because
plumesare stationary
withinthe mantle)
relativeto the spinaxis,whichis truepolarwander.(Note
that the low viscosityof the fluid outer core allows the
geomagnetic
fieldto remaincoupledwith thespinaxis.)
HargravesandDuncan[1973]firstnotedthesystematic
northwardmotionof hotspots
in the Atlanticregionand
simultaneous
southwardmotionof hotspotsin the Pacific
region,relativeto thegeomagnetic
pole,overthepast--50
m.y. Thisobservation
wasexplained
by a 12ø clockwise
rotationof themantleaboutanequatorial
axisemerging
in
the westernIndianOcean. Subsequent
studies[Morgan,
1981;GordonandCape,1981;HarrisonandLindh,1982;
Livermore et al., 1983; Andrews, 1985; Gordon and
Livermore, 1987; Courtillot and Besse, 1987] have
history of plate convergenceacrosssubductionzones, confirmedthis surprisingconclusionand havedetermined
where the record of relative plate motion is largely a historyof truepolarwandersince200 Ma.
29, 1/REVIEWSOF GEOPHYSICS
Duncanand Richards:HOTSPOTVOLCANISM ß 39
Figure7 [fromCourtillotandBesse,1987]summarizes
thelatestcomparison
of platemotionsmeasured
in thetwo
reference frames. In this illustration,apparentpolar
wanderpaths (paleomagnetic
data) for all plates were
transferredto the African apparentpolar wanderpath by
removing relative plate motions. This would be the
apparentmotionof the geomagnetic
pole with respectto a
(long-lived)observeron the African plate. The apparent
motion of the hotspotreferenceframe with respectto
Africa can alsobe describedby a polar wanderpath. (In
polarwanderpriorto Jurassictime,but its magnitudeand
directionare less certain becauseof the paucity of
identifiablehotspottracksolderthan 120 Ma.
Intriguingly,the dramaticchangein directionof true
polarwanderat 40 Ma occurredwithina periodof global
platemotionreorganization
to significantlymoreeast-west
collisionalplateboundaries[Ronaand Richardson,1978].
The abrupt change in direction of the Pacific plate,
reflectedin the Hawaiian-Emperorbend (43 Ma), and the
"hard"collisionof IndiaandAfrica/ArabiaagainstEurasia
this case the rotation of the mantle relative to Africa, as
occurred at this time. Courtillot and Besse [1987]
concludedthat thesechangesin subductionzone location
inferredfrom the many hotspottracks,is appliedto the
spinaxis.) Grossfeaturesof thetwo pathsaresimilar,but
significantdifferencesare seenby subtractingone curve
from theother,to derivetruepolarwander.
180
•ots•ot•
and activityperturbedthe torquebalanceof the coupled
uppermantle-lithosphere
system,which then alteredthe
directionof truepolarwander.The earlier60-m.y.period
when no true polar wanderoccurredis correlatedwith a
time of decliningmagneticfield reversalfrequency
and
low lithosphericvelocities which, Courtillot and Besse
[1987] speculated,indicate a reduced state of mantle
convectionand decreasinginstability in the outer core,
probably linked by a lower heat flow acrossthe coremantleboundary.
4.
DO PLUMES SWAY IN THE MANTLE
WIND?
Having surveyedthe surface,volcanicexpressionof
hotspots,
let us now turnto the deeperconvective
regime
of mantleplumes. The fixity of hotspots
with respectto
plate motionsis difficult to explainwith simplemantle
convection
models. In fact, no numericalor laboratory
convection experiment has ever demonstrated such
0
behavior,that is, plumesthat are independent
of upper
boundarylayermotions.Apparently,thesourceregionfor
mantleplumesis recoupledfrom mantleflow associated
with plate motions,presumablyas a resultof chemical
Figure7. Comparison
of thesynthetic
paleomagnetic
apparent and/orrheologicalstratificationin the mantle. However,
polar wanderpath (triangles,which are paleomagnetic
pole the lack of any self-consistent
model is (and shouldbe)
positions
for all platesrotatedto the Africanplate,in present troubling.
coordinates)
with the synthetic
pathdeduced
fromthe hotspot
Oneapproachto thisproblemis to considertheeffectof
referenceframe (circles). True polar wander(squares)is
horizontal
shearin the uppermantlebeneatha moving
calculatedby subtracting
one from the other;this represents
plate
(the
"mantle
wind")upona cylindricalplumerising
globalmotionof themantlewithrespect
to theEarth'sspinaxis.
throughthe mantleto form a hotspot. Shearflow bends
the plumeover horizontally,and someremarkableresults
order
of4ø [from
Courtillot
andBesse,
1987].
have been obtainedfrom laboratoryfluid dynamical
simulations
of this flow interaction.Horizontallytilted
Approximately
12ø of the true polar wanderhas plume conduitswere first studiedby Skilbeckand
occurredin the past40 m.y., that is, clockwiserotationof Whitehead[1978] and Whitehead[1982], who discovered
the mantle about a pole on the equator,emergingnear thata tiltedconduitof low-density
materialcanbreakup
60øE(western
IndianOcean).Between
about110Ma and into a seriesof individualrisingblobs. This gravitational
40 Ma themantlerollednearly20ø aboutapproximatelyinstabilityoccursfor conduitsdeflectedto greaterthan
the samepole but in the oppositedirection,forming a about60ø fromthevertical
(Figure8). Whitehead
[1982]
hairpinturn in the truepolarwanderpathat about40 Ma suggested
thatthisbehaviormayoccurwithina shallow
(Figure 7). The period from 170 Ma to 110 Ma was shearzonebeneathan oceanicplate,perhaps
accounting
remarkablebecausethere was virtually no true polar for individualislandvolcaniccentersalongsuchhotspot
wander;hotspots
did notmovewithrespectto thespinaxis tracksastheHawaiian-Emperor
chain(Figure1la). Olson
duringthis time. There is evidencefor significanttrue andSinger[1985]andOlsonandNam [1986]suggested
Pointson eachcurveare20-m.y.increments,
from200 Ma to the
present. Confidenceintervalsof 95% (not shown)are of the
40 ß Duncan and Richards: HOTSPOT VOLCANISM
29, 1 /REVIEWS OF GEOPHYSICS
The aboverelationcan 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 combination
of thesimplelinearshear
flow in the experimentaltank and the constantStokesrise
velocityof the plume conduitresultsin the parabolic
trajectories
shownin Figure9.
Althoughthe experiments
shownhave a particularly
simpleflow geometry,it is possibleto use (1) to model
plumedeflectionundermorerealisticconditions.Figure
10 (top) illustratesagaina simpleshearflow, but with a
large increasein viscosityat the level of the dashedline.
The horizontalflow velocityis muchlarger in the lowviscosityregion immediatelybeneaththe plate, but the
Figure 8. Photographof a weak plume injectedinto a shear
flow. The tilted conduithasdevelopeda numberof instabilities
thatare separatingfrom theplume.
that a chainof risingblobsof mantle(diapirs)as largeas
400 km in diametermay resultfrom the sametype of
plume instabilityin the deepmantle. Olsonand Singer
[1985] alsopointedout that the relativefixity of hotspots
was not necessarilyinconsistentwith deflected,unstable
plumes,at leastfor steadystatemantlereturnflow.
In light of thesephenomenait is naturalto ask under
what conditionsplumesmay or may not suffersignificant
horizontal deflection. Deflection of buoyant plume
conduitsby mantleshearflow beneathmovingplateswas
studiedin detailby Richardsand Griffiths[1988]. Figures
9a-9c show a series of increasinglystrongermantle
plumes deflectedin a simple shear flow in laboratory
experiments.In eachof thephotographs
a cylindricaltank
of glycerol is fired with a lid which rotatesand createsa
shearflow in the tank. Motionis from left to fight in the
side-viewphotographs.A buoyantplume(80% glycerol
and20% waterwith red dye) is injectedat a constantrate
througha smallhole in the bottomof the tank to the left of
the dark verticalbandformedby the rotationalaxis of the
tank lid. The movinglid ("plate") generates
a cylindrical
shearflow, with velocityincSeasing
linearlyfromzeroat
the bottomof the tank. The plumetrajectories
are seento
be almostperfectparabolas,and Richardsand Griffiths
[1988] were able to model thesetrajectoriesaccurately
usinga simplerelation:
v = k$pga2/q
(1)
•½....
..........
..........................
.., ..
'::..;?':'.':"•.':::.::,:""•':'"•
. .•½•
.....
•::::ii•;ii:•.::•¾.;•:.,:'
.... .'-•¾..
::•:.:
"-'
........
.......................................
........
-'-i•:
.•:
•
...
'*-'
......
..::•:•:•.:
•:; ;:--',•:?'•....
:•-:
...... •
..............
•. ......
.
..............................................................................................
....
....
•::::•:::•
...........
.;•
....
.j......½½...•.,
..•,.....½•
.•.....•
-•
..•
..•
-m.....•...:'.X'•;;•
................. .'•:•
............
(!::•::
':•:'
•.,•..•.%.•;1;•?•:.:?::.:::...•.•.*`..::%7•.::C.•i5;•*•*•;•;•;•;•;•i•"
%.•.•.•";.;:•:•:•:i:•
.................................................................
'*.............................
•:•:'•:•:•:'•:'::•:•
..............
' "::•:
:"•
..:';:•';"•.:':;;:?:.•.;...:.-;.'
•,:
.....,.
::..
'.:::
'-•;;;;.•.;::•;;:'..'::.....:.;:;-:;;;•::::;.;:;:':....;.'
'.;•??:.•:.'....'......:::.,::,...:.::....:
.:..:;,,..:.:.::
..:.........:...;;.:..:.;:;....
:..•.:.::.,:•::....:.:.:..•;:.:;::.•::;•::....•:.....:.:.....::..:.;:.:
.:.:.:.:.:.
..............
::•.•:•.:..:.:::
.,::
;:;;;;-•;;;:..'.•,;;;
,:½...::
.... ;...:•::•
..... .:..:.::ß.........................................................
;..............................................................................................................
;:;•
...................
.
...-:•;•
.,:::-;.;;-.:,.--;;--:,{;:.:.:7.½?•;:;
;..•:4.•..•::...:..::•½•
..
....
. :..........................
.................
...........
.........................
,...................................
•:;½:::::;::.A.,•.
-.......
. ....................
.;•
...... ,..
.....
?•?
......•.:•;
...............
.-..:}-.•;
...............
:...........................
•,,Z:•;:::•.;•-.-.;::,;:•:;::;,::;•;•;,;•:---..::'.:•:-•,•,,.•::.;
;':"
::.5..:;•;•?..•:`.;?.:•;•..•..•.:*•.•..`.?:•.•;:•;?::•:•.:5
................
;:?.?:::•;:::•;•?--4:½-;,-;;;?;--::,.,---:,-,•?;';;:::.•::•;:;•:.;•?:½:::•?:.•.;•.?.?
----'•'•••;;....:...}•;::
"•'•---.....
•.:
.......................................................
½.....•..•._..•:
.......•....••..••,..,.,:.:,.-.:•...,
............
.::;;•
.........................
I..
.4 ½•½:;
..................
'.;
•
....
where v is an effectiveStokesrise velocity for a given
plumeconduitelement,•5pis thedensitycontrast
between
the plume and surroundingfluid, g is the gravitational
acceleration,
a is theplumeradius,andfl is theviscosityof
Figure 9. Photographs
of laboratorytankexperiments
showing
the surrounding
fluid; k is a constantdeterminedempiri- plumedeflectionby horizontalshearflow exertedby rotating
cally from theexperiments
(k = 0.54 + 0.02).
surfaceplate[Richardsand Griffiths,1989].
29,1/REVIEWS
OFGEOPHYSICS
Duncan
andRichards:
HOTSPOT
VOLCANISM
ß41
N-S andE-W spatialscales
in Figure1lb arenormalized
uo
by thefactorUh/2v,whichis themaximum
horizontal
deflectionof the plume. With this normalization
the
approximate
radius
of curvature
of thebendin thesurface
I
"ridge"
uo •_
•"trench"
•
75
!
.•_.•p/•xh
zC
!
!
'•
Emperor
Seamounts
I
x
•Hawaiian
Ridge
Figure10. (Top)Diagram
ofplumedeflection
in a plate-driven
shearflow with verticalviscositystratification
and (bottom)
diagram
ofplumedeflection
in a simple
return
flowmodel.
0
steadystateplumeprofileis unaffected.The higher
horizontal
flow velocityis justoffsetby theeffectof the
lowerviscosityon the Stokesrisevelocityof the plume
conduit
(see(1)). For steadystateflowthelow-viscosityFigure11a. Thevolcanic
trailof theHawaiian
hotspot
onthe
zonedoesnotprevent
plumedeflection.
(However,
it must Pacificplate(present
direction
indicated
by arrow).Agesalong
be bornein mind that the part of the plumein the low-
thelineamentarein millionsof years;an abruptchangein plate
viscosity
layerwill adjust
muchmorerapidlytochanges
in
motion). In the exampleof Figure10 (bottom)a return
directionoccurredat 43 Ma, anda lesseroneoccurredat 75 Ma.
flow is addedto accountfor trench-ridge
massbalancein
accordwith the constant
pressure
modelof Turcotteand
Schubert[1982]. A more complicatedplumetrajectory
results,with maximumhorizontaldeflectionat aboutone
fourthof thelayerdepthbeneath
theplate. (Thebottom
no-slipboundaryconditionin Figure10 (bottom)is
probably
unrealistic.
A freeslipcondition
wouldresultin
a finitevelocityat thebottomof thelayer,withnovertical
velocitygradientthere.)
The Stokes velocity characterization,
which was
developed
for steadystateplumedeflection,
alsoaccurately describes
the transientadjustment
of plumesto
changes
in the background
shearflow [Richards
and
Griffiths,1988]. Thisleadsto a veryspecific
application
of (1) above,namely,the adjustment
of the Hawaiian
plume to the changein Pacificplate motionwhich
occurredabout 43 million years ago, resultingin the
Hawaiian-Emperor
bend(Figure1la). Figure1lb showsa
mapview of the theoretical
surfacetraceof a deflected
mantleplume,subject
to a 60ø change
in platemotion
-1
-4
,
I
-3
,
I
-2
,
I
-1
,
I
0
,
1
(2v/Uh)X
(comparable
to the changein Pacificplatemotionat 43 Figure11b.A theoretical
plumetrackrelative
to a moving
plate
Ma). The "track"star•sat the top left with steadyplate whichundergoes
an instantaneous
changein direction
through
motion(speedU) at N30øW,andthenadjusts
to due 60ø withno change
in speedis alsoshown.Theplumerises
westerlyplate motion(also speedU) with the newest througha layer of constantshear. Spatialcoordinates
are
"volcano"at thefar right. In thismodel,thereis simple nondimensionalized so that unit distance is the maximum
shearflow beneaththeplatein a fluid layerof depthh. horizontaldeflection. This becomesthe radius of curvaturefor
The Stokesrisevelocityof the plumeconduitis v. The the bend.
42 ß Duncan and Richards: HOTSPOT VOLCANISM
29, 1 /REVIEWS OF GEOPHYSICS
trace is seento 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 tracein responseto the changein plate motionis
approximately
equalto the maximumhorizontaldeflection
of theplume. This theoreticalresultwasshownto be quite
accurate in laboratory experimentssimulating a plate
motionchange[GriffithsandRichards,1989].
The Hawaiian-Emperorbend itself has a radius of
curvatureof no morethanabout100-200 km (Figure1la).
(See for example, Jackson et al. [1972] for detailed
bathymetry.) Of course,someapparentcurvaturemust
resultfrom the finite width of the traceitselfand,perhaps,
a noninstantaneous
change in plate motion. From the
east of the Australiancontinentalmargin the Tasmanfid
Seamountshave beenshownto representAustralia-Indian
platemotionoverthreefixed hotspots
sinceat least24 Ma,
formingthreeparallelseamount/volcano
chains(Figure6
[McDougallandDuncan,1988]). Sincethesehotspots
are
at leastan orderof magnitudeweakerthanHawaii (e.g., in
terms of volcanic production rates or associated
bathymetricswells),it is even moreremarkablethat these
seamount
chainsare consistent
with the hotspotreference
above
theoretical
considerations
it
is clear
that
the
frame.
Although it is plausiblethat plumes remain vertical
beneathfastplates,we havenot justifiedthe fixity of the
plume sources. Thermal plumesmust originatefrom a
thermal boundary layer, and either the core-mantle
boundary(for mantle-wideconvection)or a hypothetical
boundarylayer in the mantle transitionzone (for chemically layered convection)is the only plausiblesource
region. Motions at the core-mantleboundarymay be
effectivelydecoupledfrom plate-relatedconvectiveflow
by an increasein mantleviscositywith depthby abouta
factorof 100-1000 (see,for example,Gurnisand Davies
[1986]). A boundarylayer at 670 km depthpresentsa
greaterdifficultysincetrench-ridge
returnflow is confined
to the uppermantle,regardlessof the viscositystructure.
Global modelsfor deep mantleflow derivedfrom plate
motions[Hagerand O'Connell,1981] andfrom attempts
to modelsesimically
imagedmantleheterogeneity
andthe
geoid[ForteandPeltier, 1987;RichardsandHager, 1988]
yield midmantlevelocitiessomewhatsmallerthan plate
velocitiesbut generally greater than about 5 mm/yr.
Thereforeit is notclearhow a midmantleboundarylayer
source for hotspot plumes could provide an absolute
referenceframe with respectto plate motions[Richards,
observed sharpnessof the Hawaiian-Emperor bend
constitutes a f'trst-order observation concerning the
dynamicsof the Hawaiian plume. The Hawaiian plume
cannotbe horizontallydeflectedmore than about200 km
in theregionof theuppermantlethatis stronglycoupledto
plate motions. This was first pointedout by Whitehead
[1982], who suggested
that the Hawaiianplumemightbe
stronglydeflectedwithin a shallowshearzone (-200 km
deep)beneaththe Pacific plate, giving rise to individual
islandvolcaniccentersvia conduitinstability. However,
an additionalconditionfor suchan instabilityis that the
plume diameter be much less than the layer depth;
otherwisethe diapirs would not have sufficientroom in
which to form. Alternatively,the plume may be vertical
through the entire upper mantle, and the formation of
individual volcanic centers might be controlled by
lithosphericstrengthandheterogeneity.
Using (1), it is instructiveto calculatethe sizeof plume
necessary
to remainverticalin the faceof the mantlewind
beneaththe Pacificplate. An appropriate
criterionwould
be that the verticalStokesrise velocityv associated
with
the Hawaii plume is not much less than the overriding 1991].
platevelocityof about100mm/yr.Setting
v•> 10mm/yr
andusingreasonable
valuesfor uppermantleviscosityq =
10zzP andplume
density
contrast
5p= 0.05g/cm
3,this
requiresa plume diameterd = 2a > 70 km. This is
consistentwith Morgan's [1972] original estimatefor
plume width basedon the gravity anomalyover Hawaii
and is certainlya reasonablevalue.
More remarkable perhaps than the spectacular
Hawaiian-Emperorbend is the fact that much weaker
hotspottracks also appear to represent"absolute"plate
motions quite reliably. Although there are no other
similarly distinctbends,our ability to formulatea fixed
hotspotreferenceframe showsthat other hotspotsin the
Pacific, Atlantic, and Indian oceansare not undergoing
significanttransientadjustmentto overlyingplatemotions.
In additionto the well-knownmajorhotspotsmentionedin
previoussections,severalseamountchainswhichare tiny
by comparisonalso appearto recordplate motion faithfully. The Cobb-Eickelbergseamountsin the northeast
Pacific have been shown by radiometricdating to be
explainableby a fixed hotspotactivesinceat least10 Me,
5. ARE FLOOD
INITIATION?
BASALTS THE RESULT OF PLUME
The origin of continentalflood basalt eruptionshas
remaineda major geologicalmysterywhich hasnot been
solvedby any aspectof the theory of plate tectonics.
Recently,muchattentionhasbeenfocusedon thegeological, geochemical,and geodynamicalnature of these
extraordinary
events,and thisinterestresultsin largepart
from the clear association of flood basalt events with the
beginningof hotspottracksas well as the coincidenceof
the spectacularDeccan flood basalt eruptionswith the
Cretaceous-Tertiary
mass extinctionevent [Duncanand
Pyle, 1988; Courtillot et al., 1988]. The associationof
floodbasaltsandhotspots
[Morgan,1972, 1981]is shown
in Figure12,andspatialandtemporal
relationships
suchas
thatbetweentheR6unionhotspotandtheDeccaneruptions
are now firmly documentedby precise K-Ar and
40Ar_39
Ar dating[Duncan
andHargraves,
1990].These
29,l/REVIEWS
OFGEOPHYSICS
Duncan
andRichards:
HOTSPOT
VOLCANISM
ß43
i
i
i
i
Yellowstone
i
i
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i
!
!
,,
i
!
i
i
,,
!,Ontong
$aval
i
i
i
i
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•.
,,
*%
I
.........
ILouisville[
I
I
I
others
haveformed
in continental
interiors
or in
Figure12. Worldwide
distribution
of floodbasalt
provincesnentalmargins,
erupted
in thelast250m.y.andassociated
hotspots
(where oceanbasins. CRB are ColumbiaRiver basalts,and NATB are
known
orconjectured).
Notethatwhilemany
occur
along
confi- North ArianticTertiarybasalts.
in termsof the geological
record. However,
hotspot/flood
basalt
correlations
alsoinclude
Iceland/Northguishable
Atlantic Tertiary basalts,Tristan da Cunha/Parana-their implicationsconcerningthe relationshipbetween
Etendekabasalts, Marion/Karoobasalts,Yellowstone/ continentalbreakupand plumes,the origin of plumes
and the processes
of meltingin flood basalt
ColumbiaRiverbasalts,andKerguelen/Rajmahal
traps.In themselves,
events
are
different
in
important
ways. White and
addition,some major oceanicbasalt plateaus(e.g.,
McKenzie
[1989]
have
recenfiy
developed
a detailed
Ontong-Java,
Caribbean)
maymarktheinitialactivityof
formulation
of
the
more
passive
and
uniformitarian
rifting
hotspots
(Louisville,
Galapagos).
Of all themajorflood
model,
providing
an
elegant
explanation
for
the
presence
basaltprovinces
only the Siberiantrapslack a clear
along rifted continental
association
witha knownhotspot
track,perhaps
because
of of huge basaltaccumulations
margins
but
placing
little
emphasis
on howplumesbegin
ourpoorknowledge
of theArcticOceanbasin.
and reach the base of the lithosphere. Richardset al.
[1989]haveemphasized
themeritsof theplumeinitiation
Passive
Riflingor PlumeInitiation?
model
in
explaining
the
extraordinary
eruptionratesand
Many of the continental
flood basalteruptions
are
sudden
onset
of
flood
basalt
volcanism,
also suggesting
associated
withtheearlystages
of continental
breakup
and
prerequisite
to the initial
rifting(No• Atlantic,Parana,
Karoo,Deccan),
andthis that rifting is not a necessary
observation
hasgivenriseto a number
of models
fortheir stagesof volcanism. Campbellet al. [1989] further
that Archeangreenstone
terranes(thick seoriginbased
onrifting[e.g.,Cox,1980;Bott,1982;Wtu'te suggested
of low-temperature
alteredbasaltic
rocks)maybe
et al., 1987;Meissner
andKopnick,1988].Morgan[1981] quences
proposed
two relatedhypotheses:
(1) thatcontinentaltheremnantsof plumeinitiationevents.
Bothmodelsrequirethepresence
of a broadreservoirof
lithosphere
wasweakened
by thelong-term
activityof a
anomalously
hot mantlebeneaththe lithosphere. The
hotspot,
whichwarmed
theuppermanfie(asthenosphere)
by WhiteandMcKenzie[1989]is
beneaththecontinentandled to massiveeruptionsderived riftingmodeldeveloped
fromthismaterialduringextension
andrifting;and(2) that very specificabouthow melt is generated.A plume
warmsthesublithospheric
mantleto
thelarge"head"of a newmantleplumeresultsfirstin a (steadyor otherwise)
asmuchas200øCin excess
of thenormal
massive
meltingevent,followedby moremodestactivity temperatures
potential
temperature
of 1280
ø, whichis typically
alongtheensuing
hotspot
trackbecause
of theremaining mantle
steady
plume.Thesetwomodels
arecloselyrelatedand, found beneathmid-oceanspreadingridges far removed
uponsuperficial
examination,
mayseemalmostindistin- from hotspots.Largedegreesof extension(factorsof at
44 ß Duncan and Richards: HOTSPOT VOLCANISM
least 2-4) in the overlying continentallithospherecause
decompression
meltingof the hot plumematerial,producing a melt thicknessof about25 km (as comparedwith 7
km for normal melting at spreadingridges)that eruptsto
fill the rift. Thusa hugeamountof melt is generatedwhen
a continentrifts abovean activemantleplume,as opposed
to normal mantle. This model has been applied to the
developmentof the volcanicrift marginsof the North
AtlanticTertiaryProvince(Figures4 and 12 [Whiteet al.,
1987]) and is most successfulin explaining the great
thicknesses
of onlappingbasalt (and perhapsunderlying
intrusive)structures
mappedin seismicreflectionprofiles
of the Hatton Bank and V0ring Plateaumarginson the
easternmarginsof the North Atlantic [Eldholmet al.,
29, 1 / REVIEWSOF GEOPHYSICS
tion of hot plume material beneath the plate due to
"ordinary"R6unionplumeactivity. (2) The mainsequence
of tholeiitic basalt lavas in the Parana event in South
America preceded significant extension in that area
[Piccirillo et al., 1988a, b], again with smaller, more
alkalic basalteruptionsaccompanying
the extensionthat
eventuallyled to continentalbreakup. (3) The large
~195-Ma central area tholeiitic basalt eruptionsof the
Karooprovince(Lesotho)are associated
with geochemically identicalfeederdike complexeswhichare pervasive
throughout
a very broadarea of what is clearlyunrifted,
stablecontinentallithosphere[Marsh, 1987]. "Associated"
rifting about20 m.y. later betweenAfrica and Antarctica
was accompaniedby huge alkalic basalt eruptionsthat
1989]. Cox [1989] has also invoked this model for the shiftedtowardthe rifting margin. (4) The GrandeRonde
evolutionof volcaniccontinentalmarginsto explain the eruptionsof the Columbia River Basalt Group were
developmentof drainagesystemsfollowing the Deccan, accompanied
by at most~1% crustalextension[Hooper,
Parana, and Karoo flood basalt events. Despite the 1990], with subsequent
extension(~20%) associatedwith
objections
we raisebelow,theWhite andMcKenziemodel more silica-richeruptions. (5) The SiberianTrapswere
providesa usefulquantitativeframeworkfor understanding precededby a long "geosynclinal"period,but structures
the evolutionof volcanic continentalmarginssubsequent disruptedby thesehugeintracontinental
eruptionsshowno
to rifting(for example,seeCox [1989]).
evidenceof significantextensioneitherprior to or during
The plume initiation hypothesiswas originally based flood volcanism [Makarenko, 1976]. Hence the Siberian
upon the largely circumstantial
evidence(1) that flood Trapsvolcanismdid not coincidewith major continental
basaltsoften representthe earliestactivity of hotspotsand rifting. (6) Even in the North AtlanticTertiaryProvince,
(2) thatthe truly catastrophic
natureof floodbasaltevents wheretheriftingmodelhasfounddetailedapplication,it is
requiredsomespecialcauseotherthanoneassociated
with not obviousthat all of the flood basalt eruptionswere
the time scale for plate motions. The first of these accompanied
by largedegreesof extension.For example,
observations
is questionable,
sinceit is possiblethat such the famousdike complexof Skye (westernScotland),
hotspotsasR6unionand Tristanda Cunhacannotnormally which fed someof the tholeiiticflood basalteruptions,
penetratestable continentallithosphere. The second indicatesonly mild extensionorientedperpendicular
rather
observationis more compelling becausethe alternative thanparallelto thedirectionof subsequent
rifting.
From this evidence we conclude that most flood basalt
rifting mechanism
described
abovepredictsthedurationof
floodbasalteventsto be of theorderof~10 m.y.,clearlyin events result from melting events at the base of the
disagreement
with evidencefor ~1 m.y. (or smaller)time lithosphere,
whetheror not suchthermaleventsprecipitate
scalesfor some events such as the Deccan basalts[Duncan subsequent
rifting. Thereforewe must appeal to an
pulseof mantleplumematerialfar in excess
and Pyle, 1988; Courtillotet al., 1988;Baksi and Kunk, extraordinary
1988]. Also, as we describe in some detail below, of the flux represented
by the subsequent
hotspottrack.
laboratory and numerical models suggestthat plume Thisgivesriseto an interesting
andfundamental
question:
initiation may be expectedto result in a huge volcanic Canlargedegrees
of partialmelting(say10-20%) occurat
eventat thebeginningof a hotspottrack.
thebaseof normalcontinental
lithosphere
in theabsence
of
Althoughthis evidenceis suggestive,
we find that the significantmechanicalthinning(rifting)? Althoughwe
observedtectonicsettingsfor flood basalteventsprovide will not attempta full discussion,a partial answerto this
compelling evidence that these eruptions do not, in questionis foundby referringto recentmantlemelting
general,resultfrom decompression
meltingin responseto models based upon melting experimentson candidate
lithosphericextension. In fact, manyflood basaltevents mantlerocksand upon observations
of major and trace
are neitherprecedednor accompanied
by largedegreesof elementvariationsin oceanridge basaltsand peridotites
extension. Many examplesmay be cited: (1) The main (Mg,Fe silicate
rocksthoughttoberepresentative
of upper
tholeiiticeruptionsof the DeccanTrapsprecededsignifi- mantlematerial). Both McKenzie and Bickle [1988] and
cant extensionalong the west coast of India [Hooper, KleinandLangmuir[1987]conclude
thatexcesstempera1990], while much smaller(volumetrically)eruptionsof tures
of about250øCaboveambient
arerequired
toobtain
more alkalic lavas (typical of volcanismassociatedwith large melt fractions at depths of about 70-100 km.
extensionin other areas)accompaniedthe extensionand Although this value is somewhatlarger than some
rifting eventwhichcarriedthe SeychellesBank westward. estimates
of 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[Jacquesand Green, 1980]. Hence the differthe Deccaneruptions,thusprecludingany largeaccumula- encewith regardto melt production
betweenthe plume
29, 1/REVIEWSOF GEOPHYSICS
Duncan and Richards: HOTSPOT VOLCANISM
ß 45
initiationmodeland the rifting model for melt production
is thatwe assumea slightlyhighertemperature
for at least
the leadingdiapir of a startingmantleplume [Richardset
al., 1989] than that advocatedby White and McKenzie
[1989]. (It should also be noted that these inferred
temperatures
resultfrom experimentson water-freemelts
significantlylower viscositythan the material through
whichtheyrise. Figure 13a showssucha plumeformed
by steadyinjectionof dyedwaterinto higher-viscosity
and
higher-density
glucosesyrup. A very large"head"forms
at the top of the plume, followed by a thin, connected
conduit("tail") of buoyantfluid. Hence the possible
andthatthepresence
of volatiles,
particularly
COo.,in the analogyto floodbasaltsandhotspottracksasplumeheads
more enrichedplume sourcematerial may enhancethe and mils is noted[Richardset al., 1989; Campbellet al.,
degreeof meltingat depthor producemeltingat lower 1989].
temperatures
[Willie, 1977].
We emphasize that neither the tectonic nor the
petrologicalconstraintson the nature of flood basalt
eruptionsare conclusive. More field work neexlsto be
directedtoward elucidatingthe field relationswe have
attemptedto summarize,and thisaspectof theproblemhas
beenneglectedin comparisonto the enormousamountof
work on the petrologyand geochemistry
of 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 reviewis to emphasizethe importanceof field
observationsconcerning the relations between crustal
extension and flood basalt volcanism.
In additionto modelsbasedon mantleplumeactivity,
Rampinoand Stothers[1988] have suggestedthat flood
basalteruptionsare triggeredby meteoriteimpact. This
remarkablehypothesisresultsmainly from the association
of some flood basalt events with extinction events, in
particular,the Deccanbasaltswith the Cretaceous-Tertiary
(K-T) boundaryeventsand, possibly,the Siberianbasalts
with the Permo-Triassic
extinctions.This impactorigin
for floodbasaltsis inferredlargelyfrom thecoincidence
of
the Deccan volcanismwith the K-T boundary,but a
generalizedmodel for flood basaltvolcanismand associated post impact hotspot track activity has not been
formulated.In our opinionthe impacthypothesis
for flood
basaltand hotspotvolcanismlacks physicalplausibility
(andcraters).However,theapparentcoincidence
of two of
the largestflood basaltevents(Deccan,Siberian)with the
two largestextinction
eventsfurthersuggests
thepotential Figure 13a. Photographof a plume head followedby a trailing
importanceof developinga derailedunderstanding
of the conduit.The plumeis formedby injectionof water(coloredwith
dynamical origins of flood basalt volcanism. In the
remainingdiscussion
we brieflydiscuss
thefluid dynamics
of the plume initiation model. Having stated our
preferencefor the plumeinitiationmodel[Richardset al.,
1989], the followingdiscussion
reflectsour bias against
thepassiverifting model.
StartingPlumes
Given that mantle plumes have for some time been
widely acceptedas the likely causeof hotspottracks,it
seemsstrangethat little work has been directedat the
question,
How do plumesstart?The answerappears
to be
"Spectacularly!"
at leastin thecasesinvolvingfloodbasalt
eruptions.Simplelaboratoryexperiments[Whiteheadand
Luther, 1975] showthat this behavioris to be expected
from startingplumes, especiallyif the plumes have
red dye) at a constantrate into the bottomof a large tank of
glucose.
The starkcontrastbetweenplumeheadsizeand feeder
conduitwidthis mainlya resultof the viscositycontrast
betweenthe plume and the surrounding
material. The
largerthe viscositycontrast,the thinnerthe conduit. This
canbe understood
easily. The riseof theplumethrough
the fluid is governedalmostentirelyby the "Stokesian"
rise of the plumeheadthroughthe surrounding
material
[OlsonandSinger,1985],whiletheconduitmustsupply
the growingplumeaccordingto the volumetricfeedrate
(an artificeaddressed
below). The lower the plume
viscosity,the narrowerthe plumeconduit[Whiteheadand
Luther, 1975]. Returningto the hotspotanalogy,this
relationship
mayexplaintheverynarrowapparent
widthof
46 ß Duncan and Richards: HOTSPOT VOLCANISM
mantleplumes(-10-100 km)comparedto the largeareal
extentof flood basaltevents. Of course,a largeviscosity
contrastbetweenthe plumeand the surrounding
mantleis
expectedif the plumebuoyancyis derivedfrom temperaturesexceedingthe surrounding
mantleby perhapsseveral
hundred degrees Celsius [McKenzie and Bickle, 1988;
Sleep,1990].
On the basisof thissimplemodelof diapiricinstability
29, 1 /REVIEWS OF GEOPHYSICS
surface
mustbe about50øChotter(withonlya small
fractionof originalplumesourcematerial).
The experiments
shownin Figure13 areartificialin the
sensethat the plumesare injected(at a constantrate) and
are not the result of the type of thermal convective
instabilitylikely to occurat an internalboundarylayer
such as the core-mantleboundary. Unfortunately,no
laboratoryor numericalconvection
experiments
havebeen
and rise we have shown, for reasonablechoicesof mantle conductedin the regime thought appropriateto the
viscosity
(1022
P)andplume
density
contrast
(0.1g/cm3),formationof manfie plumes. Such startingplumesare
thata startingplumefed by a conduitsufficientto account expectedto arise in time-dependent,
three-dimensional,
for volcanismalongtheR6unionhotspottrackwill achieve thermalconvection
at very highRayleighnumberin fluids
a minimum
volume
of about5 x 106km3 [Richards
etal, with stronglytemperature-dependent
viscosity,but these
1989] (see Griffiths and Campbell [1990] for a more conditionshave not yet been reached in simulations.
detailexldiscussionof this type of estimate). Extensive Thereforethis represents
a gap in our ability to model
melting of such a plume head beneaththe lithosphere startingplumessatisfactorily
andto understand
fully how
wouldaccountfor the largevolumeof lavasin the Deccan to relateobservation
and theory,especiallyin regardto
eruptions
(1-2 x 106kma).In thiscontext
it isimportanthotspotsandfloodbasalts.
to notethatthe resultingmagmawouldhavea higholivine
Perhapstheexperiments
thatcomeclosestto exhibiting
content[Campbelland Griffiths, 1990] and that basaltic he expectedstartingbehaviorare those of Olson et al.
magmas would result from olivine crystallizationand [1988] in whichtwo-dimensional
plumeswere generated
removal at a density trap somewhereabove the crust- in a pulseheatingmode. Plumedevelopment
in onesuch
mantleboundary(Moho) at a typicaldepthof 30-40 km. numericalexperimentis shownin Figure 14 [from Olson
Such a fractionationprocessis well documentedfor the et al., 1988,Figure18],wherea layercooledinitiallyfrom
tholeiitesof the ColumbiaRiver basalts[Hooper, 1988],
and the plume initiation model suggests,at least in the
caseswherehugeeruptionsprecedelarge-scaleextension,
thatfloodbasaltsmaynotrepresent
primarymagmas.
One interestingaspectof thermalplumesis the process
of entrainmentof surroundingmantleduring their ascent
[Griffiths, 1986; Richards and Grij•ths, 1989]. Such
mixing has importantconsequences
for the geochemical
signaturesof both hotspotand flood basalts,and it is of
particular interest in light of the often enrichedtrace
elementcontentsand isotoperatios associatedwith these
lavas [gindler and Hart, 1986]. In plumesdriven by
compositional
buoyancyas in Figure 13a, there is little
mixing between the two fluids. However, in plumes
driven by thermal buoyancy,thermal diffusion causes
surroundingmaterial to becomebuoyantand rise along
with the plume. This is shown in Figure 13b [from
Gri•'ths and Campbell,1990], wherea hot, dyedstarting
plume of glucosewraps laminae of clear surrounding
glucoseinto the head as it rises. If the originalplume
materialis fertile lower mantle,then the compositionally
distinct(depletedin certainelementsby repeatedmelting
episodes)uppermanfiematerialwill be entrainedinto the
plumehead,resultingin a complicatedmixingprocessas
melting and volcaniceruptionsoccur. A particularly
interestingaspectof Gfiffiths and Campbell'sanalysisis
themannerin whichthe mantleactsas a plumebuoyancy
flux filter in thermally dissipatingweak plumes (i.e.,
plume instabilitieswith effectively low Rayleigh numbers). They concludedthat the strongeststartingplumes
maintain
temperature
excesses
of-250øCinrisingthroughFigure13b. Photograph
of a starting
thermal
plumeformed
by
the mantle (with only a small fraction of entrained injection
of hot,dyedglucose
intoa tankof cooler,clearglucose
material),while the weakestplumesthat make it to the [fromGriffithsandCampbell,1990].
..
.
ß
.
.:
:
...
ß
..
29, 1/REVIEWSOF GEOPHYSICS
Duncan and Richards: HOTSPOT VOLCANISM ß 47
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Figure 14. Isothermsfromnumericalpulse-heating
experiments theologyat approximately
10-m.y.intervals[from Olsonet al.,
on plume-lithosphere
interactionwith temperature-dependent1988].
above(yieldinga stiff lithosphere)is suddenlyheated (Figure12), are floodbasaltprovinces,
equivalentto the
uniformly from below. The fluid has a mildly continental
examples.We expectthatbasaltagesacross
temperature-dependent
viscosity,with maximumcontrast the entire Ontong-Java
plateauare contemporaneous.
of a factorof 1000. Plume instabilitiesform andcoalesce, Recentlycompleteddeep sea drilling (OceanDrilling
then rise to the top, followed by narrower,connected Projectleg 130) has recoveredthick, flood-typebasalt
plume "conduits." This studyshowsthat the trailing flowsthatare contemporaneous
with submarine-erupted
plumeconduits
becomethinnercompared
withthestarting flow sequences
exposedat theislandsof MalaitaandSanta
plumesas the viscositycontrastis increased,
as expected Isabel,at the southernmarginof the plateau[Kroenkeet
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 strongercontrastsin rheological 6. CONCLUSIONS
behaviorand probablydoes not conductpulse-heating
experiments. (This is not a criticism of Olson et al.'s
More than a peculiarvolcanicphenomenon
that has
experiments,
sincetheyweredirectedtowardunderstand- defied plate tectonicclassification,
hotspotvolcanism
ing plume-lithosphere
interactionmore than plume appearsto be a manifestationof whole-mantleconvection
formation.) Clearly,thereis a needfor betterconvection via plumeflow. While we havefocusedon the kinematics
experiments
in orderto understand
even this very basic and dynamicsof hotspots
and plumes,the geochemical
phenomenon
of plumeinitiation.
characteristics
of hotspotprovinces
indicatethatplumes
There is no doubt that some flood basalt events are
delivermaterialfromthelowermantlethatisprimordial
or
related to continentalfiring, but the causeand effect has beenaccumulating
over billionsof yearsof plate
relationshipbetweenvolcanismand rifting is unclear. subruction
[e.g.,ZindlerandHart, 1986]. Thepatternof
Becausenot all continental
rift marginsshowa phaseof hotspots
is stablewithinusefultimescales
because
plumes
flood volcanism,even a modelin whichrifting playsa originate
in thedeepmantlewheretheviscosity
structure
majorrole in melt generation
mustinvolveextraordinarypermitsonly horizontalvelocitiesthatare muchlower than
circumstances.
A crucialdistinction
betweenthepassive platevelocities;
plumes
arenotsignificantly
deflected
by
rift andthe activeplumeinitiationmodelswill be whether uppermantlehorizontal
flow. Plumeinitiationis governed
certainoceanicplateaus,
suchas theOntong-Java
plateau by heatflowingfromthecoreto thelowermantle.Diapir
48 ß Duncan
andRichards:
HOTSPOT
VOLCANISM
29, 1/ REVIEWS
OFGEOPHYSICS
formation
andriseresultin vastamounts
of melting
from Duncan,
R. A., Geochronology
of basalts
fromtheNinetyeast
Ridgeandcontinental
dispersion
in theeastern
IndianOcean,
theplumehead.Thisfloodbasalt
phase
is followed
byan
J. Volcanol.Geotherm.
Res.,4, 283-305, 1978.
age-progressive
hotspot
trackthatissupplied
bymelting
of Duncan,
R. A., Hotspotsin the southernoceans--Anabsolute
the plume tail. Furtherstudiesof continentaland oceanic
flood basalt provincesshouldbetter constrainthe time
frameof reference
for motionof theGondwana
continents,
Tectonophysics,
74, 29-42, 1981.
scale,fluid dynamics,
andheatflux of theseimportant Duncan,
R. A., Ageprogressive
volcanism
in theNewEngland
seamounts
andtheopeningof the centralAtlanticOcean,J.
events.It is clearthathotspottracesforma richsourceof
constraints
on thedynamical,
compositional,
andthermal
Geophys.
Res.,89, 9980-9990, 1984.
Duncan,
R. A.,Theagedistribution
ofvolcanism
along
aseismic
histories
of thedeepmantlewhichwehavehardlybegun
ridges in the easternIndian Ocean, Proc. Ocean Drill.
Program
Sci.Results,
121,in press,1991.
Duncan,
R. A., andD. A. Clague,
Pacificplatemotion
recorded
to utilize.
by linearvolcanic
chains,
TheOceanBasins
andMargins,
vol.7A,edited
byA. E.M. Nairn,F. G.Stehli,
andS.Uyeda,
ACKNOWLEDGMENTS.
Thisresearch
wassupported
bythe
NationalScience
Foundation
andtheOfficeof NavalResearch.
pp. 89-121, Plenum,New York, 1985.
DavidScholl'wastheeditorresponsible
for thispaper.He
Duncan,R. A., andR. B. Hargraves,
Platetectonic
evolution
of
the Caribbean
regionin the mantlereference
frame,The
thanks David Engebretsonand Richard Carlson for their
assistance
in evaluatingthe technicalcontentandEric Woodfor
servingasa cross-disciplinary
referee.
Caribbean-South
American
PlateBoundary
andRegional
Tectonics,
editedby W. E. Bonini,R. B. Hargraves
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