ELSEVIER Earth and Planetary Science Letters 158 (1998) 1–18

The geometric relationship between hot spots and seamounts:implications for Pacific hot spots

Paul Wessel Ł, Loren W. KroenkeSchool of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA

Received 28 July 1997; revised version received 29 December 1997; accepted 6 February 1998

Abstract

Hot spots and the seamounts produced by them provide both geometric and temporal evidence for changes in absoluteplate motion. The main limitation in using hot-spot-produced seamounts in plate tectonic reconstructions arises fromthe multiple sources of error and ambiguity that plague radiometric age estimates. In particular, unless the hot spot hasmaintained a steady and voluminous flux rate over long periods of time, the exact location of a hot spot (which representsthe zero age origin along the hot spot trail) is poorly known. Here, we discuss a unique geometric relationship between ahot spot and the seamounts produced by it that we recently have discovered, i.e. hot-spot-produced seamounts have seafloorcrustal flow lines that intersect at the hot spot location. Furthermore, we obtain images of cumulative volcano amplitudes(CVA) by convolving seamount shapes with their flow lines; hot spots correspond to clear local maxima in this image andthe amplitudes are proportional to cumulative hot spot flux. This technique, dubbed ‘hot-spotting’, allows us to determinehot spot locations based only on a set of seamount locations; no age information is required. We use the hot-spotting tech-nique to examine the Pacific plate hot spots in general and the Bowie and Cobb hot spots in the Gulf of Alaska, in particular.We find that the Hawaii, Louisville, Caroline, Cobb, and Bowie hot spots have clear representations in the CVA images, Ru-rutu and=or Rarotonga are close to a large CVA high, while the other French Polynesian hot spots in general exhibit a muchmore subdued and blurred expression. We also conclude that the Cobb hot-spot plume may have been entrained by the Juande Fuca Ridge about 2 Ma ago, or, alternatively, is in a waning phase and cannot penetrate Juan de Fuca plate lithosphere.The Bowie hot spot appears to have encountered the ridge more recently. 1998 Elsevier Science B.V. All rights reserved.

Keywords: hot spots; seamounts; plate tectonics

1. Introduction

The modern theory of plate tectonics was in itsinfancy when Tuzo Wilson suggested that the Hawai-ian seamount chain could have been formed as thePacific plate moved over a hot spot in the Earth’smantle [1]. However, several more years passedbefore Jason Morgan expanded on Wilson’s idea

Ł Corresponding author. Fax: C1 (808) 956-4778.

and proposed the hot spot hypothesis to explainthe origins of several, apparently collinear Pacificseamount chains [2]. Morgan’s hypothesis assumedthat hot spots were deep-seated thermal anomaliesfixed with respect to the surrounding mantle. Plumesfrom the lower mantle would supply hotter materialto the surface which would manifest itself as surfacevolcanism. Because plates move over the Earth’ssurface, the volcanism would delineate and recordthe history of plate motion. Moreover, individual

0012-821X/98/$19.00 1998 Elsevier Science B.V. All rights reserved.PII S 0 0 1 2 - 8 2 1 X ( 9 8 ) 0 0 0 4 3 - 0

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seamounts should form at the time they were directlyabove the hot spot, hence the radiometric age ofseamounts should show a simple progression awayfrom the present-day hot spot locations.

Despite the success of the hot spot hypothesis[3–7] there are several unresolved problems pertain-ing to mid-plate tectonics. One of the most impor-tant involves the notion of hot spot fixity. If hot spotsare stationary, then seamount chain geometries canbe modeled as series of small circles about a set ofstage (Euler) poles. However, if hot spots migrate,then there will be a directional bias in the geometry.A test for hot spot fixity necessarily requires accu-rate locations of present-day hot spots. Unfortunately,placing much confidence in these locations appearsunwarranted, for a number of reasons. Apart fromthe strong, continuous volcanic activity at the Hawai-ian hot spot (thought to be located in the vicinity ofKilauea and Loihi) most hot spots show more inter-mittent activity, making it difficult to determine theexact present location. Moreover, radiometric datesfor seamounts in a hot-spot-produced chain are sub-ject to several sources of uncertainty, including: (1)analytical errors; (2) incomplete knowledge of whichstage of the volcanism a sample represents; (3) move-ment of the same area of the plate over multiple hotspots aligned in the direction of plate motion caus-ing chains to intertwine; and (4) later over-printingby tectonic or posterosional volcanism. For instance,despite the steady volcanism at Kilauea, there wasa historical eruption in 1790 on Maui, almost 200km from Kilauea. Consequently, when seamountsare ‘back-tracked’ to their point of origin, the back-tracked positions show considerable scatter, givingrise to extremely elongated error ellipses whose ma-jor axes (often several hundred km long) align withthe seamount trail (reflecting the age uncertainties)while the minor axes are normal to the trail (reflect-ing the width of the trail). Furthermore, when theage progression along a hot spot trail is examined indetail, we rarely find unequivocal trends. Even thearchetypal example of the Hawaiian–Emperor chainshows less than impressive age-distance correlations[8]. The mounting difficulties in establishing unequiv-ocal and convincing age-progressions have led severalscientists to express doubts about the validity of thehot spot hypothesis [9,10]. Likewise, and for many ofthe same reasons, attempts to determine whether or

not hot spots migrate have proven both controversialand inconclusive [11–14].

Recently, the combination of declassified Geosataltimetry and ERS-1 altimetry has provided the firstdetailed view of all oceans at a resolution of about10 km [15]. In the satellite-derived gravity field wecan discern more than 10,000 seamounts on the Pa-cific plate alone [16–18]. Many of these underseavolcanoes are hot-spot-produced, but, without know-ing their individual ages, conventional back-trackingcannot be utilized. While studying seamounts re-motely characterized from the satellite-derived grav-ity field, we uncovered a geometric relationship be-tween hot spots and the seamounts they produce thathad remained undiscovered since the conception ofplate tectonic theory [19]. This relationship now al-lows us to precisely locate hot spot positions fromthe present locations of seamounts produced by thesehot spots. The seamount ages (or crustal ages) arenot required, as the method is purely geometrical.Due to the novelty of our technique, we will firstreview principles in some detail, then use syntheticdata to demonstrate the potential of the hot-spottingtechnique to improve absolute plate motion param-eters, and, finally, apply the method to a large dataset of Pacific seamounts. In particular, we will studythe Cobb–Eickelberg and Pratt–Welker (or Kodiak–Bowie) seamount chains in the far northeastern re-gion of the Pacific plate, and use our technique toassess the present positions of the hot spots believedresponsible for these hot spot trails.

2. The hot-spotting principle

Fig. 1 shows 8 snapshots of a simplified modelfor the Hawaiian–Emperor seamount chain throughtime. We restricted seamount production to one vol-cano per 10 Ma (with one exception, discussed be-low). At 70 Ma, the Pacific plate was moving almostdue north, following a recent change in plate mo-tion at 76 Ma [20,21]. The solid circle represents aseamount formed at 80 Ma, while the open circle rep-resents the fixed location of the Hawaiian hot spot.At¾62.8 Ma, the Emperor seamount Suiko is formed(open triangle). The plate continued its northerly mo-tion until 43 Ma, the time of the 120º bend in theHawaiian–Emperor chain. The solid lines show the

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Fig. 1. Snap-shots at 8 different times of a model for the evolution of the Hawaiian chain. The seamounts (solid circles) produced overthe hot spot (open circles) are carried passively on top of the plate. It is a common misconception that seamounts reached their currentposition by moving along the hot spot tracks (dashed gray line for t D 0 Ma). A seamount’s path over the mantle is instead given by oneof the solid lines. These lines represent the loci of positions the seamounts occupied in the past. Suiko, one of the Emperor seamounts,has a radiometric age of ¾62.8 Ma [8]. To reach its current location it moved along the heavy solid line.

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actual motion of each seamount over the underlyingmantle as they passively ride the Pacific plate. Fol-lowing the change in motion at 43 Ma, the Pacificplate began a west–northwesterly journey. The final(0 Ma) panel illustrates the present configuration. Thelocus of all the seamounts delineate the hot spot trail(dashed line). Note that, with the exception of post-43-Ma volcanism, the trajectories of seamount traveldo not correspond to the hot spot trail.

Fig. 2a explores this difference in more detail.Presently, the Suiko seamount is located in the Em-peror seamount chain, sitting on older (100 Ma)oceanic crust [22]. The heavy solid line representsthe path that Suiko followed by riding passively ontop of the Pacific plate. Given Suiko’s age of ¾62.8Ma [8] we apply the appropriate stage rotations todraw the dashed line which terminate at the presenthot spot location. This technique is commonly re-ferred to as ‘back-tracking’, which may be somewhatmisleading since past locations of a seamount (solidline) are not actually retraced but rather locations aremapped where seamounts of progressively youngerages might be found (dashed line). Because bothpaths lead to the hot spot when the correct seamountage is used, the latter path is chosen because of itsobvious geological significance. Note that the solidline reflects motion of the plate, hence there are nofeatures on the seafloor along these lines (other thanby pure coincidence).

Next, let us assume that Suiko’s age is not yetknown. By trying all possible ages for Suiko (whichmust be between 0 and the age of the seafloor,100 Ma) and back-tracking Suiko for these ages weobtain theoretical hot spot locations which togetherdefine a line from the present location of Suiko tothe Hawaiian hot spot and beyond. This line repre-sents the flow line of the segment of crust directlybeneath Suiko. Thus, if the crustal age and stagepoles are correct, this crust was formed at a ridgesegment which, at 100 Ma, was located at the termi-nal point of this flow line. This particular segmentof seafloor traveled along its flow line until 62.8Ma when it found itself on top of the Hawaiianhot spot. At that time, Suiko was formed and wascarried along with the plate following the flow lineuntil reaching its current location. The dashed linesrepresent the back-track paths for every 10 Ma in-crement in Suiko’s hypothetical age. Note that only

Fig. 2. (a) Summary of the concepts presented in Fig. 1: Suikomoved along the solid line to reach its current location (opentriangle). Conventional ‘back-tracking’ is a technique that usesthe stage poles for a plate’s absolute motion to trace a seamountback to its origin (i.e. the hot spot that produced it; open circle).The conventional back-tracked path (dashed line) is the locusof all the younger seamounts created by that hot spot. (b) Webacktrack Suiko for each possible age between 0 and 100 Ma(the crustal age). The path (solid line) linking these backtrackedlocations (dashed lines terminating at solid circles) represents thelocus of all possible hot spot locations; it is a seafloor flow lineand the stationary hot spot must lie somewhere along this path.

the path for the correct seamount age (heavy dashedline) terminates at the actual hot spot location. Thisdemonstrates the important distinction that whereasthe flow line always contains the (stationary) hot

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spot location, the back-track path requires a correctseamount age to have that property.

While the flow line in Fig. 2b contains the hot spotlocation, we still have no information that would al-low us to determine where along this path it mightbe without knowing the seamount age. However, be-cause all crustal flow lines associated with seamountsproduced by a single hot spot must contain the lo-cation of that hot spot, the inescapable conclusion isthat these flow lines will all intersect at that uniquelocation. Fig. 3 illustrates this concept: the foursolid circles represent seamounts in the Hawaiian–Emperor seamount chain; their individual flow linesare drawn as solid lines which do indeed intersect atthe hot spot. The flow lines of other seamounts (graylines), like the Brahms seamount in the Musicianchain or Johnston Atoll in the Line Islands, will, ingeneral, not pass through the hot spot. We note thatsome of these lines also intersect elsewhere; how-ever, it is only at the hot spot that all flow lines fromthe same chain converge.

Although crustal ages are far better known thanseamount ages, there are still some uncertainties as

Fig. 3. Flow lines for four seamounts from the Hawaii–Emperorseamount chain (solid circles) and for two other seamounts notpart of that seamount chain (open circles). Flow lines of seafloorbeneath seamounts created by the same (stationary) hot spotmust intersect at the hot spot location. Flow lines associated withother seamounts do not share that property and will randomlyintersect the other flow lines. The dashed lines represent theflow lines when we assign 145 Ma as the upper age limit toall 6 seamounts. While the portions of the flow lines older thanthe actual crustal age are imaginary, they do not obscure thegeometric relationship outlined above.

well as large regions for which the crustal ages arespeculative or unknown (i.e. particularly in the Cre-taceous quiet zone). However, seamounts producedon crust formed during that period represent our bestevidence for intraplate volcanism as well as the na-ture of past absolute plate motion. As long as weassign an age that is much older than the seamount,we may use any seamount regardless of what isknown about its crustal age. In Fig. 3, we have re-placed the upper age limit (i.e. the age of the crust)for each seamount with 145 Ma. Because the pathis drawn from the present seamount location back-wards in time, the only effect of the older age is toextend the lines beyond their terminal point (dashedlines). Hence neither seamount age nor crustal age isrequired to precisely determine the hot spot locationprovided the hot spot has remained stationary andour stage pole parameters are accurate.

In reality, seamounts are not points, but each havea finite radius and height. In Fig. 4, we show twoseamounts in a hypothetical seamount chain. The up-per panel uses the line geometry as in previous il-lustrations. In the bottom panel, however, we allowthe lines to have thickness given by the seamount di-ameters and intensity (or amplitude) proportional toheight. Mathematically, we convolve the seamountsignature (i.e. gravimetric or bathymetric shape) withits flow line, and add up the resulting intensities. Theintersection of lines will result in higher intensity thanelsewhere. The sum of all such intensities on a gridgives rise to a surface which represents the strengthof flow line convergence at any location. If the in-tensity of a flow line is proportional to the amplitudeof the corresponding seamount then, at hot spots, thesum reflects the strength of the hot spot and repre-sents a measure of its cumulative volcanic output. Wetherefore refer to quantities such as those imaged inFig. 4b as cumulative volcano amplitude (CVA) andthe technique of locating hot spots by determininglocal maxima in the CVA image as ‘hot-spotting’.These positions are the optimal geometric locationsof stationary hot spots given the stage poles.

3. Hot-spotting using synthetic data

Because the hot-spotting concept is a relativelyrecent discovery, we will first explore the concept

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Fig. 4. Extending the flow line concept to account for variationsin seamount size. (a) The two seamounts (solid circles) havetheir own flow lines (solid lines) that intersect at the hot spot(open circle); dashed line is the seamount chain trend. (b) Thesum of all convolutions of each flow line with the correspond-ing seamount signature yields an image of cumulative volcanoamplitude (CVA). Stationary hot spot locations will generallycorrespond to local maxima in the CVA image.

using synthetic data before addressing several platetectonic questions using actual seamount data. Fig. 5shows the location of three hypothetical hot spots(open circles) and the seamounts they would haveproduced since 90 Ma. During this interval, each hotspot produced seamounts every 1 Ma; however, hotspot B first became active at ¾60 Ma, while hotspot A ceased its activity around 25 Ma and is nowextinct. We have perturbed the seamount locationsand dimensions somewhat and added a second set ofrandomly distributed seamounts of all sizes (gray cir-cles). During the life of these hot spots the plate mo-tion changed from due west to northwest (at 60 Ma)and back to due west again (at 30 Ma). Convolvingthese seamounts with their flow lines gives the CVAimages in Fig. 6. The exact image (Fig. 6a) has clearintersections at the prescribed hot spot locations,

but numerous other intersections are also visible.These are the results of interference between flowlines for seamounts from different hot spot chains.Clearly, if we did not know the locations of thesehot spots we would be hard pressed to tell these in-tersections apart. When inaccurate (perturbed) stagepole rotations are applied (Fig. 6b), most of the in-tersections become blurred and difficult to identifyuniquely. We also note that the shutdown of hot spotA at 25 Ma does not affect the CVA intersectionas long as seamounts were produced during the twoearlier stages that had different plate motion direc-tions. Hot-spotting is therefore theoretically capableof locating extinct hot spots.

There is necessarily a strong connection betweenthe stage pole parameters and the brightness of theCVA image. By modifying the stage poles we mayseek those changes that will focus the image opti-mally. In this simple demonstration we only perturbedthe intermediate stage pole (the northwesterly mo-tion). In Fig. 7, we show the results of a grid searchfor the best stage pole location that would brightenthe CVA image. As our measure of success, we choseto find the pole location that would maximize thestandard deviation of the image. We caution that thisquantity may not prove to be the most diagnostic in-dicator of a bright and focused image, but it does rep-resent a measure of high variability. Fig. 7a presentsa contour plot of the standard deviation which at-tains a maximum at longitude 43ºE and latitude 45ºN.This location is very close to the prescribed location(45ºE, 45ºN); the misfit being attributable to the noiseassociated with the random seamounts as well as theacross-track scatter of the hot spot trails. Next, we usethe best-fit stage poles to recalculate the CVA image(Fig. 7b.) A comparison with Fig. 6a reveals that theimage is still slightly blurred and offset, although it ismuch improved from the starting point (Fig. 6b). Nev-ertheless, the intersections are not as strong as in thenoise-free image, and the apparent hot spot locationsare still offset from the prescribed positions.

Whether we examine the noise-free (Fig. 6a) oroptimized CVA image (Fig. 7b), we find that thereare still several local maxima caused by interfer-ence between flow lines associated with seamountsproduced by different hot spots. Fig. 8 suggests apossible approach to unravel the hot spots hiddenin the CVA image. We apply an iterative technique

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Fig. 5. Series of synthetic seamount tracks. The open circles indicate the location of the hot spots and we have used three stage polesto create the seamounts (solid circles). Each hot spot generated a seamount every 1 Ma. We added normally distributed noise to eachseamount’s location and size, and randomly added seamounts of various sizes throughout the area (gray circles). While hot spot C hasbeen active for the entire 90-Ma period, hot spot B first became active at¾60 Ma, while hot spot A became extinct at ¾25 Ma.

which first finds the global maximum in the CVAimage and then identifies all the seamounts whoseflow lines came within 100 km of the CVA maxi-mum. Next, these seamounts are removed from thedata base and the exercise is repeated. Fig. 8a showsthe identification of the global maximum in the im-proved CVA image (from Fig. 7b). After removingthe seamounts as described above, we recalculate theCVA image (Fig. 8b). Notice that several of the in-terference patterns evident in Fig. 8a have now beeneliminated. The global maximum moves elsewhereand gives us our second hot spot location. We re-peat the procedure and find a new global maximumin Fig. 8c. Continuing the iterations further gives ablurred image (Fig. 8d) whose maximum amplitudeis significantly lower than the previous maximum,indicating that we have found all the hot spots inthe image. Fig. 8d also shows the hot spot locationsdetermined by hot-spotting (open double circles) aswell as the true locations (labeled circles).

4. Hot-spotting using actual seamounts

The geographic positions and physical character-istics of seamounts can be inferred remotely using

satellite-derived gravity data [23]. Wessel and Lyons[16] recently reported a compilation of seamountson the Pacific plate. In addition to determining itslocation, each seamount was characterized by its ap-proximate radius (in km) and amplitude (in mGal orEotvos). This data set was used to study the distri-bution of intraplate volcanism [17] and indirectly ledto the discovery of the geometric principle centralto our presentation [19]. Here, we take these 8882seamounts, assign an upper age to each based onavailable crustal ages [22] (or 145 Ma where agesare not given, like in the Cretaceous quiet zone),and calculate a CVA image using the stage polesof Wessel and Kroenke [19] (hereafter referred toas WK97.) Fig. 9a shows the distribution of theseamounts used for this study, while Fig. 9b presentsthe CVA data as a 3-D surface relief. Clearly, theHawaii hot spot has been the most voluminous ofall Pacific hot spots, despite only having a preservedvolcanic record for the last 80 Ma. The Louisville hotspot has a cumulative output ¾25% of the Hawaiianproduction, while the French Polynesian hot spotshave too blurred CVA signatures to allow an indi-vidual assessment of strengths at this time. Fig. 9cpresents the CVA image resulting from the totalsummation of all the individual convolutions of each

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Fig. 6. (a) CVA image based on the seamounts in Fig. 5. The bright spots in the CVA image correspond to multiple crossings of flowlines. The actual hot spot locations are indicated by the circles. While each hot spot has a bright spot associated with it, bright spots alsooccur where there are no hot spots; some of these can be brighter than the one over an actual hot spot. (b) Weion of the second stagepole by ¾7 spherical degrees before redoing the convolution. The resulting CVA image is blurred, has lower amplitudes, and the brightspots no longer coincide with the actual hot spot locations (circles).

seamount’s gravimetric signature with its flow line.A dramatic ‘X’ over Hawaii testifies to the powerof the hot-spotting technique to detect hot spots. Anoblique crossing defines the Louisville hot spot in thesouth Pacific. This hot spot was recently relocatedusing the hot-spotting method to the position indi-cated here [19]. The new location coincides with theHollister Ridge, a site of intense underwater acousticactivity [24], a local geoid high, and zero-age lavas[25]. The Hollister Ridge itself is probably a resultof hot spot–ridge interaction [26]; the geochem-istry of the Hollister lavas suggests mixing betweenLouisville plume material and depleted MORB [27].

The various patterns in the CVA image are, to acertain extent, dependent on the stage poles used inthe calculations. Consider Fig. 10a which shows the

CVA image portrayed obliquely in Fig. 9c. Super-imposed on the image are the location of numeroushot spots that have been proposed in the literature.Clean CVA maxima occur at Hawaii and Louisvillehot spots, and even an oblique intersection near theCaroline hot spot (although it does not show up wellat this map- and gray-scale). Strong CVA intensitiesalso occur in French Polynesia, possibly associatedwith the proposed hot spots at Rurutu and Raro-tonga [5], but, other than a broad intersection, we areunable to uniquely identify individual hot spots.

We next use the very different and minimalisticset of stage poles from Duncan and Clague [3] (here-after referred to as DC85). This simple plate motionmodel has no second-order changes in motion be-tween the Hawaiian–Emperor bend at 43 Ma and

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Fig. 7. (a) Results of a numerical search for the best pole location that maximized the standard deviation (¦ ) of each correspondingCVA image. The maximum s occurred for a pole (45ºN, 43ºE) slightly to the west of the actual pole (45ºN, 45ºE). A flow line inversiontechnique may therefore determine the set of stage poles that are most compatible with the given seamount locations. (b) The CVA imagefor the best-fitting stage poles. Now, the bright spots are much closer to the actual hot spots (circles). However, there are still severalbright spots of higher amplitude than the one associated with the easternmost hot spot (C).

the present, and yields the CVA image in Fig. 10b.The hot spot locations are superimposed for ref-erence. Comparing the two images reveals several

interesting similarities as well as differences: (1) theHawaii maximum is the global maximum and ex-tremely clear in both images. However, when using

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Fig. 8. Results of an iterative approach to hot-spotting, which follows the steps outline in the text. (a) Maximum in raw image is used topick the first hot spot (circle and cross hair). (b) After applying the iterative technique once we locate the new maximum which becomesthe second hot spot (circle and cross hair). (c) Final hot spot (circle and cross hair) located at new maximum after the second iteration.(d) The located hot spots are very close to the actual locations (labeled circles). Several of the superfluous bright spots disappear whenthe seamounts associated with a located hot spot are removed. Once all hot spots are found, the maximum amplitude of the CVA imagedrops off significantly.

the DC85 model, the maximum is located 135–180km north of the site of current volcanic activity atKilauea and Loihi, while it is within 35 km whenusing the WK97 poles (which include a change inplate motion near Molokai); (2) for the DC85 model,the Louisville CVA maximum is poorly resolved andappears to be offset from the new WK97-derivedlocation by almost 1000 km; (3) the DC85 modelgives a strong, north–south elongated CVA high witha maximum close to Rurutu, while the WK97 modelgives two local CVA maxima between Rarotonga andRurutu; (4) the oblique intersection at the Carolinehot spot (Fig. 10a) is not seen when using the DC85poles since this model has no post-43-Ma changes

Fig. 9. (a) Distribution of seamounts on the Pacific plate from satellite altimetry [16]. (b) 3-D CVA surface whose maxima reflect thecumulative output at hot spots, generated by convolving each seamount’s Gaussian shape (given by gravimetric amplitude and radius)with its flow line and summing the contributions. The WK97 poles are modified from earlier models [20,21,43] with the addition of arecent (¾3 Ma) change in motion [19]. (c) The CVA surface presented as an image.

in plate motion; (5) neither model gives convincingmaxima that with confidence can be associated withany of the other French Polynesian hot spots; (6)both models indicate that significant CVA ampli-tudes are associated with the microplate regions onthe East Pacific Rise, perhaps suggesting that manyseamount-like features in the gravity field may havehad their origin in this evolving environment.

Several circumstances make the straightforwardinterpretation of a CVA image difficult [19,28]; here,we will simply point out that inaccurate stage poles (inparticular for older stages), multiple hot spots alignedin the direction of plate motion, intraplate deforma-tion, mis-identification of cracks as hot spots [10], hot

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Fig. 10. (a) CVA image from Fig. 9. Hawaii (CVA maximum at 154º550W, 19º550N), Louisville (141º120W, 53º330S), and Caroline(165º300E, 4º150N) hot spots are resolved, while most south Pacific hot spots are not (Marquesas, Pitcairn, Society, Macdonald, Samoa).The broad intersection near Rarotonga may represent both Rarotonga and=or Rurutu. (b) CVA image based on the same seamounts, butusing the DC85 stage poles. Hawaii is again obvious, while Louisville is no longer clear and displaced ¾1000 km further to the east.Rurutu is associated with the broad CVA intersection, while other hot spots appear to have little or no CVA representation.

spot–fracture zone interaction [29], as well as possi-ble hot spot migration are among the most likely ex-planations for the blurred image. Also, a large differ-ence in volcanic flux between strong and weak plumesmeans that flow lines associated with large seamountsmay obscure flow lines connected to seamounts fromother, weaker hot spots and thus make the detectionof smaller hot spots problematic. Ridge jumps mayalso be problematic, by transferring material fromone plate to another. Consider Fig. 11 which illus-trates the effect of hot spot migration on the CVAimage. The small white circles indicate the positionsalong the flow lines that correspond to the actual agesof the seamounts (if they were known). As discussedearlier, all these points coincide with the actual (sta-tionary) hot spot location. Now consider what hap-pens if the hot spot is migrating (Fig. 11b). Again, the

small white circles represents the true position alongeach flow line, but now that the hot spot has movedwhile forming these seamounts, they no longer coin-cide with the present location of the hot spot. Instead,the intersection is spread out in the direction of hotspot migration, causing a smeared or blurred CVAimage. To us, the French Polynesian CVA maximumappears to be smeared in this manner, but we hesi-tate to attribute this feature unequivocally to hot spotmigration since it may also be a consequence of in-accurate older stage poles. Furthermore, note that noattempt was made to remove the many non-hot-spotseamounts which we believe to be included in the database. While the inclusion of these seamounts (mostlyof low amplitude) should not interfere significantlywith the CVA maxima it is likely to contribute noisethat may mask the finer details of the CVA image.

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Fig. 11. Effect of hot-spot drift on CVA images. (a) For astationary hot spot (gray circle), the flow lines associated withthe seamounts (solid circles) all intersect at the hot spot location.Heavy dashed line outlines the seamount chain. (b) If the hotspot is migrating with respect to the mantle, then the points onthe flow lines that correspond to actual seamount ages (opencircles) no longer overlap, but are distributed along a smallcircle.

5. The northeast Pacific

The WK97 poles appear to locate successfullythe Hawaiian, Louisville, and Caroline hot spotsand describe adequately the absolute Pacific platemotion since the Late Cretaceous, in particular theTertiary stages recorded in the well-resolved Hawaiiand Louisville seamount chains. However, if the Pa-cific is moving as a single rigid block, then thesestage poles should apply to all other Pacific plate hotspots too. We have already discussed the problematicsituation in French Polynesia, and we now focus ourattention on the far northeastern corner of the Pacificplate. Having calibrated the WK97 poles and open-ing angles on the Hawaii–Emperor and Louisvillechains, we seek to test the model using data from thenortheast Pacific. Here, we find two seamount chainsbelieved to be created by separate hot spots [2]. The

southernmost chain (Cobb–Eickelberg) is attributedto a hot spot suggested to be located near the Cobbseamount [3], while the more northerly chain (Pratt–Welker or Kodiak–Bowie) is believed to have beenproduced by the Bowie hot spot [30]. Like many ofthe other Pacific hot spots, there are several areas ofrecent volcanic activity, but none provide the steadyand voluminous output of a Hawaiian-type hot spot.Therefore, the exact locations of the hot spots inthe Gulf of Alaska are not well defined. Considerthe source for the Pratt–Welker hot spot chain. Theresponsible hot spot has been proposed to be locatednear the Tuzo Wilson seamounts [31,32], the Bowieseamount [30], or the Dellwood Knolls [33]. TheCobb hot spot, on the other hand, is believed, atpresent, to be located beneath the Axial seamount onthe Juan de Fuca Ridge [34].

Fig. 12a is a detailed CVA image (a subset ofFig. 9c, shaded using the slopes in the gravity field)for the Gulf of Alaska. Unless hot spots migrate, thepredictions using these stage poles should provideinsight into the situation in the northeastern Pacific.The prominent CVA maxima associated with theseamounts in this region are located at two obliqueintersections. These suggested hot spot locations dif-fer from previously published accounts, as detailedbelow.

5.1. Cobb hot spot

The CVA maximum in the vicinity of the Axialseamount is actually located over 260 km furthersoutheast, centered on the Blanco transform faultat 128º400W, 43º480N. This location implies that thewestward-migrating Juan de Fuca Ridge encounteredthe Cobb plume approximately at 2 Ma, consis-tent with earlier conclusions [35]. Although severalseamounts are observed just north of the Blancotransform, there is no documented evidence of large-scale magmatic activity on the Juan de Fuca platedirected away from our CVA maximum. We suggest,therefore, that plume material either has been chan-neled to the ridge (entrained) since ¾2 Ma and ispresently responsible for the on-axis volcanism at theAxial seamount, or that the hot spot is in a waningphase and is unable to penetrate Juan de Fuca platelithosphere. However, we do note that the Blancotransform shows evidence of intra-transform spread-

14 P. Wessel, L.W. Kroenke / Earth and Planetary Science Letters 158 (1998) 1–18

155˚W 150˚W 145˚W 140˚W 135˚W 130˚W 125˚W

40˚N

45˚N

50˚N

55˚N

CANADA

COBB

BOWIE

GMT Dec 22 10:24:49 1997 Wessel and Kroenke

155˚W 150˚W 145˚W 140˚W 135˚W 130˚W 125˚W

40˚N

45˚N

50˚N

55˚N

CANADA

Mendocino FZ

Surveyor FZ

Sedna FZ

Sila FZ

Aja FZ

Blanco FZ

Sovenco FZ

AX

HE

TW

CO

EI

EX

DKUN

OS

GRBO

HKDA

DE

DI

WEDU

PR

SUQN

GIKO

MIMU

PT

PK

PF

HO

b)

a)

12

P. Wessel, L.W. Kroenke / Earth and Planetary Science Letters 158 (1998) 1–18 15

ing [36] and has recent volcanic activity; the sampledfresh pillow basalts are unusual since most trace el-ement concentrations and ratios (e.g. La=Sm) areover-enriched relative to predicted crystal fractiona-tion models [37]. The locations of the back-trackedseamounts (Fig. 12a) using published radiometricages are compatible with our new location for theCobb hot spot, but also display the expected scatter,in particular in the along-trail direction.

5.2. Bowie hot spot

The CVA image (Fig. 12a) exhibits a clear,oblique intersection which we interpret to signify theoptimal geometric location of the Bowie hot spot.Our new location is centered on the Sovanco trans-form fault, at 130º00W, 49º300N, and is significantlyfurther south than the other proposed locations at theDellwood knolls (225 km), Tuzo Wilson seamounts(310 km), and in particular the Bowie seamount(600 km). We note there are no hints of CVA max-ima anywhere near these locations, testifying to theuniqueness of the new location given our stage rota-tions. Interestingly, our new location for the Bowiehot spot is close to the Heckle melting anomaly [38]which is believed responsible for the Heck, Heckle,and Springfield seamounts. These features (as wellas the Explorer seamount) may, therefore, actuallybe related to the Bowie hot spot. Furthermore, wenote that the Bowie hot spot appears to have haltedits seamount production around the time of the re-cent plate motion change (¾3 Ma) since there islittle evidence of large-scale volcanic activity alongthe youngest section of the predicted trail. Perhapsthe increased plate velocity prevented the weak andwaning Bowie plume from penetrating the litho-sphere until the migrating ridge entrained the plumeand facilitated seamount production in the ridge and

Fig. 12. (a) CVA image for the northeastern Pacific; texture is provided by illuminating the slopes in the gravity field [15]. Twohigh-amplitude, oblique CVA intersections signify the present locations of the Cobb and Bowie hot spots. Blue circles are seamountswith radiometric dates [30,34,41]. Green ‘back-track’ paths lead to the predicted hot spot location (pink circles). The CVA intersectionsfor the two hot spots are somewhat different, reflecting different seamount configurations and ‘gaps’ along each trail. (b) Satellite-derivedgravity field of the northeastern Pacific [15]. The cross-hairs show the locations of the Cobb and Bowie hot spots, and the predictedswaths (width D 250 km) are indicated with lighter shading. AX D Axial; BO D Bowie; CO D Cobb; DA D Davidson; DE D Denson; DID Dickens; DK D Dellwood Knolls; EI D Eickelberg; EX D Explorer; GI D Giacomini; GR D Graham; HE D Heckle; HK D Hodgkins;HO D Horton; KO D Kodiak; MI D Miller; MU D Murray; OS D Oshawa; PF D Pathfinder; PK D Parker; PT D Patton; PR D Pratt;QN D Quinn; SU D Surveyor; TW D Tuzo Wilson; UN D Union; WE D Welker.

fracture zone environment where the Explorer andHeckle seamounts are found.

6. Discussion

Despite the potential of the hot-spotting tech-nique, our results must still be considered prelimi-nary until a rigorous inversion for the optimal stagepoles as well as hot spot migrations has been un-dertaken. Nevertheless, we do find it significant thatthe WK97 model correctly ‘locates’ the Hawaiianhot spot as well as relocates the Louisville hot spotnear a site of recent underwater volcanism on theHollister Ridge. In addition, the model also findsthe Caroline hot spot [39]. Another consequence ofthe new (¾3 Ma) stage pole is a predicted, signifi-cant increase in absolute Pacific plate motion in thenortheast Pacific (by a factor of 2) as well as a largechange in differential motion along all Pacific platespreading centers. There is abundant evidence fordramatic reorganizations along the entire divergentPacific plate boundary around ¾3 Ma, all of whichhave been attributed to the ongoing interaction be-tween the Ontong Java plateau and the northernmargin of the Australian plate [40]. It is thereforeof interest to examine the predictions of the WK97model in the Gulf of Alaska.

Both the Cobb and Bowie hot spots, likeLouisville [19,26], may be located much furthersouth than previously suggested. Fig. 12b shows thepredicted hot spot trails for the Gulf of Alaska, withHawaii for reference in Fig. 13. The predicted Bowietrail provides a reasonably good fit to the seamountsin this area, including all the dated seamounts inthe Pratt–Welker chain. We note that the trend of theyoungest part of the Cobb–Eickelberg chain parallelsthe prediction of our post-3-Ma model (Fig. 12b).

16 P. Wessel, L.W. Kroenke / Earth and Planetary Science Letters 158 (1998) 1–18

Fig. 13. Bathymetry of the Hawaiian seamount chain. The gray line is the predicted trail based on the present location of the hot spot andthe WK97 model. The dashed line suggests a better fit may be obtained by allowing for an offset near 175ºW.

The Cobb and Eickelberg seamounts are all withina 250-km-wide hot spot swath, as are the Horton,Pathfinder, Murray, and Patton seamounts. This ge-ometry suggests that the Miller seamount is notpart of either chain, contrary to conventional pro-posals [3]. Based on geochronology and petrology,Dalrymple et al. [41] concluded that Miller was con-structed on younger crust, probably near a spreadingridge, whereas Horton, Pathfinder, Murray, and Pat-ton all clearly are products of mid-plate volcanism.Notwithstanding, the predicted swath back in timefrom Pathfinder is obviously not accommodating allthe larger seamounts in that area (e.g. Parker andothers nearby). Unfortunately, data from the Parkerseamount and others just north of the Sila fracturezone are not available, although their oblique align-ment and association with the fracture zone led us tospeculate that they formed in a near-ridge environ-ment [42].

Our preliminary analysis of the northeast Pacificis purely geometric since the hot-spotting techniqueallows us to separate the age progression from thegeometry. However, at the heart of the hot spothypothesis lies the notion of age-progressive volcan-ism. Here, we simply note that the reported ages arein general agreement with our model, but that indi-vidual ages may differ from the predicted values. Wehave made no attempt to adjust the opening rates inour plate model to minimize such misfits. Dalrympleet al. [41] concluded that the Denson, Davidson, andHodkins seamounts formed near a spreading ridgeand have simply been incorporated into the chain;their radiometric ages are too large to fit the Pratt–Welker age progression and they back-track to the

Olympia Peninsula (Fig. 12a). The CVA image ap-pears to corroborate this possibility as the only otherCVA intersection attributable to this seamount chainis found very close to these back-tracked positions.Although we find it most likely that this spot simplyidentifies a location on the paleoridge where severalridge-produced seamounts were spawned, we cannotexclude the possibility that a third hot spot (nowextinct and overrun by the north America plate) waslocated near 124º400W, 46º580N.

Several earlier investigations have suggested thatthe shape of the Bowie hot spot trail (convex pole-ward) is opposite the trend generally observed atother Pacific hot spot trails and, therefore, arguedagainst a single hot spot origin for the chain [41] orproposed drift between the Bowie and Hawaii hotspots [33]. While it is true that the first order shapeof the Pacific hot spot trails to the south are concavepoleward since 43 Ma, there are numerous second or-der plate motion changes in that time period [20,29].Fig. 13 shows the Hawaiian chain which has severalminor trail adjustments leading to stage poles thatlocally produce a convex poleward geometry. Thus,the observed convexity of the Pratt–Welker chain isentirely consistent with more detailed plate tectonicmodels for the Pacific plate (from which the WK97model was derived). We note, however, that the timeperiod 27–23 Ma apparently is not well representedin available absolute plate motion models: a possiblenorthward offset in the Hawaiian chain at 175ºW(dashed line in Fig. 13) may correspond to a possibleoffset of the Cobb chain at 144ºW where a smallchain of seamounts trend northward to Miller andthen westward toward Patton.

P. Wessel, L.W. Kroenke / Earth and Planetary Science Letters 158 (1998) 1–18 17

7. Conclusions

Seafloor segments with hot-spot-producedseamounts share a fundamental geometric charac-teristic: their crustal flow lines all intersect at thelocation of the (stationary) hot spot. This propertydecouples the age-dependency from the geometryand allows us to determine optimal geometric hotspot locations based on seamount positions and stagepoles and opening angles alone.

The convolution between seamount shapes andflow lines give rise to a surface of cumulative vol-cano amplitude (CVA). Hot spot locations corre-spond to local maxima in the CVA images; we callthe technique of identifying hot spots in CVA images‘hot-spotting’.

Preliminary investigations suggest that a blurredand out-of-focus CVA image may be used to improvethe stage poles by determining the perturbationsto the plate tectonic model as well as hot spotmigrations that optimally brightens the CVA image.

The Hawaii, Louisville, Caroline, Cobb, andBowie hot spots are well resolved in the CVA im-age calculated using the stage poles of Wessel andKroenke [19].

Apart from a broad CVA high in the vicinityof Rurutu and Rarotonga, other French Polynesianhot spots (Marquesas, Pitcairn, Society, Macdonald)have blurred or no apparent CVA maxima associatedwith them. Whether or not this is a failure of thehot spot hypothesis, strong evidence for hot spot mi-gration, inaccurate Cretaceous stage poles, intraplatedeformation, or non-hot-spot volcanism remains tobe determined.

Cobb and Bowie are relocated to more southerlylocations on the Blanco and Sovanco transformfaults, respectively. The predicted hot spot trailsinclude all major seamounts visible in the satellite-derived gravity field (except a few that formed ata spreading ridge or are of unknown, but possiblyridge origin) and are thus compatible with rigid platemotion over two fixed hot spots.

Acknowledgements

This work was supported by Grant OCE-9729253from the National Science Foundation. We thank

Marcia Maia and Chris Small for thoughtful reviews.This is SOEST contribution 4570. [CL]

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