Seamounts As Recorders of Hot-Spot Epeirogeny

Seamounts as recorders of hot-spot epeirogeny

S. THOMAS CROUGH* Department of Geosciences, Purdue University, West Lafayette, Indiana 47907

ABSTRACT In this case also, seamounts accurately reflect the vertical motion of the lithosphere beneath them. Observations in the central Pacific suggest that older seamounts are The subsidence of seamounts is fairly well understood, and so it is uplifted if plate motion carries them over mantle hot spots. A depth- now possible to use seamount depths as indicators of epeirogenic uplift. anomaly map of the area reveals several volcanic chains now located on Given the ages of both a seamount and the sea floor it sits on, the expected younger hot-spot swells. Uplift is estimated by comparing the observed depth of the seamount's top can be calculated as the amount of subsidence depth of a seamount summit or the observed stratigraphic section in an expected of standard sea floor in the time between its present age and its atoll with that predicted assuming standard subsidence of the sea floor age when the seamount formed. For seamounts that originally developed after the seamount formed. Cretaceous seamounts on the margin of the on older crust, the effect of lithospheric reheating can be accounted for. Hawaiian Swell have standard summit depths, whereas those on the crest Therefore, one can compare observed seamount depths with expected of the swell are at least 1,100 m shallower. Summit depth correlates with depths based on known ages and make inferences about postformational sea-floor depth anomaly, implying that the Hawaiian Swell has caused the episodes of uplift. Of course, an expected depth calculation assumes that a uplift. Eocene fossils are exposed at the surface in the Tuamotu Atolls, seamount was planed at sea level soon after its origin; if it never reached whereas, according to the standard subsidence relation, Eocene deposits sea level, it will at present be deeper than otherwise predicted. should be covered by 1,000 to 2,000 m of more recent material. The inferred uplift is consistent with the atoll's present position atop the Society HOT-SPOT EPEIROGENY IN THE CENTRAL PACIFIC Swell. Possible evidence for 600 m of uplift in the Southern Cook Islands, however, is unexplained by swell height. The observed uplifts of the Cre- Although many forms of mid-plate uplift may be recorded by sea- taceous seamounts near Hawaii and of the Tuamotu Atolls put a limit on mounts, the central Pacific region indicates that hot-spot swells are likely when the Hawaiian and Society Swells formed at their present locations of to cause the largest and most frequent effects. Figure 1 is a depth-anomaly maximum elevation. The swells must have developed within the past map of the area made by combining previous maps of the Society Islands several million years and probably formed at the same time as did the region (Crough, 1978) and the Marquesas-Line Islands region (Crough younger volcanic edifices on their summits. and Jarrard, 1981) with more recent work north of 15°N. The details of map construction, including all the input data, will be published separately INTRODUCTION in a Pacific-wide study of depth and age (P. A. Schroeder and S. T. Crough, unpub. data). Four of the five recently active hot-spot volcanic It is well established that seamounts are affected by the subsidence of chains—Hawaii (Dairymple and others, 1981), Marquesas (Duncan and their underlying sea floor, thereby providing a record of epeirogenic sink- McDougall, 1974), Society (Dymond, 1975), and Tubuai (Dalrymple and ing. The flat guyot tops of the Pacific become progressively deeper with others, 1975)—all cap rises that are more than a thousand kilometres increasing age; these gave one of the first indications that the sea floor itself across and several hundred metres high. The fifth hot spot, Pitcairn (Dun- subsides with age (Menard, 1964). The subaerially eroded crests of can and others, 1974), has little or no swell, as is typical for hot spots on aseismic ridges drilled by the Deep Sea Drilling Project (DSDP) also sea floor less than 30 Ma old (Crough, 1978). It should be noted that there become deeper with age and their age-depth relation closely parallels the are no depth anomalies of similar amplitude that are not associated with age-depth curve estimated for standard sea floor (Detrick and others, hot spots. 1977). These volcanic ridges apparently form near oceanic spreading cen- The central Pacific region contains at least five locations where an ters, are quickly truncated, and then sink solely due to the cooling of the older volcanic chain migrated over a hot spot and was perhaps uplifted by upper mantle beneath them. Seamounts that are originally formed in a the formation of a new swell beneath it. As labeled by number in Figure 1, mid-plate setting usually subside faster than their underlying sea floor the current direction of Pacific plate motion (Turner and others, 1980) would be expected to sink on the basis of standard cooling (Detrick and conveyed a group of Cretaceous seamounts (1) to near Hawaii, passed the others, 1977). However, mid-plate volcanoes form on unusually shallow Line Islands (2) over a younger hot spot, probably the Marquesas hot spot swells that seem to be the result of lithospheric reheating over mantle hot (Crough and Jarrard, 1981), and carried the Tuamotus (3), the Pitcairn spots (Detrick and Crough, 1978; Crough, 1978). The swells subside trace (4), and the Tubuai group (5) close to the Society hot spot. anomalously fast for their crustal age because they have been reheated and The expected scenario of motion over a hot spot involves rapid initial the seamounts on their surfaces subside in parallel with this rapid sinking. uplift followed by gradual subsidence (Fig. 2). As can be seen in Figure 1, hot-spot swells such as the one beneath Hawaii are strongly asymmetric, *Deceased. Publication process was aided by W. J. Hinze and John G. with the side upstream of the hot spot (to the southeast in the Pacific) Sclater. being very steep compared to the downstream side. The characteristic

Geological Society of America Bulletin, v. 95, p. 3-8, 5 figs., January 1984.

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• MARQUESAS -M0C Figure 1. Depth anomalies (in metres, contour interval 250 m) of the central Pacific calculated using the age-depth curve of Par- sons & Sclater (1977), which includes a de- parture from root time behavior. Positive values signify anomalously shallow sea floor. Seamounts are indicated by dark shading. Numbers identify areas where older chains have moved onto younger swells. Makatea Island (M) and Pitcairn Island (P) are labeled by letters. The three Cretaceous seamounts near Hawaii that have been dated are identified with their ages (in Ma). The velocity of the Pacific plate relative to the hot spots is shown by arrows (Turner and others, 1980).

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mediately south of Oahu never experienced mid-plate uplift, they should be no shallower than 2,100 m and perhaps might be slightly deeper. The summits of these seamounts are, in fact, much shallower than predicted by standard subsidence. On the bathymétrie map of Chase and others (1971), the flat summits of these seamounts are only 300 to 1,000 m deep. Although there is a possibility that these depths are slightly shallow due to sedimentation atop the seamounts, igneous rocks were dredged from a depth of only 500 m (Dymond and Windom, 1968), indicating that the sediment caps are thin. Using the 2,100-m estimate of standard subsidence, the seamount summits are anomalously shallow by 1,100 to 1,800 m. Note that these seamounts lie between the 1,000-m and 1,250-m

SUBSIDENCE AND contours of depth anomaly on the Hawaiian Swell and that this rise in f REEF GROWTH sea-floor depth is sufficient to explain the minimum amount of uplift Time 3 TT UNCONFORMITY inferred. As a check on whether the swell has caused the uplift, the depths of seamounts on different parts of the swell have been compared (Fig. 3). Two of the Musicians Seamounts to the northwest of Hawaii (Fig. 1 ) have Figure 2. Sequential views of an atoll's passage over a fixed been dated as forming at 65 and 87 Ma B.P. (Clague and Dalrymple, mantle hot spot. Basement volcanic rock is ruled; the atoll's carbonate 1975), making it likely that most of the large seamounts on the eastern part section is unruled. The younger volcanics above the hot spot are not shown, so the cross section may be considered to be located to the side SEAFLOOR DEPTH ANOMALY (m) of the younger chain. O 1000

shape is explainable by fairly instantaneous reheating above the mantle hot spot followed by slower cooling of this thermal pulse. If the uplifted seamount is a guyot, its upper surface simply parallels the vertical motion of the sea floor. If the seamount is an atoll, as depicted in Figure 2, the effects are more interesting. Before uplift, the carbonate cap contains a stratigraphic record of the seamount's earlier subsidence history. During uplift, the younger deposits are removed, exposing progressively older material. With renewed subsidence, the uplift event is recorded as a large unconformity within the carbonate section (Menard, 1973).

EVIDENCE FOR SEAMOUNT UPLIFT

The Cretaceous seamounts on the crest of the Hawaiian Swell south of Oahu (Fig. 1) provide the best evidence for hot-spot uplift. Only Cross Seamount has been dated, but the uniform summit depths of this group suggest that all are of the same age. The K-Ar age for Cross of 89 Ma (Dymond and Windom, 1968) is probably accurate because, although K-Ar ages are often only minimum ages due to weathering, the seamount cannot be any older than the sea floor it sits on, which is ~95 Ma old. However, even with this rather tight control on possible age, the predicted standard depths of the seamount summits are still difficult to estimate accurately. Figure 3. The present summit depths of the older seamounts on The seamounts are nearly the same age as their underlying sea floor the Hawaiian Swell plotted versus the depth anomaly beneath the and, therefore, originally erupted near a ridge crest. Standard sea floor seamounts. The summit depths are those listed by Chase and others (Parsons and Sclater, 1977) subsides 1,100 m in its first 10 Ma of cooling, (1971) for all the seamounts shown in Figure 1 along and near the so slight changes in the ages of the seamounts relative to the sea floor will dashed trend line, and they represent the shallowest depths in each cause large changes in predicted depth. For example, if the seamounts and region. Depth anomalies were taken from the original 1° x 1° values sea floor both are 95 Ma old, which is possible given the uncertainties of used to create Figure 1. Seamounts to the south of the Hawaiian the data, the predicted subsidence is approximately 3,200 m. If the sea- Islands are indicated by crosses, to the north by dots. Seamounts with mounts are 85 Ma old and the sea floor is 95 Ma old, again within the flat tops are indicated by a bar above their symbol. The expected uncertainties, then the predicted subsidence is 2,100 m. The actual subsi- depth of seamount tops, assuming no uplift, is shown at the left. The dence probably is closer to the lower estimate than to the upper one, for solid line represents the relation between summit depth and depth volcanic edifices emplaced at ridge crests usually form linear ridges such as anomaly if formation of the Hawaiian Swell caused the uplift of the the Iceland-Faroes Ridge, whereas the seamounts near Hawaii are single seamounts closest to Hawaii and if the age offset across the Murray volcanoes indicative of offridge eruption. Therefore, if the seamounts im- and Molokai fracture zones is 8 Ma.

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of the Hawaiian Swell (excluding the Hawaiian chain itself) were formed rard and Clague, 1977), and at most 50 to 63 Ma old, based on the age of at nearly the same time, perhaps as an older hot-spot trace. If the sea floor their underlying sea floor (Herron, 1972). Morphologically, the bases of beneath these seamounts has the same age, then the seamount tops would the atolls merge to form a continuous ridge, suggesting that the islands be expected to be at approximately the same depth, provided they had formed directly over a spreading center. For an atoll near the western edge never been elevated. However, the present summit depths are not the same of the chain, where 60 Ma is a reasonable estimate for seamount and but are strongly correlated with present depth anomaly (Fig. 3). Those sea-floor age, standard subsidence would have produced the carbonate cap seamounts that currently are located on sea floor with little or no depth shown in Figure 4. However, on Makatea Island (Fig. 1), Eocene fossils anomaly have the expected summit depths calculated above. Those closer are located at sea level (Repelin, 1919), rather than at the predicted depth to the Hawaiian Islands and therefore higher on the swell have shallower of 1 to 2 km. These fossils should be re-examined and redated, but taking summits, and the difference in depth is the same as the height of the swell. this earlier work at face value indicates that Makatea is at present 1 to 2 This comparison is complicated, however, because the ocean floor km shallower than expected. The precise amount depends on the place of lies within the Cretaceous quiet zone and is dated only by extrapolation the surface fossils within the Eocene period, which is at present unknown. from neighboring magnetic lineations. The older seamount chain near Note that Makatea lies within the 1,000-m contour of the Society Swell Hawaii is crossed by two major fracture zones—the Molokai, which lies (Fig. 1), so that movement onto the swell is quantitatively sufficient to near Oahu just north of Cross Seamount, and the Murray, which lies near explain the minimum amount of inferred uplift. the northern edge of the Hawaiian Swell at Khatchaturian Seamount, Further evidence for uplift is provided by DSDP Site 318 drilled identified in Figure 1 by its age of 65 Ma. To the east, there are prominent among the Tuamotu Atolls (Fig. 1). Pliocene to Holocene turbidites with offsets in the Cenozoic magnetic anomalies at these fracture zones, such shallow-water skeletal debris overlie Miocene oozes with interbedded tur- that the crust between the Murray and Molokai transforms is about 13 Ma bidites (Schlanger, Jackson, and others, 1976). Large foraminifers as old as younger than that immediately north or south (see, for example, Figure 1 Eocene are found in the reworked debris deposits, suggesting that the of Clague and Dalrymple, 1975). If this age contrast persists through the Tuamotu Atolls have been eroded vigorously in the past few million years Hawaiian region, then the seamounts located between the two fracture and that Eocene calcareous beds have been exposed. More recent work zones originally formed on younger sea floor than did those to the north or (Premoli-Silva and Brusa, 1981) indicates that these reworked deposits south and were, therefore, perhaps 1,260 m deeper than those other sea- may be originally of late Oligocene age, which requires lesser uplift but mounts prior to the recent uplift. The seamounts between the fracture still at least 600 m of vertical motion. zones can be identified in Figure 3 as the north-side seamounts of depth Of the other possibly uplifted chains shown in Figure 1, uplift infor- anomalies greater than 250 m. They seem to be about 600 m deeper than mation is available only from the Southern Cook Islands, and it is not neighboring seamounts on opposite sides of the major fracture zones. If it is explicable by the swell-uplift mechanism. The volcanic basement of Man- assumed that the other seamounts are 85 Ma old and located on 95-Ma- gaia Island, in the Southern Cook group along the western extension of the old sea floor, then the group between the fracture zones would have to be Tubuai chain, is now about 160 m above sea level (Jarrard and Turner, 85 Ma old and located on 87-Ma-old sea floor in order to explain this 1979). According to the regional sea-floor depth, including the effect of the 600-m difference (Fig. 3). This 8-Ma crustal age contrast across the frac- Society Swell, this island should be about 400 m deeper today than when ture zones is smaller than observed to the east, but to the west of Hawaii it formed 18 Ma ago (Crough, 1978). These observations suggest either the age offsets where known, such as across the Mendocino fracture zone 600 m of uplift restricted to the island itself or else an unusual resistance to (Clague and Dalrymple, 1975), are smaller than to the east. Given the erosion such that the island was never truncated. Drilling of the marginal uncertainties, the data seem in reasonable accord with the postulated reefs might indicate which is the case, for the reefs ought to document the hot-spot effect. uplift if it occurred. The Tuamotu Archipelago (Fig. 1) also provides direct evidence for It is notable and perhaps significant that for each of the three sea- uplift. The islands are not accurately dated at present but are at least 50 Ma mount groups just discussed there may be evidence for a larger amount of old, based on a variety of paleontological evidence (summarized by Jar- uplift than explained by the hot-spot swells as shown in Figure 1. Given

TUAMOTU ATOLLS CROSS SECTION ^.sea level without uplift Figure 4. Predicted stratigraphy -PLIOCENE V— of the Tuamotu Atolls carbonate

MIOCENE \ caps, calculated assuming standard 22.5 subsidence from a ridge crest. If the Tuamotus have been elevated and .37.5 eroded by the recent formation of v with uplift caused the 1,100-m-high Society Swell, v by Society Swell then the upper part of the predicted Q. 55 column should be missing. Eocene

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e ¡xtreme age relations, the seamounts near Hawaii might be as much as in the Tuamotu region subsides approximately 25 m/m.y., so the flexural \ J,800 m shallower than expected from standard subsidence. Moreover, if uplift mechanism could not have exhumed an unconformity that was older t he surface fossils on Makatea are as old as lowermost Eocene, that island than 1 m.y. Unless some other uplift process is at work, the data require i night be 1,900 m shallower than expected. Thus, in all three cases, there that the Society Swell beneath the Tuamotu Atolls be of late Pliocene to i nay be several hundred metres more uplift than the hot-spot model pre- Holocene age. ( licts. The data currently are not accurate enough to require this view, but The Cretaceous seamounts atop the Hawaiian Swell also indicate a i t needs to be investigated further. Perhaps the hot-spot swells in the central fairly young age for swell formation. If the Hawaiian Swell developed at a 1 Pacific are higher than calculated at present, and the standard age-depth ridge crest, the guyot summits would now be at their expected depths, ( :urve needs to be revised (Heestand and Crough, 1981). Perhaps there is assuming standard subsidence. That they are much shallower implies that s some additional uplift mechanism, such as magma intrusion into pre- the swell formed after the guyots. A maximum age can be calculated by < 5xisting edifices, that functions near hot spots. assuming that when the swell was created it raised the older seamounts up to sea level. Mid-plate swells subside at a rate of about 30 m/Ma"1 ] IMPLICATIONS (Crough, 1978), so the guyots would have reached their present depths within 12 to 33 Ma. The Hawaiian Swell near Oahu cannot be older than The most important aspect of the seamount uplift data is probably the this, or the Cretaceous guyots would be deeper, but it can be younger if the i constraint they place on when hot-spot swells form. The swells in Figure 1 guyots never reached sea level during the uplift phase. The sediments atop : ire so clearly associated in space with active hot spots that no one has these seamounts could be drilled to check for late Tertiary erosion. : seriously questioned whether they are also associated in time. It is pre- When studying the tectonic history of any seamount chain, it is i sumed, for example, that the portion of the Hawaiian Swell beneath important to consider the possible effects of hot-spot epeirogeny. For Midway formed 27 Ma ago (Dalrymple and others, 1977) when the island example, McNutt and Menard (1978) recently interpreted small elevation formed and that the portion beneath the southeastern end of the chain differences among the Tuamotu Atolls as being due to volcano loading formed in the past few million years. However, it might be possible that within the past 1.5 Ma. However, in that same time interval, the atolls the swells formed significantly earlier in the history of the sea floor, per- moved about 150 km toward the Society hot spot and, according to the haps at a spreading center, and that the present volcanic chains later were contours in Figure 1, were elevated about 115 m above their expected emplaced on these older structures. depths. Their expected depths due to standard subsidence increased by 40 The seamount observations exclude this possibility and imply that m in that time, leaving a net uplift of 75 m, which is an order of magnitude swells form at approximately the same time as the hot-spot volcanoes atop larger than the presumed flexural effect. The atolls are slightly above them. If the Society Swell and the southeastern tip of the Hawaiian Swell sea level, so if the hot-spot model is correct, erosion must be very rapid. had originated earlier than in the past few million years, then the older Perhaps the present elevation variations are caused by differential erosion seamounts on those swells would now be in the subsidence phase shown in and the good fit obtained to the flexural model is fortuitous (Jarrard and Figure 2. In that case, the swell-related unconformity within the Tuamotu Turner, 1979). Atolls would be capped by late Tertiary deposits and would not be ex- Seamount uplift information also can be used to infer the origin of posed at the surface. McNutt and Menard (1978) showed that the flexural certain swells. As an example, the Line Islands (Fig. 2) north of the effects of nearby volcano formation could have elevated the Tuamotus in equator sit atop a broad swell similar in height to swells beneath the the past 1.5 Ma; however, their calculated amounts of uplift are only on Hawaiian and Society Islands. The swell appears to extend to the southeast the order of several metres. According to the standard curve, the sea floor and was interpreted by Crough and Jarrard (1981) as the Tertiary trace of

LINE ATOLLS B) Figure 5. Predicted stra- with tigraphy of the central Line Is- hotspot uplift lands: A. Assuming standard

subsidence from a ridge crest. B. fi MYBP With movement over the Mar- quesas hot spot 40 m.y. ago 22 . 5

causing uplift and then more 37.5 rapid subsidence on reheated lithosphere. The difference between these two histories should 88 be easily resolvable using con- ventional coring methods, suggesting that atoll drilling could

be crucial in deciphering the tec- 100 tonic history of parts of the Pacific.

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the Marquesas hot spot. The available geoid height and apparent subsi- plate epeirogeny and thereby of the fundamental processes that cause dence data support this interpretation, but the trend of the swell deviates vertical movement. by about 15° from that predicted assuming fixed hot spots. A single drill hole in the Line Islands might test this interpretation and ACKNOWLEDGMENTS determine whether hot spots do have small relative movements. The Line Islands and the sea floor they sit on are both about 100 Ma old, and so, if I thank R. Jarrard for showing me how reef deposits could indicate the islands experienced standard subsidence, they should have carbonate uplift, W. Menard for suggesting a comparison of seamount depths across caps similar to that in Figure 5A. This is also what would be expected if the Hawaiian Swell, and S. Schlanger for acquainting me with the latest the swell beneath the islands formed at the same time as the islands DSDP results. This work was supported by the National Science Founda- themselves. However, if the swell formed at about 40 Ma B.P. due to plate tion through Grants EAR-7904033 and EAR-8021126. motion over the Marquesas hot spot, then the carbonate caps should

resemble that in Figure 5B. The youngest stratum above the unconformity REFERENCES CITED dates the uplift, and the amount of stratigraphic section missing in Figure Chase, T. E., Menard, H. W„ and Mammerickx, J., 1971, Topography of the North Pacific: Scripps Institute of 5B compared to Figure 5A records the amount of uplift. Other possible Oceanography Map TR-17. Clague, D. A., and Dalrymple, G. 6., 1975, Cretaceous K-Ar ages of volcanic rocks from the Musicians Seamounts and scenarios produce different depositional sequences. For instance, if the the Hawaiian Ridge: Geophysical Research Letters, v. 2, p. 305-308. swell is the manifestation of a recent hot spot with no volcanic trace, then Crough, S. T., 1978, Thermal origin of mid-plate hot-spot swells: Royal Astronomical Society Geophysical Journal, v. 55, p. 451-469. the upper 1,000 m of section in Figure 5 A should be missing, and Eocene Crough, S. T., and Janard, R. D., 1981, The Marquesas-Line Swell: Journal of Geophysical Research, v. 86, p. 11763-11771. deposits should be found at the surface. Dalrymple, G. B., Jarrard, R. D., and Clague, D. A., 1975, K-Ar ages of some volcanic rocks from the Cook and Austral Islands: Geological Society of America Bulletin, v. 86, p. 1463-1467. One problem with using a stratigraphic column to infer tectonic uplift Dalrymple, G. B., Clague, D. A., and Lanphere, M. A., 1977, Revised age for Midway volcano, Hawaiian volcanic chain: Earth and Planetary Science Letters, v. 37, p. 107-116. is that eustatic sea-level changes are also recorded. A period of Eocene- Dalrymple, G. B., Clague, D. A., Garcia, M. O., and Bright, S. W., 1981, Petrology and K-Ar ages of dredged samples Oligocene erosion of the Line Atolls, as seen at neighboring DSDP sites, from Laysan Island and Northampton Bank volcanoes, Hawaiian Ridge, and evolution of the Hawaiian-Emperor chain: Summary: Geological Society of America Bulletin, v. 92, Part I, p. 315-318. might be evidence for the predicted hot-spot uplift, except that this event is Detrick, R. S., and Crough, S. T., 1978, Island subsidence, hot spots, and lithospheric thinning: Journal of Geophysical Research, v. 83, p. 1236-1244. also seen throughout the western Pacific and may therefore be a sea-level Detrick, R. S., Sclater, J. G., and Thiede, J., 1977, The subsidence of aseismic ridges: Earth and Planetary Science Letters, fluctuation (Schlanger and Premoli-Silva, 1981). If the Line Atolls were v. 34, p. 185-196. Duncan, R. A., and McDougall, I., 1974, Migration of volcanism with time in the Marquesas Islands, French Polynesia: elevated by a hot spot during this global sea-level drop, there will be more Earth and Planetary Science Letters, v. 21, p. 414-420. Duncan, R. A., McDougall, I., Carter, R. M., and Coombs, D. S., 1974, Pitcairn Island—Another Pacific hot spot?: section missing in the Line Islands than in other Pacific atolls. Hot- Nature, v. 251, p. 679-682. spot-generated unconformities should be distinguishable by their more Dymond, J., 1975, K-Ar ages of Tahiti and Moorea, Society Islands, and implications for the hot-spot model: Geology, v. 3, p. 236-240. local occurrence and their generally larger amplitude. Dymond, J., and Windom, H. L., 1968, Cretaceous K-Ar ages from Pacific Ocean seamounts: Earth and Planetary Science Letters, v. 4, p. 47-52. Heestand, R. L., and Crough, S. T., 1981, The effect of hotspots on the oceanic age-depth relation: Journal of Geophysical Research, v. 86, p. 6107-6114. Herron, Ellen M., 1972, Sea-floor spreading and the Cenozoic history of the east-central Pacific: Geological Society of CONCLUSIONS America Bulletin, v. 83, p. 1671-1692. Jarrard, R. D., and Clague, D. A., 1977, Implications of Pacific Island and seamount ages for the origin of volcanic chains: Reviews of Geophysics and Space Physics, v. 15, p. 57-76. Jarrard, R. D., and Turner, D. L., 1979, Comments on "Lithospheric flexure and uplifted atolls" by M. McNutt and H. W. Available evidence supports the idea that older seamounts are ele- Menard: Journal of Geophysical Research, v. 84, p. 5691-5694. vated if they migrate across younger hot spots that form topographic McNutt, M., and Menard, H. W., 1978, Lithospheric flexure and uplifted atolls: Journal of Geophysical Research, v. 83, p. 1206-1212. swells. The observed depths of seamounts on the Hawaiian and Society Menard, H. W., 1964, Marine geology of the Pacific: New York, McGraw-Hill, 271 p. 1973, Depth anomalies and the bobbing motion of drifting islands: Journal of Geophysical Research, v. 78, p. Swells are quantitatively consistent with this uplift mechanism. However, 5128-5137. Mangaia in the Southern Cook Islands may have been elevated more than Parsons, B., and Sclater, .1. G., 1977, An analysis of the variation of ocean floor bathymetry and heat flow with age: Journal of Geophysical Research, v. 82, p. 803-827. can be explained by swell height. There may be more than one process that Premoli-Silva, 1., and Brusa, C„ 1981, Shallow-water skeletal debris and larger foraminifers from Deep Sea Drilling Project Site 462, Nauru Basin, western equatorial Pacific, in Larson, R. L., and others, eds., Initial reports of the raises seamounts. Deep Sea Drilling Project, Volume 61: Washington, D.C., U.S. Government Printing Office, p. 439-451. Repelin, J., 1919, Sur un point de l'histoire de l'Océan Pacifique: Comptes Rendus Hebdomadaires des Séances de Searnount uplift provides one of the few possible ways in which to l'Académie des Sciences, v. 168, p. 237-239. directly date the formation of an oceanic swell. The Eocene fossils at the Schlanger, S. O., and Premoli-Silva, I., 1981, Tectonic, volcanic and paleogeographic implications of redeposited reef faunas of Late Creuceous and Tertiary age from the Nauru Basin and the Line Islands, in Larson, R. L., and others, surface of the Tuamotus indicate that the portion of the Society Swell eds., Initial reports of the Deep Sea Drilling Project, Volume 61: Washington, D.C., U.S. Government Printing Office, p. 817-826 beneath those atolls is only a few million years old, at most. The Cretace- Schlanger, S. O., Jackson, E. D., and others, 1976, Initial reports of the Deep Sea Drilling Project, Volume 33: ous guyots near Oahu constrain that section of the Hawaiian Swell to be at Washington, D.C., U.S. Government Printing Office, p. 317. Turner, D. L., Jarrard, R. D., and Forbes, R. B., 1980, Geochronology and origin of the Pratt-Welker seamount chain, most 12 to 33 Ma old. It is now fairly certain that hot-spot volcanoes and Gulf of Alaska: A new pole of rotation for the Pacific plate: Journal of Geophysical Research, v. 85, p. 6547-6556. the swells associated with them develop simultaneously. A series of well-selected drill holes in Pacific atolls might substantially MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 17, 1982 REVISED MANUSCRIPT RECEIVED OCTOBER 26, 1982 improve our understanding of the temporal and spatial pattern of mid- MANUSCRIPT ACCEPTED NOVEMBER 19, 1982

Printed in U.S.A.

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