Emperor Seamounts Research Articles

Emperor Seamounts: Hotspot Candidates

Emperor Seamounts: Hotspot Candidates

On Long-Wave Disturbances of Dynamic Height in the North Pacific

Abstract Long-wave disturbances of dynamic height in the North pacific are investigated by means of closely spaced STD (salinity, temperature, depth) stations and are related to atmospheric forcing. Wavelike disturbances with length scales between 400 and 600 km are common in the latitude range between 20° and 50°N. The wave amplitudes are larger in the western than in the central and eastern Pacific, and they depend upon season. A large eastward decrease in wave amplitude is observed near the Emperor seamount chain. The wavelike disturbances extend to several hundred meters’ depth, with an exponential-type decrease in wave amplitude. The baroclinic currents associated with the dynamic height perturbations are an order of magnitude larger than the mean currents. The perturbations are related to atmospheric forcing, in particular the curl of the wind stress field. Where the distribution of the curl is zonal, meridional waves are excited, which are reflected in north–south dynamic height sections. Where sev...

Volcanogenic effects on the rates of deposition of sediments in the Northwest Pacific Ocean

Seven piston cores, 7–16 m long, taken between the Kuril Islands and Emperor Seamounts, have been dated using radiolarian and diatom extinction levels and correlated using volcanic ash layers. The average rate of deposition in the cores decreases from 6 cm/1000 years near the Kuril Trench to about 3.5 cm/1000 years near the seamounts. Dispersed volcanic ash is the main constituent of the cores and it comprises up to 80% of the sediments. The percentage of the ash in the sediments decreases eastward from the Kuril Islands as the rates of deposition decrease. The total thickness of the sediments in the same latitudinal belt also decreases eastward. The thickness of the sediment inferred from seismic data near the Kuril trench is about 600 m and rates of deposition are approximately 6 cm/1000 years in the Pleistocene cores. Sediment thickness near the seamounts is about 300 m, and rates of deposition are approximately 3 cm/1000 years in the Pleistocene cores. Extrapolated rates of deposition in these cores suggest that the age of the base of the sediment to the east of the Kurils is only about 10 m.y. The anomalously young age for the base of the sediments obtained by extrapolation of an assumed constant rate of deposition can be explained by Deep Sea Drilling Project data from the northwest Pacific. The sediment thickness at DSDP site 192 east of Kamchatka includes sediments from all the Cenozoic epochs except the Paleocene. Rates of deposition of sediment younger than Middle Miocene are an order of magnitude higher than those prior to this time. At DSDP sites east of Japan, either Late Miocene sediments lie directly on the basement, or sediments older than Late Miocene are very thin. Post-Middle Miocene sediments are composed primarily of glass shards. Thus, about 90% of the total thickness of sediments in the northwest Pacific is composed of sediments younger than Middle Miocene with volcanic ash as the main constituent. The volcanic ash results from the present phase of explosive volcanic activity which began in the Late Miocene in the northwest Pacific volcanic arcs.

Petrography and K-Ar Ages of Dredged Volcanic Rocks from the Western Hawaiian Ridge and the Southern Emperor Seamount Chain

Alkalic basalt dredged from Yuryaku Seamount in the southernmost Emperor Seamount chain and from the western Hawaiian Ridge at Pearl and Hermes Reef and at two unnamed seamounts 160 and 380 km west of Midway is similar to the alkalic basalt that caps the volcanoes in the Hawaiian Islands. Conventional and 40 Ar/ 39 Ar K-Ar analyses give best weighted mean ages of 42.3 ± 1.6 m.y. for Yuryaku Seamount, 27.3 ± 0.4 m.y. and 26.7 ± 0.5 m.y. for the two unnamed seamounts, and 20.1 ± 0.5 m.y. for the volcano that forms Pearl and Hermes Reef. The data show that the age of the Hawaiian-Emperor bend is about 41 to 43 m.y. Although the volcanoes in the Hawaiian-Emperor chain generally increase in age to the north and west of the island of Hawaii, the measured age-distance relations along the chain are not linear in detail. A phonolite, possibly a differentiated member of a posterosional nephelinic suite and the first found on the Hawaiian Ridge, was recovered from Pearl and Hermes Reef. Samples of analcime tephrite recovered from the unnamed seamount 380 km west of Midway may also be derived from a posterosional nephelinic suite.

Gravity Anomalies and Flexure of the Lithosphere along the Hawaiian-Emperor Seamount Chain

Summary Simple models for the flexure of the lithosphere caused by the load of the Hawaiian-Emperor Seamount Chain have been determined for different values of the effective flexural rigidity of the lithosphere. The gravity effect of the models have been computed and compared to observed free-air gravity anomaly profiles in the vicinity of the seamount chain. The values of the effective flexural rigidity which most satisfactorily explain both the amplitude and wavelength of the observed profiles have been determined. Computations show that if the lithosphere is modelled as a continuous elastic sheet, a single effective flexural rigidity of about 5 x loz9 dyne-cm can explain profiles along the Hawaiian-Emperor Seamount Chain. If the lithosphere is modelled as a discontinuous elastic sheet an effective flexural rigidity of about 2 x lo3' dyne-cm is required. Since the age of the seamount chain increases from about 3 M y near Hawaii to about 70 My near the northernmost Emperor seamount these results suggest there is apparently little decrease in the effective flexural rigidity of the lithosphere with increase in the age of loading. This suggests the lithosphere is rigid enough to support the load of the seamount chain for periods of time of at least several tens of millions of years. Thus the subsidence of atolls and guyots along the chain is most likely to be regional in extent and is unlikely to be caused by an inelastic behaviour of the lithosphere beneath individual seamounts.

Formation of the Emperor Seamount Chain

THE Pacific ocean basin is attributed to a single, continuously active ridge system which is believed still to be active today in the eastern Pacific, although much of the eastern flank of the ridge and parts of the ridge itself have been subducted into a fossil trench along the west coast of North America and active trench regions such as the Aleutian trench and the Peru-Chile trench. One problem with this model is the present activity on the island of Hawaii, which according to the model is sitting in the centre of a stable plate. One explanation is based on Wilson's proposal1 that the linear ridges in the Pacific were formed as the crust moved over a localized “hot spot” in the asthenosphere or upper mantle, but the physical problems involved in creating and maintaining such localized “hot spots” over millions of years have largely gone unanswered. Although a few workers remain sceptical (see ref. 2), acceptance of the theory has been widespread.

Mantle Convection and Volcanic Periodicity in the Pacific; Evidence from Hawaii

The thermal-feedback theory of mantle melting proposed by Shaw in 1969 is found to be quantitatively consistent with data pertaining to the evolution of the Hawaiian Ridge. Applicable rate factors are estimated from relations between lava volumes and position along the ridge given in this paper and the radio-metric age distributions given by Jackson and others in 1972. Rate curves derived from these data provide a new method of age extrapolation or interpolation; results indicate that previous methods used to estimate the age of the Hawaiian-Emperor Bend are in error. No definite age is established, but calculations suggest an age greater than 50 m.y. Much more extensive radiometric data are required to define kinematic relations between the Hawaiian Ridge and Emperor Seamount chain. It appears to be firmly established from the work of Jackson and others and from the present study that the evolution of the Hawaiian Ridge has been episodic, with episodes of several different time scales. Average growth rates of the entire ridge system are divided into two regimes with a discontinuity at a position roughly 1,000 km northwest of Kilauea; the estimated age of this discontinuity is about 10 m.y. Other episodes relate to the durations of eruptive sequences along individual or contiguous lines of volcanoes within the en echelon set of locus lines defined by Jackson and others. The latest of these episodes, beginning about 6 m.y. ago, is marked by accelerating volume rates of eruption and accelerating rates of ridge propagation; this episode appears to be approaching a culminating stage represented by the present activity of Kilauea Volcano. The calculated rate of eruption of Kilauea (0.11 km 3 per yr) is virtually identical with a rate independently estimated by Swanson in 1972 using different data. Calculated durations for older locus lines are generally greater than 6 m.y., but major time overlaps occur that are not adequately understood. Episodic behavior of shorter durations also exists relative to growth of individual shields or to synchronous activity on neighboring shields (for example, Mauna Loa and Kilauea). Some of these shorter term effects are partly explained in terms of isostatic factors acting on the lithosphere and asthenosphere. The longer episodes are explained in terms of variations of melting rates in the asthenosphere, governed by viscous heating produced by the interaction of lithosphere translation and both vertical and horizontal shear flows in the subjacent mantle. Accelerations of eruption and propagation rates are explained by melting instabilities in the upper zones of the asthenosphere as a result of thermal feedback. During the latest melting episode, shear stresses in the asthenosphere derived from the rate data as interpreted by the thermal feedback model are in the range 100 to 200 bars; apparent viscosities range from 2 × 10 21 to 4 × 10 20 poise, decreasing with increasing melting rate. In general, a thermomechanical model is shown to be consistent with the idea that oceanic melting spots can be fixed relative to the deep mantle, although this invariance is not completely established. The thermal plume model of Morgan is not definitely ruled out but does not seem to be required for internally consistent interpretations of oceanic chains of volcanism. It is concluded that motion vectors of the Pacific plate cannot be inferred directly from rates of propagation of volcanic chains, because these rates reflect local, not average, relative velocities of lithosphere versus mantle flow. During growth of the Hawaiian Ridge, propagation speeds calculated on the basis of rate data for the southeastern Hawaiian Islands ranged from less than 1 cm per yr near the Hawaiian-Emperor Bend to nearly 30 cm per yr at the present ridge front.

Age of Kōko Seamount, Emperor Seamount chain

K Ar ages obtained by the conventional isotope-dilution and the 40Ar/39Ar techniques on two sanidine trachytes, four basalts, and a phonolite dredged from the top of Ko¯ko Seamount, 300 km north of the Hawaiian-Emperor bend, show that the seamount is 46.4 ± 1.1 my old. These data indicate that the volcanoes in the Hawaiian-Emperor chain continue to increase in age to the west and north beyond Midway Atoll, as predicted by the melting-spot hypothesis for the origin of the chain, and that the rate of volcanic migration along the chain was nonlinear between the time of formation of the island of Hawaii and Ko¯ko Seamount.

Kōko seamount: A major guyot at the southern end of the Emperor seamounts

Kōko seamount is an exceptionally large guyot situated at 35°30′N, 171°45′E, at the southern end of the Emperor seamount chain. An accurate survey of Kōko seamount and two successful dredge hauls from near its summit were made by R/V “Thomas Washington” in August 1971. The dredge hauls included samples of a wide variety of unusual igneous rocks and coral reef debris. The significance of the occurrence of coral reef debris capping a guyot which is now both too deep and too far north for active reef growth is discussed briefly.

Seamount Reflections from the CANNIKIN Underground Explosion

Hydroacoustic signals were recorded from the five megaton Amchitka Island underground explosion (code name CANNIKIN) of 6 November 1971. SOFAR depth hydrophones of the Missile Impact Location System (MILS) off the islands of Midway, Wake, and Oahu in the Central Pacific were used as detectors. In the low-frequency (<20 Hz) band, the main signal was followed by reverberations that persisted for several hours after the shot. Arrival times of some of these reverberations were correlated with those computed for reflections from seamounts in the Emperor Seamount Chain and the Hawaiian Arch. Reverberations from more distant reflectors were separated by quiet periods or “windows” of several minutes duration. The azimuthal and temporal distribution of the “windows” versus the reverberation maxima is investigated in relation to low-frequency ambient noise in the ocean.

LONG-RANGE DETECTION AND LOCATION OF SOME SHALLOW-DEPTH EXPLOSIONS IN THE NORTHWEST PACIFIC.

Abstract During a survey of underwater explosions, a set of 382 closely-spaced events was located in the Northwest Pacific near the Emperor Seamounts. Later, it was learned that the shots were from a joint U.S.-Japan science cooperation seismicrefraction program. After-the-fact correlations were then made between the SOund Fixing And Ranging (SOFAR) fixes and shot-log data. Shots were composed of 3 to 288-lb Therytol or Seismogel demolition charges. Detonation depths varied from 60 to 420 ft. Results are that: (1) If only shots of 8.6 lb or greater are considered, a larger percentage (87.7) of shots composed of Seismogel was successfully located than of those composed of Tetrytol (59.5); (2) the threshold of detection was lower (9 lb at 90 ft as compared to 12 lb at 110 ft) in the northern part of the profile (37°N) than in the southern part (32°N); and (3) signals from shots detonated over the southern edge of the Shatsky Rise were received at a higher level than those detonated off the Rise. The reason for (1) is the higher detonating velocity of Seismogel; for (2) it is the shoaling of the SOFAR channel with increasing latitude, and for (3) it is the effect of a highly reflective bottom which sloped toward the hydrophones.

Hawaiian-Emperor Chain and Its Relation to Cenozoic Circumpacific Tectonics

The Hawaiian Ridge and Emperor Seamounts appear to form a single chain of tholeiitic shield volcanoes that erupted sequentially on the sea floor of the central Pacific Ocean during Tertiary and Quaternary time. The chain cuts obliquely across the older Cretaceous structural patterns of that sea floor. While the pattern of the chain as a whole is linear, the individual volcanoes lie on short, sigmoidal, en echelon loci that are subparallel with respect to each other and that may represent extensional features in the crust and upper mantle. In general, the order of eurption progressed from northwest to southeaśt along the chain, but the rate of progression of volcanism along individual loci is nonlinear where best studied in the southeastern part of the chain. Furthermore, simultaneous eruptions appear to have occurred within a distance along the chain of about 200 to 400 km. The available data are consistent with a genesis related to the motion of the Pacific crust over a melting spot in the mantle. This melting spot, which may be due to either excess heat or pressure release, appears to have a diameter of about 300 km and is presently centered slightly north of the island of Hawaii. We concur with the idea that the bend in the Hawaiian-Emperor chain probably reflects a significant change in the motion of the Pacific plate. Our best estimate of the age of the Hawaiian-Emperor bend, based on the existing radiometric data, is 24.6 ± 2.5 m.y., which correlates with a time of increased tectonic activity in the western Pacific island arcs and along the northern and eastern boundaries of the Pacific plate. The vector change in the motion of the Pacific plate (with respect to the melting spot) that is required to produce the bend is about 12 cm/yr in a west-southwest-ward direction.

Emperor Fracture Zone: a Newly Discovered Feature in the Central North Pacific

SOUNDINGS by ESSA's Pacific Oceanographic Research Laboratory during 1968 and 1969 reveal a large north-west trending fracture zone east of the Emperor seamounts in the central North Pacific (Fig. 1). Bathymetric charts, based on rather sparse data, show traces of the feature as isolated deeps1 or as possible continuations of a southwest trending Chinook trough2.

Seismic-refraction measurements in the Northwest Pacific Basin

Seismic-refraction measurements made on Shatsky rise in the Northwest Pacific basin show crustal layering appreciably different from that in the basin on either side. Beneath the crestal zone of the rise and its broad western flank at water depths less than 5 km, oceanic basement (layer 2) is about 3–6 km thick and contains two refracting layers, 4.7 and 5.5 km/sec, overlying the main crustal layer (layer 3) of velocity averaging 7.0 km/sec. Pronounced crustal thickening beneath the rise is associated mainly with a 7.3- to 7.8-km/sec layer intermediate between layer 3 and the upper mantle; the thickness of layer 3 is approximately normal. The depth to mantle is at least 22 km near the crestal zone and perhaps even greater below the crest where comparatively shorter range profiles did not determine it. Additional measurements between two large seamounts of the Emperor seamount chain, on the eastern flank of a low rise in the basin floor, also indicate two layers within layer 2, but with velocities different from those found beneath Shatsky rise. Here, layer 2 is 2–4 km thick and composed of material with velocities of 3.7 and 6.2 km/sec. Below these layers the crust seems normal, except that the average thickness of layer 3 is somewhat lower than regional average.

Submarine topographic echoes from Chase V

Hydroacoustic waves from the 1-kiloton Chase V shot detonated off Cape Mendocino, California, were recorded 240 km (150 miles) north of Hawaii on R. P. Flip. Reverberations were received for 2½ hours after the direct arrival. Some of the prominent reflectors have been identified as the continental slope of North America, the Hawaiian arch, the Aleutian ridge, the Emperor seamounts, the Volshouki ridge, the Kuril Islands, and the Japanese Islands.

MARINE GEOLOGY OF NORTHWESTERN PACIFIC: DESCRIPTION OF JAPANESE BATHYMETRIC CHART 6901

This paper is a geological discussion of the northwest Pacific, to accompany the Japanese Hydrographic Chart 6901 (a bathymetric chart of the northwest Pacific) which is presented as Plate 1. A special edition of this chart was published under an arrangement made by the writer between the Japanese Hydrographic Office and The Geological Society of America in the belief that it will provide useful topographic data in a previously little-known portion of the sea floor and that it will stimulate further interest in Pacific marine geology. A comparison of the land surface with the sea floor, both of which are contoured at 500-m intervals, emphasizes the variety of sea-floor features and the grand scale of this submerged topography where erosional leveling is subdued. The region covered by the chart can be divided into (1) the Pacific Basin proper, (2) the Philippine Sea Basin, including the arcuate ridges extending from Japan to Palau, and (3) the submerged portion of the Asia continental block, including the continental shelf and slope and the encompassed “intra-continental” basins. Large-scale undulations or swells and swales of low relief are the dominant structures of the Pacific Basin, but the five great groups of seamounts are the most striking features. A study of the original soundings shows some abrupt changes in level and grabenlike depressions which probably mark fracture zones in the earth9s crust. The seamounts are considered to be volcanoes, many of which are capped with coral. Smooth bottom characteristizes most of the Basin, suggesting a thick sedimentary blanket. Two of the seamount groups (the Emperor Seamounts and the Magellan Seamounts) are entirely submerged; another group pierces the surface at only two places (Marcus I. and Wake I.); the Caroline and the Marshall-Gilbert groups form numerous atolls. A large number of the seamounts have deeply submerged flat-topped summits and, thus, are guyots or tablemounts. However, because of the low seismicity and the rarity of raised islands, the Pacific Basin is considered a stable area, and the sinking of the guyots is thought to be a local isostatic subsidence in response to the load of the seamounts. The Philippine Sea Basin is bounded by the Asia continental block on the west and by the great geanticlinal ridges, extending from Japan to Palau, on the east. Trenches, presumably marking downbuckles in the earth9s crust, lie seaward of these ridges. These geanticlinal ridges are active belts surmounted by volcanoes and a string of raised islands. The Kyushu-Palau Ridge, another large geanticlinal ridge, bisects the basin. Soundings give little evidence of guyots on any of the ridges or in the Philippine Sea. The Daito Mountains, nested between the Kyushu-Palau Ridge and the Ryukyu Trench, are probably orogenic rather than volcanic. The Philippine Sea Basin appears to be non-sialic and a region with normal Pacific crustal structure which has been deformed by the same compressional forces that have affected the margin of Asia. The continental slope, marking the boundary of the Asia sialic block, extends from Kamchatka, along the Kurils, Japan, and the Ryukyus to Formosa, and probably along the east side of the Philippines. Extensive featureless shelves with shelf-breaks at normal depths fringe China and Siberia. The continental slopes are canyoned and irregular in detail. In the Okhotsk Sea there is a remarkable deep terrace which is probably a subsided normal shelf. The Kuril Basin, the Japan Sea Basin, and the Ryukyu Basin are similar abyssal pear-shaped basin within the continental framework. En bloc drifting of the associated island arcs, opening up simatic pools in the continental block, may account for these basins. Their smooth bottoms suggest that they are sediment traps for a great amount of sediment eroded from Asia.