Volcanic Seamounts Research Articles

First record of Hyalascus (Hexactinellida: Rossellidae) from the Indian Ocean, with description of a new species from a volcanic seamount in the Andaman Sea

First record of Hyalascus (Hexactinellida: Rossellidae) from the Indian Ocean, with description of a new species from a volcanic seamount in the Andaman Sea

First record of Hyalascus (Hexactinellida: Rossellidae) from the Indian Ocean, with description of a new species from a volcanic seamount in the Andaman Sea

A new species of Hyalascus is described from the submarine volcanic crater seamount of Andaman Back-arc Basin, Indian Ocean. The genus was previously known in the Pacific Ocean only.

Mineralogical evidence for a local volcanic origin of the parent material of Bermuda Quaternary paleosols

The alternation of carbonate deposits and paleosols compose the emerged part of the Bermuda archipelago. The pedological units present a complex and diversified mineralogy. Former studies demonstrated that the paleosols are not primarily a product of the unique dissolution of the surrounding carbonates, but contain a massive input of allochthonous non-carbonate detrital material. Researchers during more than the past three decades have attributed this flux of insoluble residues (IR) to Saharan dusts. We carried out systematic field and mineralogical analyses on the Quaternary paleosols from the Bermuda archipelago. Their mineralogical assemblage predominantly includes carbonates, clay minerals (kaolinite, chlorite and chlorite/vermiculite), phosphates, and aluminium and iron oxides/hydroxides. This assemblage is strikingly close to the mineralogy of the weathered volcanic substrate of Bermuda, but noticeably different from the mineralogy of Saharan dust. Moreover, we found volcanic lithoclasts in numerous paleosol profiles all over the archipelago and in all the recorded time intervals. We thus consider the volcanic seamount underlying Bermuda as the main source of non-carbonate minerals detected in the paleosols. This hypothesis further resolves the anomalous maturity of Bermudan paleosols compared to their southern counterparts in the Bahamas and Barbados.

P and S velocity structures of the Santorini–Coloumbo volcanic system (Aegean Sea, Greece) obtained by non-linear inversion of travel times and its tectonic implications

One of the most prominent tectonic features of the Eastern Mediterranean region is the Hellenic volcanic arc in the Southern Aegean Sea, with the Santorini Island being one of the most active volcanic centers. Recent seismic studies show that the main seismic activity of the Santorini volcanic center is strongly associated with the volcanic processes, as well as with the seismo-tectonic regime of the broader Southern Aegean Sea area. The main cluster of local seismicity is located near the northeastern edge of the Santorini Island, beneath the Coloumbo Reef, which is a submarine volcanic seamount of the Santorini Island volcanic system. The P and S wave velocity structures of the Santorini–Coloumbo volcanic system is studied by inverting travel times of local earthquakes recorded by two independent dense seismic arrays installed in the broader area of Santorini islands during the period of September 2002–September 2005. In particular, 137 local earthquakes with 1600 P phases and 1521 S phases recorded by 25 seismological stations have been selected for the inversion. The inversion technique applied is non-linear, since three-dimensional ray tracing is incorporated. The reliability of the final tomographic results is demonstrated through resolutions tests using synthetic seismic data. The obtained results confirm the strong variations of the P and S wave velocity structures in the area of Santorini–Coloumbo volcanic system, as well as the connection between the tectonic setting of the study area with the magmatic processes taking place beneath the two volcanoes. The tomographic models show that a low-velocity zone extends along the northeastern edge of the Santorini Island, parallel to the “Kameni–Coloumbo” fracture zone (NE–SW direction), which corresponds to the western termination of the major ENE–WSW Santorini–Amorgos Fault Zone. Evidence is presented that this structural lineament corresponds to a tecto-volcanic fracture zone, which probably links the volcanic center of Santorini with the submarine volcano at Coloumbo Reef. Furthermore, the tomographic results show indications that the magmatic chamber beneath the Coloumbo volcanic seamount is located at a depth of 6–7 km in good agreement with recent independent studies in the area, though this feature is not within the resolving power of the employed data set.

Origin of volcanic seamounts at the continental margin of California related to changes in plate margins

Volcanic samples collected with the Monterey Bay Aquarium Research Institute's ROV Tiburon from eight seamounts at the continental margin offshore central to southern California comprise a diverse suite of mainly alkalic basalt to trachyte but also include rare tholeiitic basalt and basanite. All samples experienced complex crystal fractionation probably near the crust/mantle boundary, based on the presence in some of mantle xenoliths. Incompatible trace elements, poorly correlated with isotopic compositions, suggest variable degrees of partial melting of compositionally heterogeneous mantle sources, ranging from MORB‐like to relatively enriched OIB. High‐precision 40Ar/39Ar ages indicate episodes of volcanic activity mainly from 16 to 7 Ma but document one eruption as recent as 2.8 Ma at San Juan Seamount. Synchronous episodes of volcanism occurred at geographically widely separated locations offshore and within the continental borderland. Collectively, the samples from these seamounts have age ranges and chemical compositions similar to those from Davidson Seamount, identified as being located atop an abandoned spreading center. These seamounts appear to have a common origin ultimately related to abandonment and partial subduction of spreading center segments when the plate boundary changed from subduction‐dominated to a transform margin. They differ in composition, age, and origin from other more widespread near‐ridge seamounts, which commonly have circular plans with nested calderas, and from age progressive volcanoes in linear arrays, such as the Fieberling‐Guadalupe chain, that occur in the same region. Each volcanic episode represents decompression melting of discrete enriched material in the suboceanic mantle with melts rising along zones of weakness in the oceanic crust fabric. The process may be aided by transtensional tectonics related to continued faulting along the continental margin.

The volumes of volcanic seamounts as a factor determining the formation of the ocean floor’s morphostructure

The volumes of formed volcanoes are an important indicator of the formation intensity of the ocean floor volcanogenic morphostructure under different geodynamic conditions. The distribution of the volumes of volcanoes that formed within the oceanic lithosphere of different geological ages is considered to analyze the spatial and time dynamics of the scopes of the seamount formation. The greatest volumes of effused rocks concentrate in volcanoes of different heights on the different-age segments of the lithosphere. The total volumes of volcanogenic substance growing with increasing age and thickness of the lithosphere is caused by the increasing range of the heights of the formed volcanoes and the growing number of big seamounts. Despite that the volcanic seamounts formed within the mid-oceanic ridges, on the divergent boundaries of the lithospheric plates and in the transform faults most often, half of the volcanogenic material was emitted to the earth’s surface during the spreading of the intraplate volcanism. A leading part in the formation of the volcanogenic morphostructure of the ocean floor belongs to the formation of big seamounts with heights of 4–6 km and volumes from 1500 to 5100 km3, respectively, in the provinces with ages from 60 to 90 mln years.

Seismically active column and volcanic plumbing system beneath the island arc of the Izu-Bonin subduction zone

A detailed spatio-temporal analysis of teleseismic earthquake occurrence (mb > 4.0) along the convergent margin of the Izu-Bonin-Mariana arc system reveals an anomalously high concentration of events between 27° and 30.5°N, beneath a chain of seamounts between Tori-shima and Nishino-shima volcanoes. This seismicity is dominated by the 1985/1986 earthquake swarm represented in the Engdahl—van der Hilst—Buland database by 146 earthquakes in the body wave magnitude range 4.3–5.8 and focal depth range 1–100 km. The epicentral cluster of the swarm is elongated parallel to the volcanic chain. Available focal mechanisms are consistent with an extensional tectonic regime and reveal nodal planes with azimuths close to that of the epicentral cluster. Earthquakes of the 1985/1986 swarm occurred in seven time phases. Seismic activity migrated in space from one phase to the other. Earthquake foci belonging to individual phases of the swarm aligned in vertically disposed seismically active columns. The epicentral zones of the columns are located in the immediate vicinity of seamounts Suiyo and Mokuyo, recently reported by the Japanese Meteorological Agency as volcanically active. The three observations—episodic character of earthquake occurrence, column-like vertically arranged seismicity pattern, and existence of volcanic seamounts at the seafloor above the earthquake foci—led us to interpret the 1985/1986 swarm as a consequence of subduction-related magmatic and/or fluid activity. A modification of the shallow earthquake swarm magmatic model of D. Hill fits earthquake foci distribution, tectonic stress orientation and fault plane solutions. The 1985/1986 deep-rooted earthquake swarm in the Izu-Bonin region represents an uncommon phenomenon of plate tectonics. The portion of the lithospheric wedge that was affected by the swarm should be composed of fractured rigid, brittle material so that the source of magma and/or fluids which might induce the swarm should be situated at a depth of at least 100 km in the aseismic part of the subduction zone.

The upper lithostratigraphic unit of ANDRILL AND-2A core (Southern McMurdo Sound, Antarctica): Local Pleistocene volcanic sources, paleoenvironmental implications and subsidence in the southern Victoria Land Basin

We report results from the study of the uppermost 37 m of the Southern McMurdo Sound (SMS) AND-2A drill core, corresponding to the lithostratigraphic unit 1 (LSU 1), the most volcanogenic unit within the core. We present data on the age, composition, volcanological and depositional features of the volcanic sedimentary and tephra deposits of LSU 1 and discuss their source, mechanisms of emplacement and environment of deposition. Sedimentary features and compositional data indicate shallow water sedimentation for the whole of LSU 1. Most of LSU 1 deposits are a mixture of near primary volcanic material with minor exotic clasts derived from the Paleozoic crystalline basement rocks. Among volcanic materials, glassy particles are the most abundant. They were produced by mildly explosive basaltic eruptions occurring in subaerial and subaqueous environments. The Dailey Islands group, 13 km south-southwest of the SMS drill-site, has been identified as a possible source for the volcanics on the basis of similarity in composition and age. 40Ar– 39Ar laser step-heating analyses on a lava sample from Juergens Island yields an age of 775 ± 22 ka. Yet because of the minimal reworking features of vitriclasts, preservation of fragile structures in volcaniclastic sediments and evidence for volcanic seamounts to the north of the Dailey Islands, it is likely that some of the material originated also from vents close to the drill-site. Evidence for local volcanic sources and for deposition of sediments in a shallow marine environment provides indications about the local paleogeography and implications for the subsidence history of the southern Victoria Land Basin from Pleistocene to Recent.

Geology and hydrocarbon potential of the offshore Indus Basin, Pakistan

ABSTRACT The offshore Indus Basin is a rift and passive margin basin offshore Pakistan and northwest India. Rifting associated with break up of the India/Madagascar/Seychelles plates began during the Late Cretaceous and was accompanied by a major period of volcanism associated with the Deccan volcanic event. A major volcanic centre was located along the south of the basin adjacent to the Saurashtra Arch transform fault and resulted in the deposition of up to 7 km of extrusive basalts interbedded with Late Cretaceous–Paleocene marine sediments. The basalts show stacked prograding reflection patterns on seismic data. A chain of northeast–southwest-trending volcanic seamounts in the central part of the deep-water basin formed topographic highs for the development of shallow-water carbonates during the Eocene post-rift phase. The passive margin stratigraphy includes up to 9 km of Oligocene–Recent age clastic sediments from the Indus River system. The proximal part of the Indus Fan contains spectacular large-scale channel-levees in the Miocene and Plio-Pleistocene intervals. Fourteen major channel-levee systems have been identified in the Plio-Pleistocene and represent potential reservoir targets. Trap types include extensional rollover anticlines at the shelf edge, drape structures over the Eocene carbonate highs, stratigraphic traps along the Murray Ridge and folds associated with strike-slip faults along the Murray Ridge. A key challenge for future exploration is to determine whether source rocks are present in sufficient quality for commercial discoveries.

The Moín High, East Costa Rica: Seamount, laccolith or contractional structure?

The back-arc area of the southern Central American arc–trench system in East Costa Rica is characterized by a complex basin system. An extensional back-arc area (the North Limón Basin) and a compressional retro-arc foreland basin (the South Limón Basin) are closely related. Both basins are separated by an approximately 50 km long and 30 km wide mound-shaped structure referred to as Moín High, which evolved in Eocene times. The Moín High has previously been interpreted as a basement structure or paleo-high. The modern geothermal gradient is 3 °C/100 m. There is no evidence for thermal anomaly or higher heat flow in that area. A mean heat flow of 56–60 mW/m 2 implies that an origin as a volcanic seamount or magmatic intrusion is unlikely. 3D static models show that the Moín High trends NNE–SSW and has an antiformal shape in cross-section and an elliptic outline in map view. The trend of the Moín High coincides with the orientation of folds in West Costa Rica that formed in response to an Eocene deformation phase. The seismic lines show that Miocene reflectors onlap against the structure. Based on this data set it is likely that the Moín High is an anticline formed due to contraction.

Seamount volcanism along the Gakkel Ridge, Arctic Ocean

SUMMARY The Gakkel Ridge in the Arctic Ocean is the slowest spreading portion of the global midocean ridge system. Total spreading rates vary from 12.8 mm yr −1 near Greenland to 6.5 mm yr −1 at the Siberian margin. Melting models predict a dramatic decrease in magma production and resulting crustal thickness at these low spreading rates. At slow spreading ridges, small volcanic seamounts are a dominant morphologic feature of the rift valley floor and an important mechanism in building the oceanic crust. This study quantitatively investigates the extent, nature and distribution of seamount volcanism at the ultraslow Gakkel Ridge, the manner in which it varies along the ridge axis and the relationship of the volcanoes to the larger scale rift morphology. A numerical algorithm is used to identify and characterize isolated volcanic edifices by searching gridded swath-bathymetry data for closed concentric contours protruding above the surrounding seafloor. A maximum likelihood model is used to estimate the total number of seamounts and the characteristic height within different seamount populations. Both the number and size of constructional volcanic features is greatly reduced at the Gakkel Ridge compared with the Mid-Atlantic Ridge (MAR). The density of seamounts (number/area) on the rift valley floor of the Western Volcanic Zone (WVZ) is ∼55% that of the MAR. The observed volcanoes are also much smaller, so, the amount of erupted material is greatly reduced compared with the MAR. However, the WVZ is still able to maintain a MAR-like morphology with axial volcanic ridges, volcanoes scattered on the valley floor and rift valley walls consisting of high-angle faults. Seamount density at the Eastern Volcanic Zone (EVZ) is ∼45% that of the WVZ (∼25% that of the MAR). Seamounts are clustered at the widely spaced magmatic centres characteristic of the EVZ, although some seamounts are found between magmatic centres. These seamounts tend to be located at the edge of the rift valley or on the valley walls rather than on the valley floor. Seamounts in the Sparsely Magmatic Zone (SMZ) are located almost entirely at the 19 ◦ E magmatic centre with none observed within a 185 km-long portion of the rift valley floor. The EVZ and SMZ appear to display a mode of crustal accretion, characterized by extreme focusing of melt to the magmatic centres. Magmas erupted between the magmatic centres appear to have ascended along faults. This is very different from what is observed at the WVZ (or the MAR), and there is a threshold transition between the two modes of crustal accretion. At the Gakkel Ridge, the location of the transition appears to be localized by a boundary in mantle composition.

Plate tectonics, radiogenic isotopic tracers and paleoceanography: The case of the manganese crusts in the Pacific

Hydrogenous ferromanganese crust deposits on seafloor and volcanic seamounts are archives of past isotopic compositions of the ocean. Isotopic compositions of long-life radiogenic isotopes that are dissolved in the ocean such as Nd and Pb, are mixtures of different erosion products (continents, volcanics arcs, ridges etc…) transported by oceanic currents or winds. Isotopic studies on Mn crusts are thus potential paleogeographic and paleoceanographic tracers. However, global paleogeography can drastically evolve over the period of growth of crust, as well as the latitude, longitude and water depth of the Mn crust. Thus each crust sampled a succession of geographic sites related to its spatial evolution through time. For this reason, valid information about the past ocean isotopic composition can only be extracted from Mn crust under the condition that the successive paleogeographic and paleodepth positions can be reconstructed for each Mn crust. We followed this approach for six hydrogenous Mn crusts from the Pacific Ocean that have recorded Pb and Nd isotopic signals for the past 80 Myr. The evolution of the isotopic data was considered within a proper geodynamical framework that takes into account the paleopositions of the Pacific plate and surrounding continents, of the hot spots and ridges as well as the paths of the six crusts. The differences and similarities between the Pb and Nd revealed by our analysis emphasize their complementarity as paleoceanographic tracers. The Nd isotopes are strongly influenced by the exchange between Indian and Pacific waters, specifically by the progressive closure of the Indonesian Gateway. The closed central Pacific characteristics are defined from the extrapolation of the isotopic trend. This highlights the important role played by erosion of volcanic arcs in the Nd isotopic value of seawater. Our approach also stresses the influence of local volcanic activity (ridges or active hot spot) for lead isotopes.

Magmatism associated with Gondwanaland rifting and Neo-Tethyan oceanic basin development: evidence from the Mamonia Complex, SW Cyprus

Volcanic and intrusive rocks of the Dhiarizos Group, occurring within the Mamonia Complex of SW Cyprus, comprise mainly tholeiitic basalts, ocean island tholeiites (OIT) and other alkalic varieties, with minor alkaline lamprophyre intrusive rocks. These rocks were highly tectonized and dismembered during the collision with the Troodos oceanic lithosphere. Trace element data for the most primitive mafic igneous rock samples suggest that the tholeiitic basalts are derived from a depleted mantle source whereas the OIT and the alkalic basalts originated from variable degrees of partial melting of a spinel peridotite mantle. Alkaline lamprophyres are products of a small degree of partial melting of a deep-seated garnet lherzolite. The bulk of the tholeiites and alkalic varieties are Late Triassic in age and are interpreted as having formed during Neo-Tethyan sea-floor spreading and associated seamount volcanism. Amphibole separates from a lamprophyre intrusion have 40 Ar– 39 Ar ages of 140.7 ± 0.4 Ma. In this paper, we report for the first time an Early Cretaceous alkaline lamprophyric magmatism, which is probably associated with a renewed episode of Gondwanaland rifting. These results show that the development of the Neo-Tethyan oceanic basin in the Eastern Mediterranean was largely controlled by a two-phase rifting–drifting process.

Mantle Plumes: A Multidisciplinary Approach

Mantle plumes are blobs of relatively hot, low‐density mantle that because of their buoyancy, rise through the surrounding mantle. The existence of mantle plumes in the Earth was first suggested in 1963 by J. Tuzo Wilson at the University of Toronto as an explanation of oceanic island chains, such as the Hawaiian and Emperor volcanic chains, which change progressively in age along the chain. Wilson proposed that as lithospheric plates move over fixed hot spots (the mantle plumes), volcanism is recorded as a linear array of volcanic seamounts and islands parallel to the direction the plate is moving.In 1971, W. Jason Morgan at Princeton University suggested that flood basalts were formed by the melting of plume heads, whereas hot spot volcanic chains were derived from partial melting of plume tails. He also showed that closely spaced hot spots on the same plate had not moved significantly relative to each other, and he suggested this was evidence that the plumes had come from the core‐mantle boundary.

Seamount volcanism associated with the Xigaze ophiolite, Southern Tibet

Basaltic lavas at Renbu, Southern Tibet are associated with the Xigaze ophiolite in the Yarlung-Zangbo suture zone. They are alkaline lavas rich in large ion lithophile elements (LILE, Ba, Rb and Sr) and high field strength elements (HFSE, Nb, Ta, Zr and Hf), but poor in Cr, Co and Ni. All of the rocks have chondrite-normalized REE patterns enriched in light rare earth elements (LREE), comparable to modern basalts of the Society Islands, Kerguelen Plateau and Broken Ridge. Abundances of some immobile or moderately immobile elements (Nb, Ta, Zr, Hf, Y, Ti and REE) are also comparable to Kerguelen alkaline basalts. The Renbu basalts are geochemically similar to oceanic island basalts (OIB) and have some elemental ratios, such as Nb/Ta ratios = 15.7–18.1, Th/Nb = 0.06–0.10, La/Nb = 0.59–0.83 and Th/Ta = 1.03–1.52, similar to the primitive mantle. Their 87Sr/ 86Sr ratios (0.70453–0.70602) are relatively high, similar to OIB. In the 87Sr/ 86Sr vs. εNd(t) diagram, the Renbu basalts plot along a trend from N-MORB to EMII (enriched mantle II), suggesting the involvement of at least two mantle sources in their generation. The Renbu basalts represent seamount volcanism associated with the Xigaze ophiolite. They formed from an OIB-type mantle source within the Neo-Tethyan Ocean that had a composition similar to the modern Indian Ocean mantle.

Erratic continental rocks on volcanic seamounts off the US west coast

Sampling of volcanic seamounts with dredges and the remotely operated vehicle Tiburon recovered erratic rocks in surprising abundance as far as 500km offshore of the US West coast. The erratics usually have continental lithologies and appear to have been weathered in nearshore environments. They are probably transported by kelp holdfasts, drift logs, and pinnipeds to the seamounts, where they accumulate over time. The erratics are concentrated as lag deposits and kept from becoming buried in sediment by currents that sweep the seamounts. The erratics often have thinner manganese-oxide crusts than rocks of the seamounts because they were delivered to the seafloor more recently and manganese-oxide crusts precipitate over time. The thinner crusts make erratics easier to collect. While most of the erratics clearly did not originate by the volcanic processes that formed the seamounts, careful evaluation of some is necessary to distinguish them as erratics. Failure to recognize the presence of erratics may result in unrealistically complex interpretations of regional geology.

Pacific seamount volcanism in space and time

SUMMARY Seamounts constitute some of the most direct evidence about intraplate volcanism. As such, when seamounts formed and into which tectonic setting they erupted (i.e. on-ridge or off-ridge) are a useful reflection of how the properties of the lithosphere interact with magma generation in the fluid mantle beneath. Proportionately few seamounts are radiometrically dated however, and these tend to be recently active. In order to more representatively sample and better understand Pacific seamount volcanism this paper estimates the eruption ages (tvolc) of 2706 volcanoes via automated estimates of lithospheric strength. Lithospheric strength (GTRrel) is deduced from the ratio of gravity to topography above the summits of volcanoes, and is shown to correlate with seafloor age at the

Vulnerability of deepwater demersal fishes to commercial fishing: Evidence from a study around a tropical volcanic seamount in Papua New Guinea

The deepwater fishes around the Lihir Island group, Papua New Guinea, were assessed for their vulnerability to exploitation from a potential commercial fishery. Three dropline surveys were made between 1999 and 2002 to estimate catch rates and life-history parameters. Growth parameters were obtained using the von Bertalanffy growth function fitted to length-at-age data from annuli counts in sectioned otoliths. Mean catch rates and total mortality rates were then used to estimate total fish biomass and maximum sustainable yields for the deepwater species around the island group. A total of 975 fish of 98 species were caught during 886 line hours of fishing. Most of these species are commercially valuable: 17 species are commonly fished commercially in other tropical Pacific regions. However around the Lihir Island group, their catch rates (1.38 ± 0.16 kg/line h) were lower and more variable than elsewhere in the Pacific, probably because of the limited deep-slope habitat. Total deepwater fish biomass between the 125 and 350 m depth zone was estimated at 17.4 metric tonnes with an annual sustainable yield of between 1213 and 3445 kg. In addition, many of the deepwater species may be long-lived, slow-growing species with low mortality rates. Our results suggest that the deepwater fish stock at the Lihir Island group is highly susceptible to overfishing at only moderate levels of fishing pressure. We therefore recommend that a precautionary approach should be taken, including accurate long-term monitoring of catches, if these stocks are to be commercially exploited.

New species of Galatheidae (Crustacea: Anomura: Galatheoidea) from volcanic seamounts off northern New Zealand

Two new species of Munida (M. gordoni and M. grieveae) and one new species of Agononida (A. nielbrucei) are described from volcanic seamounts off northern New Zealand (RV ‘Tangaroa’, National Institute of Water and Atmospheric Research [NIWA], New Zealand). Description of new species and preliminary examination of NIWA collections reveal unusually high endemism of volcanic seamount populations of Galatheidae.

Miocene volcanic seamounts on northern Lord Howe Rise: Lithology, age and origin

Multibeam sonar swath‐mapping has revealed small submarine volcanic cones on the northeastern Lord Howe Rise, a submerged ribbon continent. Two such cones, aligned north‐northwest and 120 km apart, were dredged at 23–24°S. Water depth is about 1150 m nearby: the southern cone rises to 750 m and the northern to 900 m. Volcanic rocks dredged from the cones are predominantly highly altered hyalodastites with minor basalt. The clasts are mostly intensely altered vesicular brownish glass with lesser basalt, set in zeolitic, clayey, micritic, or ferruginous cement. Lavas and hyalodastites contain altered phenocrysts of olivine and plagioclase, and fresh clinopyroxene. The latter have compositions between acmite and Ti‐augite, matching clinopyroxene phenocrysts in undersaturated intraplate basanitic mafic lavas. Interbedded micrites in the volcaniclastics represent calcareous ooze that was deposited with (or later than) the volcanic pile. Foraminifers indicate that the micrite is dominantly Early Miocene (ca 16 Ma), and that the ooze was deposited in cool water. Upper Miocene to Pliocene micrites, presumed to be later infillings, all contain warm‐water forms. This evidence strongly suggests that both cones formed in pelagic depths in the Early Miocene. Ferromanganese crusts from the two cones are up to 7 cm thick and similar physically, but different chemically. The average growth rate is 4 mm/106 y. Copper, nickel, and cobalt content are relatively high in the north, but copper does not exceed 0.08 wt%, nickel 0.65%, and cobalt 0.25%. The Mn:Fe ratio is high in the south (average 13.7) suggesting strong hydrothermal influence. Such small volcanic cones related to intraplate hot‐spot‐type volcanism may occur in extensive fields on the Lord Howe Rise, like those off southern Tasmania. On the Lord Howe Rise, the known small volcanic cones coincide with broad gravity highs in areas of shallow continental basement. The highs are aligned parallel to the Lord Howe Rise margins, and it is probable that the Miocene volcanism is related to jostling on older faults. Other fields of small volcanic seamounts may occur on the Lord Howe Rise on such fractured gravity highs.