 | Submarine volcanism - near-ridge seamounts |
Three chains of near-ridge seamounts off Oregon and California: Vance, President Jackson, and Taney Seamounts Map MBARI 2000 |
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From MBARI - http://www.mbari.org/volcanism/Seamounts/Seam-NearRidge.htm
Seamounts erupted near mid-ocean ridges
There are many linear chains of seamounts that originate near mid-ocean ridges and are somehow due to excess magmatic activity intermittently but profusely over extended periods at that same point of the ridge. They are especially common near fast spreading ridge segments, and seem to be preferentially located near bends or offsets in the ridge crest. The chains are often asymmetric, with many more seamounts located on one side of a ridge than on the other. The cones are often flat-topped with pronounced calderas.
A new family, genus, and species of enteropneust worm from the President Jackson Seamounts is reported on the eclectic topics page. Go to the MBARI mapping program for more maps and information about these seamounts.
Tectonomagmatic relationship with ridge volcanism
GORDA RIDGE - The President Jackson Seamounts are a 65km long, linear volcanic chain west of the northern Gorda Ridge. Dredged basaltic lavas and hyaloclastites are normal mid-ocean-ridge basalts (MORB). The seamount lavas are similar in many ways to those erupted at the adjacent ridge but have some important geochemical differences: they have more primitive compositions, with higher MgO than the ridge lavas; they have lower TiO2 and FeO and higher CaO, Na2O, and Sr at comparable MgO than the ridge basalts; they contain phenocrysts ("conspicuous crystals") in equilibrium with the melt; and they lack the compositionally diverse glass inclusions and compositional zoning common in phenocrysts of most ridge basalt. The seamounts have multiple, nested calderas or pit craters, stepping downward toward the ridge axis, indicating formation in the active, near-ridge, extensional environment. The predominantly primitive nature of the lavas suggests that they pass through crustal magma chambers underlying the calderas very rapidly. The lack of evidence for magma mixing suggests that batches of magma are delivered to the seamounts episodically and either solidify or are drained into ridge-parallel faults before the next batch arrives.
In contrast, lavas from the ridge axis show evidence for magma mixing: they have more fractionated melts that show evidence for clinopyroxene fractionation. Despite a lack of seismic evidence for magma chambers under slow spreading centers, continuous melt zones must be present under the Gorda Ridge axis to give the ubiquitous imprint of magma mixing.
Reference: A.S. Davis and D.A. Clague (2000) President Jackson Seamounts, northern Gorda Ridge: tectonomagmatic relationship between on- and off-axis volcanism, Journal of Geophysical Research, 105(B12): 27,939-27,956. [Abstract] [Article]
Geology from high resolution bathymetric data
NORTHEAST PACIFIC - High-resolution bathymetry and side-scan sonar data of the Vance, President Jackson and Taney near-ridge seamount chains in the northeast Pacific were collected with a hull-mounted 30 kHz sonar. The central volcanoes in each chain consist of truncated cone-shaped volcanoes with steep sides and nearly flat tops. Several areas are characterized by frequent small eruptions that result in disorganized volcanic regions with numerous small cones and volcanic ridges but no organized truncated conical structure. Several volcanoes are crosscut by ridge-parallel faults, showing that they formed within 30-40 km of the ridge axis where ridge-parallel faulting is still active. Magmas that built the volcanoes were probably transported through the crust along active ridge-parallel faults.
The volcanoes range in volume from 11 to 187 km3, and most have one or more multiple craters and calderas that modify their summits and flanks. The craters (<1km diameter) and calderas (>1km diameter) range from small pit-craters to large calderas over 8km across. Crosscutting relationships commonly show a sequence of calderas stepping toward the ridge axis. To form these calderas, the volcanoes must overlie crustal magma chambers at least as large as those under Kilauea and Mauna Loa Volcanoes in Hawaii, perhaps 4-5 km in diameter and ~1-3 km below the surface. The nearly flat tops of many of the volcanoes have remnants of centrally located summit shields, suggesting that their flat tops did not form from eruptions along circumferential ring faults but instead form by filling and overflowing of earlier large calderas. The lavas must retain their primitive character by residing in magma chambers for only short time periods prior to eruption. Stored magmas are withdrawn, probably as dikes intruded into the adjacent ocean crust along active ridge-parallel faults, triggering caldera collapse, or solidified before the next batch of magma is intruded into the volcano, probably 1000 to 10,000 years later.
The chains are oriented parallel to subaxial asthenospheric flow rather than absolute or relative plate motion vectors, and models yield rates of volcanic migration of 3.4, 3.3, and 5.9 cm/yr for the Vance, President Jackson, and Taney Seamounts, respectively. The modeled lifespans of the individual volcanoes in the three chains vary from 75 to 95 kyr. Magma supply rates differ from chain to chain, as expressed by the size of calderas, infilling of calderas, and formation of late cones on volcano summits and flanks.
Reference: D.A. Clague, J.R. Reynolds, and A.S. Davis (2000) Near-ridge seamount chains in the northeastern Pacific Ocean, Journal of Geophysical Research, 105(B7): 16,541-16,561
From http://www.mbari.org/volcanism/Seamounts/Seam-Eclectic.htm
the sea floor result in biological as well as geological observations and collections. An enteropneust worm found at the President Jackson Seamounts is of a new family, genus, and species, and we have observed others like it at Rodriguez Seamount and off Hawaii. In another study, deep sea coral distributions on Davidson Seamount were examined with video and GIS.
On all our dives to seamounts, we have found that erratic rocks are surprisingly abundant and can be from the continent and even from other seamounts. The erratics were probably transported by biological means, rather than icebergs or turbidity flows.
Isotopes in deep-sea corals record growth history and paleoclimate
Deep-sea bamboo corals hold promise as long-term climatic archives, yet little information exists linking bamboo coral geochemistry to measured environmental parameters. This study focuses on a suite of 10 bamboo corals collected from the Pacific and Atlantic basins (2502136 m water depth) to investigate coral longevity, growth rates, and isotopic signatures. Calcite samples for stable isotopes and radiocarbon were collected from the base the corals, where the entire history of growth is recorded. In three of the coral specimens, samples were also taken from an upper branch for comparison. Radiocarbon and growth band width analyses indicate that the skeletal calcite precipitates from ambient dissolved inorganic carbon and that the corals live for 150300 years, with extension rates of 9128 mm/yr. A linear relationship between coral calcite 18O and 13C indicates that the isotopic composition is influenced by vital effects (18O:13C slope of 0.170.47). As with scleractinian deep-sea corals, the intercept from a linear regression of 18O versus 13C is a function of temperature, such that a reliable paleotemperature proxy can be obtained, using the "lines method." Although the coral calcite 18O:13C slope is maintained throughout the coral base ontogeny, the branches and central cores of the bases exhibit 18O:13C values that are shifted far from equilibrium. We find that a reliable intercept value can be derived from the 18O:13C regression of multiple samples distributed throughout one specimen or from multiple samples within individual growth bands.
Reference: Hill, T. M., H. J. Spero, T. Guilderson, M. LaVigne, D. Clague, S. Macalello, and N. Jang (2011), Temperature and vital effect controls on bamboo coral (Isididae) isotope geochemistry: A test of the "lines method", Geochem. Geophys. Geosyst., 12, Q04008, doi:10.1029/2010GC003443. [Abstract] [Article]
Geological evolution of seamounts and their influences
The geological evolution of seamounts has distinct influence on their interactions with the ocean, their hydrology, geochemical fluxes, biology, resources, and geohazards. There are six geological evolutionary stages of seamounts: (1) small seamounts (1001000-m height), (2) mid-sized seamounts (>1000-m height, > 700-m eruption depth), (3) explosive seamounts (< 700-m eruption depth), (4) ocean islands, (5) extinct seamounts, and (6) subducting seamounts. Throughout their lifetimes, seamounts offer major passageways for fluid circulation that promotes geochemical exchange between seawater and the volcanic oceanic crust, and seamounts likely host significant microbial communities. Water circulation may be promoted by hydrothermal siphons in conjunction with the underlying oceanic crust, or it may be driven by intrusions inside seamounts from Stage 2 onward. Geochemical fluxes are likely to be very large, primarily because of the very large number of Stage 1 seamounts. Intrusive growth of seamounts also initiates internal deformation that ultimately may trigger volcano sector collapse that likely peaks at the end of the main volcanic activity at large seamounts or islands. Explosive activity at seamounts may begin at abyssal depth, but it is most pronounced at eruption depths shallower than 700 m. Wave erosion inhibits the emergence of islands and shortens their lifespans before they subside due to lithosphere cooling. Once volcanism ends and a seamount is submerged, seamounts are largely unaffected by collapse or erosion. Throughout their histories, seamounts offer habitats for diverse micro- and macrobiological communities, culminating with the formation of coral reefs in tropical latitudes. Geological hazards associated with seamounts are responsible for some of the largest natural disasters recorded in history and include major explosive eruptions and largescale landslides that may trigger tsunamis.
Reference: Staudigal, H., Clague, D.A. (2010) The geological history of deep-sea volcanoes: biosphere, hydrosphere, and lithosphere interactions. Oceanography 23(1): 5871. [Article]
Fish communities similar at three seamounts
DAVIDSON, PIONEER, AND RODRIGUEZ SEAMOUNTS - In many regions of the world, commercial fisheries for seamount-aggregating species have collapsed. Most are trawl fisheries, which have been implicated in the destruction and loss of fragile coral and sponge habitat and potentially endemic seamount species. Even with the intense fishing effort over seamounts, our understanding of processes regulating the distribution and abundance of seamount fish assemblages, which is vital to conservation policy, is weak. Here, we describe the abundance and distribution of demersal fishes found on 3 seamounts off central and southern California. Video observations were taken during 27 dives of a remotely operated vehicle (ROV), and were annotated in detail using the Monterey Bay Aquarium Research Institutes (MBARI) Video Annotation Reference System (VARS). Video analysis yielded 2151 observations of 36 identified taxa. No aggregations of fishes were observed during the surveys. Video transects were analyzed to quantify organism density. Similarity among seamounts was compared using Bray-Curtis similarity estimates. Our results indicate high similarity among seamount faunas at similar depths, a shift from provincial to abyssal/cosmopolitan species with increased depth, and no evidence of seamount endemism.
Reference: Lundsten, L., McClain, C.R., Barry, J.P., Cailliet, G.M., Clague, D.A., DeVogelaere, A.P. (2009) Ichthyofauna on Three Seamounts off Southern and Central California, USA. Marine Ecology Progress Series 389:223-232. [Abstract] [Article]
No evidence for species endemism
DAVIDSON, PIONEER, AND RODRIGUEZ SEAMOUNTS - Seamounts are unique and biologically productive deep-sea habitats that have often been described as having high levels of endemism, highly productive fisheries, and benthic communities vulnerable to trawl fishing. We describe the abundance and distribution of benthic megafaunal invertebrates found on 3 seamounts off central and southern California. Video observations were taken during 27 dives of a remotely operated vehicle (ROV) and were annotated in detail using the Monterey Bay Aquarium Research Institutes (MBARI) video annotation reference system (VARS, http://vars.sourceforge.net/). Video analysis yielded 134,477 observations of 202 identified invertebrate taxa. Video transects were analyzed to quantify organism density. Thirteen new species were observed and collected. Invertebrate communities at Davidson and Pioneer Seamounts were dominated by passive suspension-feeding cnidarians (mostly corals), but at Rodriguez Seamount, a guyot, the invertebrate community was dominated by holothurian echinoderms. We found no evidence of endemism among the megafauna at these 3 seamounts, which are all in close proximity to each other and the continental margin.
Reference: Lundsten, L., J.P. Barry, G.M. Cailliet, D.A. Clague, A.P. DeVogelaere, J.B. Geller (2009) Benthic invertebrate communities on three seamounts off southern and central California, USA, Marine Ecology Progress Series, 374: 23-32. [Abstract] [Article]
Erratic rocks are abundant at seamounts
SEAMOUNTS OFF CALIFORNIA, OREGON, AND WASHINGTON STATES - 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.
Reference: Paduan, J.B., D.A. Clague, A.S. Davis (2007) Erratic continental rocks on volcanic seamounts off the US west coast, Marine Geology, 246: 1-8, doi:10.1016/j.margeo.2007.07.007 [Abstract]
Enteropneusts refute previous assumptions
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