An outline of the interdisciplinary survey on a new type intra-plate volcanism (original) (raw)
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Subsurface Structure of the "Petit-spot" Intra-plate Volcanism, in the Northwestern Pacific
JAMSTEC Report of Research and Development
Single-channel seismic reflection surveys were conducted in the northwestern Pacific to investigate subsurface structure and nature of volcanic eruption of small volcanic knolls considered to be formed by "petit-spot" intra-plate volcanism, and also morphology of a sedimentary basin and oceanic basement around the knolls in the area. Seismic profiles reveal the sedimentary layer is acoustically transparent, having some horizontal reflectors within the layer. Oceanic crust of the Pacific Plate lies flatly beneath the sedimentary layer with nearly constant thickness of ~200-300 m. The layer beneath the small knolls is acoustically opaque, possibly caused by inhomogeneous structure due to feeder dykes or cryptodome and volcanic deposits. Strong reflectors at base of the sedimentary layer, identified in the vicinity of small knolls, are probably reflections from volcanic sills. The oceanic crust beneath the small knolls is depressed by weight of the volcanic deposits.
Small, monogenetic volcanoes: building blocks of the upper oceanic crust
… , held 2-7 May, 2010 in …, 2010
The spreading axis at many slow-spreading mid-ocean ridges is marked by an 9 axial volcanic ridge. In this study we use a combination of high-resolution remote sensing 10 methods to elucidate the detailed nature of volcanoes in such a ridge. We find that the 11 "hummocks" described in previous sidescan sonar studies are dome-or cone-shaped 12 edifices, 5 -150 m high with diameters of 30 -330 m. We estimate they form quickly, in 13 single eruptions, each of which may produce several hummocks. Hummock collapse is 14 common and hummocks of all heights are prone to failure. Collapses generally occur 15 down the regional seafloor slope, suggesting control by local topography. Around a third 16 of hummocks lose ~40% of their volume by collapse, so ~12% of all material erupted on 17
Journal of Geophysical Research: Solid Earth, 1993
SeaMarcll side-scan sonar and bathymetric data on the west flank of the East Pacific Rise (16ø-18øS) reveal a large number of seamounts, organized primarily in chains. The easternmost ends of the chains near the ridge axis appear to be active as indicated by fresh lava flows. In addition, areas of unusually high reflectivity representing recent lava flows were found as far as 60 to 80 km from the spreading axis. The flows are sometimes dammed by abyssal hill scarps formed near the ridge axis that have maximum relief of the order of 100 m. In other cases, the scarps appear to be buried by flows. The disappearance of scarps can be used to estimate the extent of old lava flows or volcanic debris that are not detected acoustically in the sonar image. Under this criterion, lava flows and volcanic debris surrounding the seamounts cover 40-50% of the survey area of about 18,000 km 2. With an estimated average thickness of about 100 m, the volume of lava flows and volcanic debris is about 0.8+0.3% of that of the total crust. The volume of the seamounts is estimated by isolating the seamounts from the background topography with an anisotropic, median-filtering technique. The volume of seamount edifices is 1.05+0.05% of the volume of the crust. Thus the total volume of volcanic material extruded off-axis is about 1.5 to 2.2% of the volume of the crust. The decreasing volcanic activity away from the ridge axis suggests that near-axis seamounts may be associated with the upwelling system beneath the ridge, perhaps by preferential melting of embedded, passive heterogeneities. However, the decrease in volcanism with increasing distance from the axis also could be caused by a decrease in lithospheric vulnerability to penetration by magma. Since no apparent east-west faults and no simultaneous volcanic activity along the length of a seamount chain are observed, north-south extension and ridge-perpendicular convection are unlikely to be major causes of seamount formation in the study area. The existence of closely spaced linear chains and the tendency for fresh flows to be found at near-ridge ends of chains suggest that there are discrete sources in the upper mantle active for extended periods (over 1.8 m.y.). Passive heterogeneities embedded in the upwelling mantle would have to be highly elongated along streamlines to produce persistent, nearly stationary melting anomalies. A variety of mechanisms may be responsible for the off-axis volcanism, but the single hypothesis most consistent with all observations is a minihotspot origin. Although fresh lavas 80 km off-axis are consistent with broad mantle upwelling beneath the ridge, the possibility of narrow upwelling cannot be ruled out, because the seamounts could be caused by independent buoyant upwelling of mini-plumes that can penetrate thin lithosphere more easily near the axis. [Cormier and Macdonald, 1993] are included in Plate 1. Most seamounts are organized into chains. The number of seamounts greater than 5 km in diameter is at least twice that expected for average East Pacific seafloor [Smith and Jordan, 1988]. The robust volcanic activity in the study area may be
Earth Planets …, 1998
The Geological Survey of India conducted a magnetic survey and geological studies in 1993 around Barren Island in the Andaman Sea on board the RV Samudra Manthan. Five ocean bottom magnetometers were deployed by the Indian Institute of Geomagnetism as a part of this cruise. The Curie isotherm profiles prepared from the seasurface magnetic data indicate a high heat flow in the east of Barren Island. These profiles also indicate the presence of a north-south structural barrier which has prevented upflow of volcanic material to the west of Barren Island. Ocean-bottom magnetometer data were recorded simultaneously at five sites for about 15 days and these have been used to determine the electrical conductivity structure beneath Barren Island. Magnetic variations recorded at the seafloor stations indicate a concentration of electric currents near the island instead of the usual effect where currents are deflected away from the island. Transfer functions, showing the relationship between the horizontal components of the seafloor stations and land station, have been computed and the quantitative estimates of the transfer functions across Barren Island indicate a high conducting zone at a depth of about 17-27 km. This zone may have been produced by an upwelling of the mantle material through the magma chamber. The structure of this conductive zone at the north and south of Barren Island seems to concentrate the flow of the subsurface electrical currents within the island and the current flow takes a sharp southward turn beneath the island. This north-south conducting zone may have caused a rise in the depth of the Curie isotherm mapped by a shipborne magnetic survey of this region. Most probably, a partial melting of this conductive zone (magma chamber) may have given rise to the volcanic activity on Barren Island.
Shallow-Level Processes in Ocean-island Magmatism: Editorial
Journal of Petrology, 1998
surface. Some of the most detailed and comprehensive Magmatism at ocean islands typically has been associated recent studies of individual ocean-island magma systems with deep-rooted mantle plumes. Rarely, these are cosuggest, however, that many of the important criteria incident with oceanic spreading centres (e.g. Iceland), that we use to infer mantle sources may be compromised but more commonly they form intra-plate hotspots (e.g. by interaction between magmas and the oceanic and Hawaii, Canary Islands). If magmatism at ocean islands island crust through which they ascend. The occurrence is indeed intimately linked with mantle plumes and of contamination at ocean islands has been deemed less these plumes are derived from great depths (670 km likely than in continental settings, where magmas pass discontinuity or core-mantle boundary), effectively conthrough thicker, lower density crust. This contention is stituting the major mode of deep upwelling in the global apparently supported by the numerous case studies of mantle convection system, then their surface manicontamination of continental magmas. The difference in festation should provide some of our best constraints on the observable chemical consequences of contamination the composition and nature of the deep mantle. In in the two environments must be appreciated in the contrast, the dominant volume of present-day volcanic context of the compositional distinctions of the crust. activity, which is concentrated along plate boundaries, The compositional contrast between continental crust provides little definitive information on the deep mantle. and mantle-derived magma is commonly large, and Over the past 20 years considerable progress in our therefore the effects of contamination, in terms of standunderstanding of the composition of the upper mantle ard indicators such as isotope ratios, are readily aphas been inextricably linked to advances in analytical preciated. In contrast, basaltic magmas erupted on ocean geochemistry, particularly the use of isotope and inislands pass through dense basaltic crust, and any comcompatible trace element ratios in oceanic basalts as positional differences between magma and contaminant tracers of their mantle sources. Early studies have now may be subtle. It is important to point out, however, that been followed up by a number of very detailed volthe potentially subtle effects of contamination do not canological-petrological-geochemical studies of inimply that such effects are irrelevant in the grand picture dividual oceanic islands. Although Hawaii has of mantle geodynamics, which itself is dependent on traditionally served as our de facto 'model' of ocean-island compositional subtlety. magmatism by virtue of the sheer volume of geochemical Given the large volume of high-quality data, and the and geophysical data accumulated there, we now realize large number of studies of individual ocean islands that that a spectrum of volcano types and compositions exists, now exist, the time has come to ask what effects shallowand these have been used to map mantle domains, to infer level processes at ocean islands may have in modifying processes of partial melting, and to calculate recycling magma compositions. This question was addressed at budgets between crust and mantle. a Chapman Conference sponsored by the American The use of ocean-island basalt (OIB) geochemistry Geophysical Union from 9 to 16 November 1996 in to define mantle sources and partial melting processes Tenerife, Canary Islands. The conference was attended depends implicitly on the assumption that the primary by 45 scientists from seven nations, representing interests magmas are not significantly modified, other than by closed system fractional crystallization, en route to the varying from isotope geochemistry to geophysics. This
Physical volcanology of the submarine Mariana and Volcano Arcs
Bulletin of Volcanology, 1989
Narrow-beam maps, selected dredge samplings, and surveys of the Mariana and Volcano Arcs identify 42 submarine volcanos. Observed activity and sample characteristics indicate 22 of these to be active or dormant. Edifices in the Volcano Arc are larger than most of the Mariana Arc edifices, more irregularly shaped with numerous subsidiary cones, and regularly spaced at 50-70 km. Volcanos in the Mariana Arc tend to be simple cones. Sets of individual cones and volcanic ridges are elongate parallel to the trend of the arc or at 110 ° counterclockwise from that trend, suggesting a strong fault control on the distribution of arc magmas. Volcanos in the Mariana Arc are generally developed west of the frontal arc ridge, on rifted frontal arc crust or new backarc basin crust. Volcanos in the central Mariana Arc are usually subaerial, large ( > 500 km3), and spaced about 50-70 km apart. Those in the northern and southern Marianas are largely submarine, closer together, and generally less than 500 km 3 in volume. There is a shoaling of the arc basement around Iwo Jima, accompanied by the appearance of incompatible-element enriched lavas with alkalic affinities. The larger volcanic edifices must reflect either a higher magma supply rate or a greater age for the larger volcanos. If the magma supply (estimated at 10-20 kma/km of arc per million years at 18°N) has been relatively constant along the Mariana Arc, we can infer a possible evolutionary sequence for arc volcanos from small, irregularly spaced edifices to large (over 1000 k m 3) edifices spaced at 50-70 km. The volcano distribution and basal depths are consistent with the hypothesis of back-arc propagation into the Volcano Arc.