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Volcanic Zone (volcanic + zone)
Selected AbstractsLand Cover Characteristics in Ne Iceland with Special Reference to Jökulhlaup GeomorphologyGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 3-4 2003Petteri Alho ABSTRACT Subglacial eruptions in Vatnajbkull have accounted for several jökulhlaups (glacial outburst floods) in the Northern Volcanic Zone (NVZ). These events and aeolian processes have had a considerable impact on the landscape evolution of the area. Most of this area is occupied by barren land cover; the northern margin of the barren land cover is advancing northwards, burying vegetation under wind-blown sediment. This paper presents a land-cover classification based on a supervised Landsat TM image classification with pre-processing and extensive field observations. Four land cover categories were identified: (a) lava cover (34.8%); (b) barren sediment cover (39.0%); (c) vegetation (25.1%); and (d) water and snow (1.1%). The mapping of sand transport routes demonstrates that a major aeolian sand transportation pathway is situated in the western part of the study area. The sedimentary formation elongated towards the northeast is evidence of active and continuous aeolian sand transportation towards the north. Interpretation of the satellite image suggests that four main areas are affected by jökulhlaups along the Jökulsáá Fjöllum: Ásbyrgi, Grímsstaðir, Herðubreið,Möðrudalur, and the Dyngjujökull sandur. In addition, jökulhlaup-related sediment cover (8%) in the study area, together with erosional features, are evidence of a severe and extensive jökulhlaup-induced process of land degradation. [source] Seamount volcanism along the Gakkel Ridge, Arctic OceanGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2008James R. Cochran SUMMARY The Gakkel Ridge in the Arctic Ocean is the slowest spreading portion of the global mid-ocean 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. [source] Lithospheric structure of an active backarc basin: the Taupo Volcanic Zone, New ZealandGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2006Antony Harrison SUMMARY Seismic data from both explosive and earthquake sources have been used to model the crustal and upper-mantle velocity structure beneath the Taupo Volcanic Zone (TVZ), an active backarc basin in central North Island, New Zealand. Volcanic sediments with P -wave velocities of 2.0,3.5 km s,1 reach a maximum thickness of 3 km beneath the central TVZ. Underlying these sediments to 16 km depth is material with velocities of 5.0,6.5 km s,1, interpreted as quartzo-feldspathic crust. East and west of the TVZ, crust with similar velocities is found to depths of 30 and 25 km, respectively. Beneath the TVZ, material with P -wave velocities of 6.9,7.3 km s,1 is found from 16 to 30 km depth and is interpreted as heavily intruded or underplated lower crust. The base of the crust at 30 km depth under the TVZ is marked by a strong seismic reflector, interpreted as the Moho. Modelling of arrivals from deep (>40 km) earthquakes near the top of the underlying subducting Pacific Plate reveals a region with low mantle velocities of 7.4,7.8 km s,1 beneath the crust of the TVZ. This region of low mantle velocities is best explained by the presence of partially hydrated upper mantle, resulting from dehydration of hydrous minerals (e.g. serpentinite) carried down by the underlying subducting plate. Within the lower crust beneath the TVZ, a region of high (0.34) Poisson's ratio is observed, indicating the presence of at least 1 per cent partial melt. This melt probably fractionates and assimilates crustal material before some of it migrates into the upper crust, where it provides a source for the voluminous rhyolitic magmas of the TVZ. [source] Crustal structure of central and northern Iceland from analysis of teleseismic receiver functionsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2000Fiona A. Darbyshire We present results from a teleseismic receiver function study of central and northern Iceland, carried out during the period 1995,1998. Data from eight broad-band seismometers installed in the SIL network operated by the Icelandic Meteorological Office were used for analysis. Receiver functions for each station were generated from events for a wide range of backazimuths and a combination of inversion and forward modelling was used to infer the crustal structure below each station. The models generated show a considerable variation in the nature and thickness of the crust across Iceland. The thinnest crust (20,21 km) is found in the northern half of the Northern Volcanic Zone approximately 120 km north of the centre of the Iceland mantle plume. Thicker crust (24,30 km) is found elsewhere in northern and central Iceland and the thickest crust (37 km) is found close to the plume centre. Velocity,depth profiles show a distinct division of the crust into two main sections, an upper high-velocity-gradient section of thickness 2,8 km and a lower crustal section with small or zero overall velocity gradient. The thickness of the upper crust correlates with the tectonic structure of Iceland; the upper crust is thickest on the flanks of the northern and central volcanic rift zones and thinnest close to active or extinct central volcanoes. Below the Krafla central volcano in northeastern Iceland the receiver function models show a prominent low-velocity zone at 10,15 km depth with minimum shear wave velocities of 2.0,2.5 km s,1. We suggest that this feature results from the presence of partially molten sills in the lower crust. Less prominent low-velocity zones found in other regions of Iceland may arise from locally high temperatures in the crust or from acidic intrusive bodies at depth. A combination of the receiver function results and seismic refraction results constrains the crustal thickness across a large part of Iceland. Melting by passive decompression of the hot mantle below the rift zone in northern Iceland forms a crust of thickness ,20 km. In contrast, the larger crustal thickness below central Iceland probably arises from enhanced melt production due to active upwelling in the plume core. [source] Quaternary tephra marker beds and their potential for palaeoenvironmental reconstruction on Chatham Island, east of New Zealand, southwest Pacific Ocean,JOURNAL OF QUATERNARY SCIENCE, Issue 7 2010Katherine A. Holt Abstract Tephras provide one of the most reliable methods of time control and synchronisation within Quaternary sequences. We report on the identification of two widespread rhyolitic tephras , the Kawakawa and Rangitawa tephras , preserved in extensive peat deposits on Chatham Island ,900,km east of New Zealand. The tephras, both products of supereruptions from the Taupo Volcanic Zone, occur as pale, fine-ash dominated layers typically 10,150,mm thick. Mineralogically they are dominated by rhyolitic glass, together with subordinate amounts of quartz, feldspar, hypersthene, hornblende, Fe,Ti oxides and zircon. Phlogopite/biotite was identified additionally in Rangitawa Tephra. Ages for each tephra were obtained via mineralogical and major element glass composition-based correlation with well-dated equivalent deposits on mainland New Zealand, and we also obtained a new zircon fission-track age for Rangitawa Tephra (350,±,50,ka) on Chatham Island. Both tephras were erupted at critical times for palaeoenvironmental reconstructions in the New Zealand region: the Kawakawa at ca. 27 cal. ka, near the beginning of the ,extended' LGM early in marine isotope stage (MIS) 2; and the Rangitawa at ca. 350 ka near the end of MIS 10. The time constraints provided by the tephras demonstrate that Chatham Island peats contain long-distance pollen derived from mainland New Zealand, which provides a reliable proxy for identifying glacial,interglacial climate conditions, in this case during the MIS 11,10 and MIS 2,1 cycles. The two tephras thus provide important chronostratigraphic tie-points that facilitate correlation and synchronisation not only across the Quaternary deposits of the Chatham Islands group but also with climatically significant terrestrial and marine records in the wider New Zealand region. Copyright © 2010 John Wiley & Sons, Ltd. [source] | ||||||||||