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121.
Peter R. Vogt 《Earth and Planetary Science Letters》1978,37(3):465-475
Anomalously high mid-plate and Mid-Oceanic Ridge seismicity apparently characterizes the flanks of the regional free-air gravity and topographic high roughly centered on the postulated Iceland hot spot; anomalously high mid-plate seismicity also occurs around the deep free-air gravity low centered just south of India. Although the correlation between gravity and seismicity is still somewhat inconclusive, it seems to suggest active asthenosphere flow associated with the regional gravity anomalies. Added to other stresses in the plate, the stress transmitted to the plate by this flow would encourage its fracture. 相似文献
122.
Manganese oxide crusts similar to those reported from the Mid-Atlantic Ridge rift valley by Scott et al. (1974) were dredged at two sites near the Galapagos spreading axis on ocean floor estimated from magnetic anomalies to be 2.4 and 0.3 m.y. old. Compared to the typical ocean-floor manganese deposits attributed to precipitation from seawater, the 2–6 cm thick manganese crusts reported here exhibit very low Fe/Mn and low232Th/238U ratios, as well as lower transition metal and higher manganese concentrations. The manganese crusts were deposited several orders of magnitude faster than the more common hydrogenous nodules; this fact together with other geochemical characteristics and the geophysical environment suggests the manganese deposits reported here are of hydrothermal origin. 相似文献
123.
Detailed bathymetric and magnetic data, complemented by nine dredge stations, define the eastern and western limits of a belt of high-amplitude magnetic anomalies associated with the Galapagos hot spot. The hypothesis of “magnetic telechemistry” was tested and locally confirmed. High amplitudes correspond to high remanence, susceptibility, FeOT, TiO2, and presumably titanomagnetite concentration. The average remanence of surface samples in the high-amplitude zone is 0.027 emu/cm3 (range, 0.009–0.085 emu/cm3), about 4 times that of the normal-amplitude zone. Magnetic amplitudes are only 2–2.5 times higher, however. If the greater TiO2/FeOT ratio of high-amplitude zone basalts also characterizes the titanomagnetites, remanence in the high-amplitude zone may fall off more rapidly with depth in the crust as a result of reheating. Alternatively, small pillows of high remanence are more common than larger pillows at the top of the high-amplitude zone crust; FeTi basalt may also be concentrated in the upper part of the crust. Anomaly amplitudes are highest at the ends of the zone, particularly in the east. As asthenosphere crystal slushes presumably flow away from the Galapagos plume, progressive crystal fractionation may enrich residual magmas in FeOT and TiO2. The Galapagos FeTi zone terminates abruptly against transform fractures at both ends, perhaps because subaxial flow is dammed at the transforms. The FeTi-producing crystal slushes have advanced east and west at speeds up to 10 cm/yr since they first appeared at the spreading axis at least 6.6 m.y. B.P. Their progressive advance was connected with the progressive southward jumps of the spreading axis east of the Galapagos hot spot, and northward jumps to the west. 相似文献
124.
An SO2 flux of 1170±400 (1) tonnes per day was measured with a correlation spectrometer (COSPEC) in October and November 1986 from the continuous, nonfountaining, basaltic East Rift Zone eruption (episode 48A) of Kilauea volcano. This flux is 5–27 times less than those of highfountaining episodes, 3–5 times greater than those of contemporaneous summit emissions or interphase Pu'u O'o emissions, and 1.3–2 times the emissions from Pu'u O'o alone during 48A. Calculations based on the SO2 emission rate resulted in a magma supply rate of 0.44 million m3 per day and a 0.042 wt% sulfur loss from the magma upon eruption. Both of these calculated parameters agree with determinations made previously by other methods. 相似文献
125.
126.
R. Vogt A. Christen M. W. Rotach M. Roth A. N. V. Satyanarayana 《Theoretical and Applied Climatology》2006,84(1-3):117-126
Summary In Summer 2002 eddy covariance flux measurements of CO2 were performed over a dense urban surface. The month-long measurements were carried out in the framework of the Basel Urban
Boundary Layer Experiment (BUBBLE). Two Li7500 open path analysers were installed at z/zH = 1.0 and 2.2 above a street canyon with zH the average building height of 14.6 m and z the height above street level. Additionally, profiles of CO2 concentration were sampled at 10 heights from street level up to 2 zH. The minimum and maximum of the average diurnal course of CO2 concentration at 2 zH were 362 and 423 ppmv in late afternoon and early morning, respectively. Daytime CO2 concentrations were not correlated to local sources, e.g. the minimum occurred together with the maximum in traffic load.
During night-time CO2 is in general accumulated, except when inversion development is suppressed by frontal passages. CO2 concentrations were always decreasing with height and correspondingly, the fluxes – on average – always directed upward.
At z/zH = 2.2 low values of about 3 μmol m−2 s−1 were measured during the second half of the night. During daytime average values reached up to 14 μmol m−2 s−1. The CO2 fluxes are well correlated with the traffic load, with their maxima occurring together in late afternoon. Daytime minimum
CO2 concentrations fell below regional background values. Besides vertical mixing and entrainment, it is suggested that this
is also due to advection of rural air with reduced CO2 concentration. Comparison with other urban observations shows a large range of differences among urban sites in terms of
both CO2 fluxes and concentrations.
Present affiliation: Swiss Federal Office for Meteorology and Climatology, MeteoSwiss, Zürich, Switzerland 相似文献
127.
128.
A longitudinal seismic reflection profile of the Reykjanes Ridge, together with earthquake seismicity patterns, is interpreted in terms of the mantle plume hypothesis. Between 52°N and 57°N Reykjanes Ridge is cut by about 12 fractures whose trend, inferred from other data, is approximately east-west. North of 57° there is little or no indication of east-west fracturing.The 57°N transition from fractured to unfractured basement occurs about 900 km southwest of the postulated Iceland mantle plume. The fractured province exhibits higher seismicity and rougher basement, on transverse profiles, than does the unfractured province. A similar transition to rougher, more seismic ridge crest also occurs 900 km northeast of Iceland. We propose that flowage of hot, basalt-rich asthenosphere away from the Iceland hot spot keeps the axial lithosphere hot, thin, sparsely fractured, and relatively aseismic out to 900 km from the plume. Similar effects are evident in the vicinity of some other plumes located near spreading axes. Some plumes also exhibit a greater number of earthquakes at some distance from the spreading axis — possibly a reflection of non-axial igneous activity or fracturing due to local, plume-generated stresses.The regional basement slope along the longitudinal profile is about 8 × 10?4. If this slope represents a balance between viscous and gravity forces in the flow, a viscosity of the order 1019 poises can be estimated from the Poiseuille equation.A peculiarly flat, opaque reflector was discovered near the Reykjanes axis, about 300 km southwest of Iceland. Several hypotheses are advanced to account for such reflectors by the exceptional volcanic activity associated with high plume discharge. 相似文献
129.
Gas hydrates that outcrop on the sea floor: stability models 总被引:4,自引:3,他引:1
A. V. Egorov K. Crane P. R. Vogt A. N. Rozhkov P. P. Shirshov 《Geo-Marine Letters》1999,19(1-2):68-75
The detection of sea-floor gas hydrates (GHs) proves that they can coexist in contact with seawater. We calculate that sea-floor
hydrates undergo rapid dissolution, yet they are maintained by the constant high rate of upward migration of gas-saturated
waters. If upward migration originates from a point source, then the thickness of sea-floor hydrates varies with distance
from the center of the upwelling (depending on heat flow). However, near-surface fracturing may control the actual points
of exit onto the sea floor above. If gas migration rates decrease from the center of the mud volcano to its periphery, a concentric
pattern in distribution of temperature, methane concentration, and GH contents can be described. 相似文献
130.