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71.
In recent months the media has been drawing attention to the possibility of a dangerous eruption of the Teide Volcano on Tenerife in the Canary Islands. Before accepting this prediction, which may well be detrimental to the tourist-based economy of the island, it would be wise to examine the evidence on which it is based. 相似文献
72.
Edifices of stratocones and domes are often situated eccentrically above shallow silicic magma reservoirs. Evacuation of such reservoirs forms collapse calderas commonly surrounded by remnants of one or several volcanic cones that appear variously affected and destabilized. We studied morphologies of six calderas in Kamchatka, Russia, with diameters of 4 to 12 km. Edifices affected by caldera subsidence have residual heights of 250–800 m, and typical amphitheater-like depressions opening toward the calderas. The amphitheaters closely resemble horseshoe-shaped craters formed by large-scale flank failures of volcanoes with development of debris avalanches. Where caldera boundaries intersect such cones, the caldera margins have notable outward embayments. We therefore hypothesize that in the process of caldera formation, these eccentrically situated edifices were partly displaced and destabilized, causing large-scale landslides. The landslide masses are then transformed into debris avalanches and emplaced inside the developing caldera basins. To test this hypothesis, we carried out sand-box analogue experiments, in which caldera formation (modeled by evacuation of a rubber balloon) was simulated. The deformation of volcanic cones was studied by placing sand-cones in the vicinity of the expected caldera rim. At the initial stage of the modeled subsidence, the propagating ring fault of the caldera bifurcates within the affected cone into two faults, the outermost of which is notably curved outward off the caldera center. The two faults dissect the cone into three parts: (1) a stable outer part, (2) a highly unstable and subsiding intracaldera part, and (3) a subsiding graben structure between parts (1) and (2). Further progression of the caldera subsidence is likely to cause failure of parts (2) and (3) with failed material sliding into the caldera basin and with formation of an amphitheater-like depression oriented toward the developing caldera. The mass of material which is liable to slide into the caldera basin, and the shape of the resulted amphitheater are a function of the relative position of the caldera ring fault and the base of the cone. A cone situated mostly outside the ring fault is affected to a minor degree by caldera subsidence and collapses with formation of a narrow amphitheater deeply incised into the cone, having a small opening angle. Accordingly, the caldera exhibits a prominent outward embayment. By contrast, collapse of a cone initially situated mostly inside the caldera results in a broad amphitheater with a large opening angle, i.e. the embayment of the caldera rim is negligible. The relationships between the relative position of an edifice above the caldera fault and the opening angle of the formed amphitheater are similar for the modeled and the natural cases of caldera/cone interactions. Thus, our experiments support the hypothesis that volcanic edifices affected by caldera subsidence can experience large-scale failures with formation of indicative amphitheaters oriented toward the caldera basins. More generally, the scalloped appearance of boundaries of calderas in contact with pre-caldera topographic highs can be explained by the gravitational influence of topography on the process of caldera formation.Editorial responsibility: J. Stix 相似文献
73.
An analysis of the factors influencing infiltration and runoff was carried out using data obtained under simulated rainfall conditions on 87 plots in arid and semi-arid areas of West Africa, arranged in a hierarchical sequence. One of the key factors to emerge from this typological classification is the type of surface crust. Together with faunal activity, vegetation cover and surface roughness, this parameter permits the inclusion of a ‘unit surface’ factor in the classification system. Each unit surface is characterized in terms of genetic, morphological and hydrological properties.
At a higher level, the combination of these unit surfaces allows a ‘soil surface features unit’ to be defined, based on an original mapping method. The samples, once defined in this system, were found to correspond to homogeneous criteria both in terms of evolution dynamics and hydrological behaviour. Furthermore, such cartography could be successfully extrapolated through remote sensing image analysis, making possible a classification of the runoff capability of small watersheds based on objective and standardized criteria. 相似文献
74.
Facies,geometry and growth phases of the Valdorria carbonate platform (Pennsylvanian,northern Spain) 总被引:1,自引:0,他引:1 下载免费PDF全文
Valentin Chesnel Elias Samankassou Óscar Merino‐Tomé Luis Pedro Fernández Elisa Villa 《Sedimentology》2016,63(1):60-104
The 14 km wide Valdorria outcrop (Pennsylvanian, northern Spain) is one of the few examples of entirely exposed flat‐topped and high‐relief carbonate platforms reported in the fossil rock record. Laterally and vertically traceable stratal patterns expose three phases of growth. Phase I is a 430 m thick platform to slope succession that prograded over 6 km, and is dated as Early Bashkirian (Akavasian–Askynbashian). Phase II aggraded and prograded, exhibiting 180 m thickness of cyclical platform top deposits, dated as Late Bashkirian (Asatauian). Phase III is a mound‐shape structure that developed over the platform top of Phase II as a new phase of platform nucleation. It is 535 m thick and 2 km wide, and dated as Late Bashkirian (Asatauian–Transition interval). The observed changes of growth styles during platform evolution, from a prograding to an aggrading–prograding system, and a rapid aggradational phase, are inferred to be controlled by flexural subsidence in the active Cantabrian foreland basin, at the Variscan orogenic front. The metre‐scale shallowing‐upward cycles of the platform top are most probably due to glacioeustasy, as evidenced by well‐recorded subaerial exposure surfaces superimposed on subtidal deposits, and by a stratal pattern recurrent in a short interval of about 160 kyr. Observations of outcropping Bashkirian cyclothems in an isolated carbonate system, devoid of siliciclastic input, are relevant for a better understanding of the impact of high‐frequency sea‐level fluctuations on the carbonate factory. Moreover, progradation of the platform margin during Phase I reaches a rate of 2500 m/Myr, and 1810 m/Myr during Phase II; rates that are high when compared to other Pennsylvanian examples. The aggradation rate of 447 m/Myr calculated for the Late Bashkirian–Transition interval (Phases II and III; uncorrected for compaction, missing beats and erosion) is uncommonly high in comparison to coeval Pennsylvanian examples. The platform exhibits a self‐nourishing prograding microbial boundstone‐dominated slope. Thus, the slope‐shedding model applies well to Valdorria. However, Phase II recorded eustatic variations able to inhibit the slope microbial boundstone factory during low sea‐level stands; this is marked by common slope red‐stained breccias synchronous to platform top subaerial exposure phases. Contrarily, periods of relative high sea‐level and rapid subsidence in Phase III registered a greater development of cemented microbial boundstone. These observed, partly opposing relationships of sea‐level stands, shedding modes and slope architecture provide an improvement of the currently used slope‐shedding model. The overall architecture of the Valdorria outcrop compares well with that of other contemporaneous platforms, such as Sierra del Cuera and Bolshoi Karatau. Valdorria shares the high‐relief and flat‐topped, steep slopes, cyclothemic patterns and occurrence of karst features with the Pricaspian Basin platforms (Tengiz, Karachaganak and Kashagan), with minor variations in facies distribution of the internal platform. Furthermore, the continuous seismic‐scale outcrop of Valdorria, together with its isolated setting and asymmetrical growth, makes it a very good candidate for potential subsurface analogues of hydrocarbon‐bearing systems. 相似文献
75.
Venera Ionescu Maria Năstăsescu Lucreţia Spiridon Valentin A. C. Bulgăreanu 《International Journal of Salt Lake Research》1998,7(1):45-80
An inventory of the aquatic and circum-lacustrine flora and aquatic fauna of 23 karst- and anthroposaline lakes formed on Miocene rock salt bodies in Romania was undertaken. Simultaneous field determinations of water and sediment pH, total dissolved solids, Secchi transparency and lake maximum depth were also made. Environmental conditions for phytoplankton, microphytobenthos, macrophyte and cormophyte species were defined. Relations between the numbers of plankton, microbenthos, macrophyte and cormophyte species differentiated pelogenous (usually, therapeutic) and non-pelogenous lakes. High densities and occurrence frequencies characterized the most representative species. For three of these species, Amphora veneta, Artemia sp. and Potamogeton pectinatus, ecological conditions were noted. In the circum-lacustrine vegetation, glycophytes were more important than halophytes. 相似文献
76.
Ingrid Anell Valentin Zuchuat Anna Daniela Rhnert Aleksandra Smyrak‐Sikora Simon Buckley Gareth Lord Harmon Maher Ivar Midtkandal Kei Ogata Snorre Olaussen Per T. Osmundsen Alvar Braathen 《Basin Research》2021,33(1):478-512
The study describes the depositional development and sediment partitioning in a prograding paralic Triassic succession. The deposits are associated with the advance of large prism‐scale clinoforms across a shallower platform area. Approaching the platform, the limited accommodation and associated relative higher rates of deposition generated straighter clinoforms with lower foreset angles. The vertical restriction across the platform is interpreted to have amplified the tidal signature. Sediment was redistributed from the coast into increasingly sandy delta‐front deposits, compared to offshore equivalents. The deposits comprise extensive compound dune fields of amalgamated and increasingly clean sandbodies up‐section. Rapid deposition of significant amounts of sand led to differential subsidence and growth‐faulting in the delta front, with downthrown fault blocks further amplifying the tidal energy through funnelling. A mixed‐energy environment created along‐strike variability along the delta front with sedimentation governing process‐regime. Areas of lower sedimentation were reworked by wave and storm‐action, whereas high sedimentation rates preserved fluvially dominated mouth bars. A major transgression, however, favoured tidally dominated deposits also in these areas, attributed to increasing rugosity of the coastline. Formation of an extensive subaqueous platform between the coast and delta front dampened incoming wave energy, and tidally dominated deposits dominate the near‐shore successions. Meanwhile formation of wave‐built sand‐bars atop the platform attest to continued wave influence. The strong tidal regime led to the development of a heterolithic near‐shore tidally dominated channel system, and sandier fluvial channels up‐river. The highly meandering tidal channels incising the subaqueous platform form kilometre wide successions of inclined heterolithic stratification. The fluvially dominated channels which govern deposition on the delta plain are narrower and slightly less deep, straighter, generally symmetric and filled with cleaner sands. This study provides important insight into tidal amplification and sand redistribution during shallowing on a wide shelf, along with along‐strike process‐regime variability resulting from variations in sediment influx. 相似文献
77.
Experiments on rift zone evolution in unstable volcanic edifices 总被引:1,自引:0,他引:1
Thomas R. Walter Valentin R. Troll 《Journal of Volcanology and Geothermal Research》2003,127(1-2):107-120
Large ocean island volcanoes frequently develop productive rift zones located close to unstable flanks and sites of older major sector collapses. Flank deformation is often caused by slip along a décollement within or underneath the volcanic edifice. We studied how such a stressed volcanic flank may bias the rift zone development. The influence of basal lubrication and lateral flank creep on rift development and rift migration is still poorly constrained by field evidence; here our analog experiments provide new insights. We injected colored water into gelatin cones and found systematic orientations of hydro-fractures (dikes) propagating through the cones. At the base of the cone, diverse friction conditions were simulated. By variation of the basal creep conditions we modeled radial dike swarms, collinear rift zones and three-armed rift systems. It is illustrated that a single outward-creeping flank is sufficient to modify the entire rift architecture of a volcano. The experiments highlight the general unsteadiness of dike swarms and that the distribution and alteration of weak substratum may become a major player in shaping a volcano’s architecture. 相似文献
78.
79.
Steffen Popp Innokenty Belolyubsky Frank Lehmkuhl Andrei Prokopiev Christine Siegert Valentin Spektor Georg Stauch Bernhard Diekmann 《Geological Journal》2007,42(5):477-497
A provenance analysis of late Quaternary deposits from tributaries of the Aldan and Lena rivers in Central Yakutia (eastern Siberia) was carried out using analysis of heavy minerals and clay mineralogy. Cluster analysis revealed one assemblage that is characterized by relatively high proportions of amphibole, orthopyroxene and garnet as well as pedogenic clay minerals, reflecting a sediment provenance from the wide catchment area of the Lena and Aldan rivers. In contrast, the three other clusters are dominated by stable heavy minerals with varying amounts of clinopyroxene, apatite and garnet, as well as high percentages of illite and chlorite that are indicative of source rocks of the Verkhoyansk Mountains. Glacial moraines reveal the local mountain source signal that is overprinted by the Lena‐Aldan signal in the oldest moraines by reworking processes. Alluvial sediments in the Verkhoyansk Foreland show a clear Lena source signal through intervals of the middle and late Pleistocene, related to a stream course closer to the mountains at that time. Loess‐like cover sediments are characterized by the dominant Lena provenance with increasing proportions of local mountain sources towards the mountain valleys. Aeolian sands in an alluvial terrace section at the mountain margin covering the time between 30 ka and 10 ka BP reflect temporarily dominant inputs of aeolian materials from the Lena Plains. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
80.
Hydrogeological characterization of groundwater storage and drainage in an alpine karst aquifer (the Kanin massif,Julian Alps) 总被引:1,自引:0,他引:1 下载免费PDF全文
Janez Turk Arnauld Malard Pierre‐Yves Jeannin Metka Petrič Franci Gabrovšek Nataša Ravbar Jonathan Vouillamoz Tadej Slabe Valentin Sordet 《水文研究》2015,29(8):1986-1998
The Kanin massif is an important trans‐boundary aquifer, which stretches between Slovenia and Italy. The groundwater is only partially exploited, mainly for water supply, but the aquifer exhibits great potential for future exploitation. Since no consistent regional overview of the hydrogeological functioning of the Kanin massif was available, the decision was made to perform a study of this area, using a pragmatic approach based on 3D geological and hydrogeological modelling. The so‐called KARSYS approach was applied, with the aim of characterizing the groundwater reserves within this karst massif and of locating the main drainage axes that carry groundwater from the recharge areas to the respective springs. Delineation of the catchment areas of the corresponding springs was carried out, and some new explanations were obtained, especially with regard to the Mo?nica spring, which is located in Slovenia and forms a potential source of drinking water. It was found that this spring's catchment area extends as far as the Italian ski resort of Sella Nevea. The conceptual model also provides a possible explanation about the underground drainage towards the Boka spring and waterfall, which has been a challenge for decades. This new explanation is based on the existence of a perched groundwater body that feeds the Boka spring via a system of conduits. Despite some limitations, the results, which consist of a visualization of the underground drainage and groundwater storage within the Kanin massif, can be used as a basis for planning the sustainable management of karst waters in the studied area. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献