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101.
Raymond A. Duraiswami Ninad R. Bondre Shreyas Managave 《Journal of Volcanology and Geothermal Research》2008
Lava flows with preserved bases and brecciated upper crusts constitute a morphological type that differs in character from typical pahoehoe and a'a: such flows have been reported from many provinces around the world. Previous studies had referred to these flows informally as ‘pahoehoe flows with rubbly tops’, ‘broken-top pahoehoe’ and ‘rubbly pahoehoe’. Recent studies have formally applied the latter term to describe parts of the well-studied Laki flow in Iceland as well as flows from the Columbia River Basalt province. Rubbly pahoehoe flows are abundant in the upper stratigraphic formations of the Deccan Volcanic Province (DVP), and are more commonly known as simple flows. This study presents detailed observations of such flows from various parts of the DVP and discusses their implications for understanding flow emplacement. These flows, which appear to be single units at the outcrop-scale, are generally much thicker and significantly more extensive than individual pahoehoe lobes that dominate the lower formations of the Deccan stratigraphy. They are characterised by preserved, gently undulating tachylitic bases but variably disrupted crustal zones that grade into flow-top breccias. The breccias are constituted of highly vesicular and oxidised fragments of varying sizes that appear to have been derived from previously formed pahoehoe crusts. Previous work has indicated that the morphology of these flows might be related to initial inflation, accompanied by rapid volatile exsolution and an increase in effusion rate and/or viscosity with time. This agrees reasonably well with the qualitative and quantitative models of emplacement developed for the Laki flow. The abundance of such flows in the upper formations of the Deccan stratigraphy clearly hints at a significant shift in the nature of the Deccan eruptions; this could be indicative of higher eruption rates during this period. This, in turn, raises the possibility of hazardous impact on the climate during the eruption of these flows, which is also discussed in the paper. 相似文献
102.
Gas emissions from Erebus volcano, Antarctica, were measured by open-path Fourier transform infrared spectroscopy to understand degassing of its magmatic system. Two degassing phonolite lava lakes were present in the summit crater during observation in December 2004. We report analyses of H2O, CO2, CO, SO2, HF, HCl and OCS, (in order of molar abundance) in the plumes. Variations in the proportions of these species strongly reflect the dynamics of degassing, and sourcing of gas from different depths in the magmatic network. The highest observed ratios of CO2 and H2O are consistent with gas extracted from the melt at a depth of up to ∼ 2 km below the lava lakes. Magma degassing above this depth contributes to a higher H2O/CO2 proportion in the airborne plume. The ratio therefore reflects the balance of deeper vs. shallower contributions of volatiles and, possibly, a combination of closed- and open-system degassing. We observe a strong contrast in HF content in emissions from the two lava lakes, which we attribute to differing levels of magma ascent and/or cooling and crystallization of the magma supply. Fluxes of all gas species were determined using independent SO2 flux determinations and measured gas ratios. In the case of CO2 and water, ∼ 1 and ∼ 0.4 m3 s− 1, respectively, of parental basanite magma are required to sustain the calculated output. The discrepancy between the two figures is readily explained by sequestration of part of the magma supply at depth such that it only partially degasses its complement of water. 相似文献
103.
104.
Robert Andrle 《Mathematical Geology》1994,26(1):83-97
A new method for characterizing the complexity of geomorphic phenomena is presented. This method, termed the angle measure technique, involves measuring the angularity of a digitized line for a wide range of scales. In this manner, the technique is capable of delineating changes in the complexity of geomorphic lines with scale, from which the characteristic scale(s) of the lines can be identified. Unlike fractal analysis, values produced by the angle measure technique correspond to single scales. Therefore, no assumptions are made concerning the relationship between complexity and scale, and the technique can resolve variations in complexity over small ranges of scale. The technique is illustrated using both computer-generated curves and natural lines, including the trace of a river channel, and is compared to fractal analysis on a contour line crossing two lava flows. 相似文献
105.
Western Anatolia, largely affected by extensional tectonics, witnessed widespread volcanic activity since the Early Miocene. The volcanic vents of the region are represented by epicontinental calderas, stratovolcanoes and monogenetic vents which are associated with small-scale intrusions as sills and dykes. The volcanic activity began with an explosive character producing a large ignimbritic plateau all over the region, indicating the initiation of the crustal extension event. These rhyolitic magmas are nearly contemporaneous with granitic intrusions in western Anatolia. The ignimbrites, emplaced approximately contemporaneous with alluvial fan and braided river deposits, flowed over the basement rocks prior to extensional basin formation. The lacustrine deposits overlie the ignimbrites. The potassic and ultrapotassic lavas with lamprophyric affinities were emplaced during the Late Miocene–Pliocene. The volcanic activities have continued with alkali basalts during the Quaternary. 相似文献
106.
107.
108.
The 3-month long eruption of Asama volcano in 1783 produced andesitic pumice falls, pyroclastic flows, lava flows, and constructed a cone. It is divided into six episodes on the basis of waxing and waning inferred from records made during the eruption. Episodes 1 to 4 were intermittent Vulcanian or Plinian eruptions, which generated several pumice fall deposits. The frequency and intensity of the eruption increased dramatically in episode 5, which started on 2 August, and culminated in a final phase that began on the night of 4 August, lasting for 15 h. This climactic phase is further divided into two subphases. The first subphase is characterized by generation of a pumice fall, whereas the second one is characterized by abundant pyroclastic flows. Stratigraphic relationships suggest that rapid growth of a cone and the generation of lava flows occurred simultaneously with the generation of both pumice falls and pyroclastic flows. The volumes of the ejecta during the first and second subphases are 0.21 km3 (DRE) and 0.27 km3 (DRE), respectively. The proportions of the different eruptive products are lava: cone: pumice fall=84:11:5 in the first subphase and lava: cone: pyroclastic flow=42:2:56 in the second subphase. The lava flows in this eruption consist of three flow units (L1, L2, and L3) and they characteristically possess abundant broken phenocrysts, and show extensive "welding" texture. These features, as well as ghost pyroclastic textures on the surface, indicate that the lava was a fountain-fed clastogenic lava. A high discharge rate for the lava flow (up to 106 kg/s) may also suggest that the lava was initially explosively ejected from the conduit. The petrology of the juvenile materials indicates binary mixing of an andesitic magma and a crystal-rich dacitic magma. The mixing ratio changed with time; the dacitic component is dominant in the pyroclasts of the first subphase of the climactic phase, while the proportion of the andesitic component increases in the pyroclasts of the second subphase. The compositions of the lava flows vary from one flow unit to another; L1 and L3 have almost identical compositions to those of pyroclasts of the first and second subphases, respectively, while L2 has an intermediate composition, suggesting that the pyroclasts of the first and second subphases were the source of the lava flows, and were partly homogenized during flow. The complex features of this eruption can be explained by rapid deposition of coarse pyroclasts near the vent and the subsequent flowage of clastogenic lavas which were accompanied by a high eruption plume generating pumice falls and/or pyroclastic flows.Editorial responsibility: T. Druitt 相似文献
109.
This study aims at quantifying the effect of rheology on plan-view shapes of lava flows using fractal geometry. Plan-view shapes of lava flows are important because they reflect the processes governing flow emplacement and may provide insight into lava-flow rheology and dynamics. In our earlier investigation (Bruno et al. 1992), we reported that flow margins of basalts are fractal, having a characteristic shape regardless of scale. We also found we could use fractal dimension (D, a parameter which quantifies flow-margin convolution) to distinguish between the two endmember types of basalts: a a (D: 1.05–1.09) and pahoehoe (D: 1.13–1.23). In this work, we confirm those earlier results for basalts based on a larger database and over a wider range of scale (0.125 m–2.4 km). Additionally, we analyze ten silicic flows (SiO2: 52–74%) over a similar scale range (10 m–4.5 km). We note that silicic flows tend to exhibit scale-dependent, or non-fractal, behavior. We attribute this breakdown of fractal behavior at increased silica contents to the suppression of small-scale features in the flow margin, due to the higher viscosities and yield strengths of silicic flows. These results suggest we can use the fractal properties of flow margins as a remote-sensing tool to distinguish flow types. Our evaluation of the nonlinear aspects of flow dynamics indicates a tendency toward fractal behavior for basaltic lavas whose flow is controlled by internal fluid dynamic processes. For silicic flows, or basaltic flows whose flow is controlled by steep slopes, our evaluation indicates non-fractal behavior, consistent with our observations. 相似文献
110.
Masamitsu Fujimoto Nobuhito Ohte Masatoshi Kawasaki Ken'ichi Osaka Masayuki Itoh Izumi Ohtsuka Masami Itoh 《水文研究》2016,30(4):558-572
Groundwater movements in volcanic mountains and their effects on streamflow discharge and representative elementary area (REA) have remained largely unclear. We surveyed the discharge and chemical composition of spring and stream water in two catchments: the Hontani river (NR) catchment (6.6 km2) and the Hosotani river (SR) catchment (4.0 km2) at the southern part of Daisen volcano, Japan. Daisen volcano is a young volcano (17 × 103 years) at an early stage of erosion. Our study indicated that deep groundwater that moved through thick lava and pyroclastic flows and that could not be explained by shallow movements controlled by surface topography contributed dominantly to streamflow at larger catchment areas. At the NR catchment, the deep groundwater contribution clearly increased at a catchment boundary defined by an area of 3.0 km2 and an elevation of 800 m. At the SR catchment, the contribution deep groundwater to the stream also increased suddenly at a boundary threshold of 2.0 and 700 m. Beyond these thresholds, the contributions of deep bedrock groundwater remained constant, indicating that the REA is between 2 and 3 km2 at the observed area. These results indicate that the hydrological conditions of base flow were controlled mainly by the deep bedrock groundwater that moved through thick lava and pyroclastic flows in the undissected volcanic body of the upper part of the catchment. Our study demonstrates that deep and long groundwater movements via a deep bedrock layer including thick deposits of volcanic materials at the two catchments on Daisen volcano strongly determined streamflow discharge instead of the mixing of small‐scale hydrological conditions. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献