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11.
The asymmetrical distribution of the welded Ata large-scale pyroclastic flow deposit in Southern Kyushu, Japan was identified. This distribution pattern was defined as depositional ramps. Depositional ramps can be identified in valleys wider than 1 km and become smaller-scale with increasing distance from the source. Upslope directions of depositional ramps are generally radially away from the source caldera, suggesting that the structure was formed by the flow of pyroclastic material radially away from the source. The original depositional surface was reconstructed based on field mapping and density measurements of the pyroclastic flow deposit. Depositional ramps having a dip angle of more than 9° were reconstructed on the vent-facing slopes of the topography underlying the valley-filling deposits in the area within 10 km of the caldera rim. Such a dip angle is much larger than previously described dip angles. The size and gradient of the depositional ramps decreases with increasing distance from the source. Depositional ramps are recognized commonly in densely welded pyroclastic flow deposits. A high emplacement temperature is required to form the depositional ramps. This suggests that the pyroclastic flow was transported as a dense, fluidized layer to minimize heat loss. 相似文献
12.
Schmincke andSwanson (1967) explained laminar flowage structures as indicators for flow direction of pyroclastic flows that show a radial flow pattern away from the source. Several other authors have reported similar examples, but the influence of pre-flow topographic relief has not been analyzed. Flow lineations were measured for the Ata pyroclastic flow deposit, southwestern Japan. This deposit has covered an undulating basement topography. Preferred orientation of crystals and lithic fragments were measured on thin sections cut parallel to sedimentary layering. The following three factors which control the flow lineation have been recognized. 1) Flow lineations oriented radially away from the source, as described by previous authors, were obtained only for samples collected from the surface of the pyroclastic flow plateau where the basement valleys were nearly filled by earlier flow units. 2) Lineations near the floor of narrow valleys were parallel to the strike of the valley. 3) Flow lineations near the wall of valleys tend to be parallel to the dip of the valley walls. These data suggest that the initial radial movement of pyroclastic flows from the source gradually changes direction to parallel the strike of deep valleys due to confining effect of valley wall. Flows which are trapped within a valley, tends to move towards the bottom of the valley just prior to the final settlement. After the basement topographic relief has been filled up with earlier flow units, the later flows maintain their original radial movement until final settlement. 相似文献
13.
Of ca. 200 Quaternary volcanic centers in the Japanese islands, ca. 120 are characterized by K2O and Na2O concentrations of their ejecta normalized to SiO2 = 60% K2O values give a geographically well-defined trend of regularly increasing away from the volcanic front. Na2O values show no geographical trend. Relationships between the SiO2-normalized K2O and the depth of the deep seismic zone (K-h relation) are different for different arcs. The Southwest Japan arc shows distinctly higher K2O and less regular geographic trend than the others probably due to crustal contamination. In the normal segments of the Northeastern Japan belt, K2O increases ca. 2.5 to 3.0% per 100 km depth of the seismic zone. Groups of volcanic centers near the junction between the Northeast Honshu and Izu-Mariana arcs give characteristically lower K2O while Rishiri volcano far behind the arc junction between the Kurile and Honshu arcs also gives a low K2O value. 相似文献
14.
Tadahide Ui Keiko Suzuki-Kamata Rumi Matsusue Kei Fujita Hideya Metsugi Mami Araki 《Bulletin of Volcanology》1989,51(2):115-122
The grain orientations within the matrix of two large-scale welded, two small-scale nonwelded and two nonwelded low-aspect ratio pyroclastic flow deposits are measured to analyze flow behavior. Preferred grain alignments are especially apparent in the middle part of layer 2 of each deposit. Preferred grain alignments do not vary laterally within a 10 m interval. The grain alignments obtained are radial from the source caldera, especially in proximal to medial and plateau-forming facies of pyroclastic flow deposits. Grain alignments are controlled by valley-channel directions for the valley-ponded facies of pyroclastic flow deposits, especially at medial to distal locations. Such local topographic factors strongly affect the data for high-aspect ratio and smallscale deposits, and weakly affect the data for widespread low-aspect ratio pyroclastic flow deposits. This work suggests that grain alignment analysis should be used with care when attempting to determine the location of an unknown source. 相似文献
15.
Tadahide Ui Norimichi Matsuwo Mari Sumita Akihiko Fujinawa 《Journal of Volcanology and Geothermal Research》1999,89(1-4)
Processes generating block and ash flows by gravitational dome collapse (Merapi-type pyroclastic flow) were observed in detail during the 1990–1995 eruption of Unzen volcano, Japan. Two different types were identified by analysis of video records and observations during helicopter flights. Most of the block and ash flows erupted during the 1991–1993 exogenous dome growth stage initially involved crack propagation due to cooling and flowage of the dome lava lobes. The mass around the crack became unstable, locally decreasing in tensile strength. Finally, a slab separated from the lobe front, fragmented progressively from the base to the top within a few seconds, and became a block and ash flow. Rock falls immediately followed, in response to local instability of the lobe front. Clasts in these rock falls fragmented and merged with the preceding flow. In contrast, block and ash flows during the endogenous dome growth stage in 1994 were generated due to local bulge of the dome. Unstable lava blocks collapsed and subsequently fragmented to produce block and ash flows. 相似文献
16.
Published chemical analyses of the Japanese Quaternary volcanic rocks are being collected to build a comprehensive data file. About 1600 selected major element analyses which include all 13 components of the bulk chemistry of the lavas and pyroclastics are considered here. Care was taken to correct misprints in the published analyses through correspondence with the original authors or analysts. Analyses are weighted against the volume of the volcano on which rock the analyses are made. Total volume of the volcanic material thus treated in this paper amounts to 3900 km3 or about 80% of the total Quaternary volcanic materials. The volume-weighted histograms for the most components show a unimodal profile when the large-scale caldera-forming pyroclastic flow and fall deposits are disregarded. SiO2 histogram shows a broad primary mode around 57 %. For many components, the large-scale pyroclastic deposits form a secondary mode at the positions corresponding to a felsic composition. This strongly suggests that the mode of generation and ascent of such calderaforming felsic magma is different from that of the ordinary cone- and dome-building basalt-andesite-dacite series of magmas. Number-of-analyses histograms for the same set of combinations reveals that number frequencies show quite similar profiles as those for the volume-weighted histograms for the Japanese Quaternary volcanics. The profiles of the histograms for the Northeast Japan arc is very much similar as those of the whole Japanese islands while those for Izu-Mariana and Southwest Japan arcs are different. The differences are due to the different proportions of the rocks belonging to tholeiitic, calc-alkalic and alkali rock series in each arc. The whole Japan histograms may be considered as a good representative of the chemical characteristics of the average Japanese Quaternary (less than the last half million years) volcanic rocks. 相似文献
17.
Depositional features and transportation mechanism of valley-filling Iwasegawa and Kaida debris avalanches, Japan 总被引：1，自引：1，他引：0
The depositional features of two valley-filling debris avalanche deposits were studied to reveal their transportation and
depositional mechanisms. The valley-filling Iwasegawa debris avalanche deposit (ca. 0.1 km3) is distributed along the valleys at the southeastern foot of Tashirodake Volcano, northern Honshu, Japan. Debris-avalanche
blocks range in size from <35 m proximally to <10 m in the distal zone and consist dominantly of fragile materials. Debris-avalanche
matrix percentages increase from 35–60% in the proximal zone to 95% in the distal zone. The debris-avalanche matrix is greater
in volume (80–90%) at the bottom and margins of the deposit. Normal grading of large clasts and reverse grading of wood logs
and branches occur within the debris-avalanche matrix. Preferred orientation of 311 wood logs and branches within the deposit
coincide with the interpreted local flow direction. The basal part of the deposit is characterized by (1) erosional features
and incorporated clasts of underlying material; (2) a higher proportion (30–50%) of incorporated clasts than the upper part;
and (3) reverse grading of clasts.
The valley-filling Kaida debris avalanche deposit (50 000 y B.P., >0.3 km3) is distributed along the valleys at the eastern-southeastern foot of Ontake Volcano, central Japan. Debris-avalanche blocks
range in size from <25 m proximally to <7 m in the medial zone. Debris-avalanche matrix percentages increase from 50–70% in
the proximal zone to 80% in the distal zone. The debris-avalanche matrix is more abundant (80–90%) at the bottom part of the
deposit. Deformation structures observed in the debris-avalanche blocks include elongation, folding, conjugate reverse faults,
and numerous minor faults in unconsolidated materials. Lithic components within the debris-avalanche matrix tend to have a
higher percentage of plucked clasts from the adjacent underlying formations.
A Bingham "plug flow" model is consistent with the transportation and depositional mechanisms of the valley-filling debris
avalanches. In the plug of the debris avalanche, fragile blocks were transported without major rupturing due to relatively
small shear stresses in regions of small strain rate. The debris-avalanche matrix was mainly produced by shearing at the bottom
and margins of the avalanche. Valley-filling debris avalanches tend to have smaller debris-avalanche blocks and larger amounts
of debris-avalanche matrix than do unconfined debris avalanches. These differences may be due to disaggregation of debris-avalanche
blocks by shearing against valley walls and interaction between debris-avalanche blocks and valley walls. Oriented wood logs
and branches, reverse grading of clasts at the base, and a higher proportion of incorporated clasts at the base are interpreted
to result from shearing along the bottom and valley walls.
Received: 25 March 1998 / Accepted: 10 October 1998 相似文献
18.
Yun‐Guo Liu Tadahide Kurokawa Masashi Sekino Toru Tanabe Kazuhito Watanabe 《Marine Ecology》2014,35(3):273-280
We found that tandem repeat (TR) arrays were present in the ark shell Scapharca broughtonii mitochondrial genome, and we demonstrated the efficiency of using this TR region as a tool for the detection of genetic differentiation. In samples collected from seven populations off the Japanese, Chinese, Russian and Korean coasts, five TR types with length ranging from 481 to 601 bp (base pairs) were found, as were 18 haplotypes (n = 155). All the Japanese samples from SD, IK, AM and KG, which represented four geographically distinct populations, had the same type of tandem repeat array, 481‐TR. All the samples from China and all but one sample from Russia had the same TR type, 511‐TR; these finding may be strong evidence that the ark shell populations in China and Russia are very similar. Based on a neighbor‐joining dendrogram of haplotypes, the Korean population was divided into two groups as follows: samples with haplotype H481‐3 and samples with haplotypes H601‐1, H601‐2, H601‐3 or H541‐1. Based on the haplotype diversity and nucleotide diversity, the Chinese, Russian and Korean populations showed a higher diversity than Japanese populations (P < 0.05). 相似文献