排序方式： 共有41条查询结果，搜索用时 15 毫秒
1.
Subaqueous pyroclastic flows and ignimbrites: an assessment 总被引：2，自引：0，他引：2
An assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary, hot gas-rich pyroclastic flows, mass-flows which resulted from the transformation of gassupported flows into water-supported ones, and secondary mass-flows carrying redeposited pyroclastic debris. Based on subaerial pyroclastic flows, the term pyroclastic flow should be restricted to demonstrably hot, gas-rich mass-flows of pyroclastic debris. Using this definition, very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described. In the majority of these cases, the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate. The only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the Ordovician nearshore, shallow marine ignimbrites of Ireland and Wales, and Miocene ignimbrites of southwest Japan, resulting from the passage of subaerially erupted pyroclastic flows into shallow water. Other possible examples are near-vent dense clast deposits in the Donzurobo Formation of Japan, possible submarine intra-caldera ponded ignimbrite successions in California and Wales, and near-vent pumiceous deposits of Ramsay Island, Wales. All other purported cases are either clearly the result of water-supported mass-flow transportation and deposition (debris avalanches, debris flows, turbidity currents), or lack adequate supporting evidence regarding the heat state or the palaeoenvironment. Only the shallow marine ignimbrites of Ireland and Wales show adequate evidence of welding, but even these could have been nearly wholly exposed above sea-level when welding occurred. We conclude that when pyroclastic flows enter water they are generally disrupted explosively and/or ingest water and transform into water-supported mass-flows, and we suggest the various scenarios in which this occurs. There is no evidence to suggest that welding in wholly subaqueous environments is common. 相似文献
2.
Joeri Kaal Yolanda Carrión Marco Eleni Asouti Maria Martín Seijo Antonio Martínez Cortizas Manuela Costa Casáis Felipe Criado Boado 《Quaternary Science Reviews》2011,30(1-2):161-175
The Holocene fire regime is thought to have had a key role in deforestation and shrubland expansion in Galicia (NW Spain) but the contribution of past societies to vegetation burning remains poorly understood. This may be, in part, due to the fact that detailed fire records from areas in close proximity to archaeological sites are scarce. To fill this gap, we performed charcoal analysis in five colluvial soils from an archaeological area (Campo Lameiro) and compared the results to earlier studies from this area and palaeo-ecological literature from NW Spain. This analysis allowed for the reconstruction of the vegetation and fire dynamics in the area during the last ca 11 000 yrs. In the Early Holocene, Fabaceae and Betula sp. were dominant in the charcoal record. Quercus sp. started to replace these species around 10 000 cal BP, forming a deciduous forest that prevailed during the Holocene Thermal Maximum until ～5500 cal BP. Following that, several cycles of potentially fire-induced forest regression with subsequent incomplete recovery eventually led to the formation of an open landscape dominated by shrubs (Erica sp. and Fabaceae). Major episodes of forest regression were (1) ～5500–5000 cal BP, which marks the mid-Holocene cooling after the Holocene Thermal Maximum, but also the period during which agropastoral activities in NW Spain became widespread, and (2) ～2000–1500 cal BP, which corresponds roughly to the end of the Roman Warm Period and the transition from the Roman to the Germanic period. The low degree of chronological precision, which is inherent in fire history reconstructions from colluvial soils, made it impossible to distinguish climatic from human-induced fires. Nonetheless, the abundance of synanthropic pollen indicators (e.g. Plantago lanceolata and Urtica dioica) since at least ～6000 cal BP strongly suggests that humans used fire to generate and maintain pasture. 相似文献
3.
Potential seasonal calibration for palaeoenvironmental reconstruction using skeletal microstructures and strontium measurements from the cold‐water coral Lophelia pertusa 
下载免费PDF全文
下载免费PDF全文 4.
Distribution and significance of crystalline, perlitic and vesicular textures in the Ordovician Garth Tuff (Wales) 总被引：2，自引：0，他引：2
Diverse spherulitic and granular crystalline fabrics, perlitic textures and fabrics related to the growth and migration of
vesicles occur in the Garth Tuff, a largely welded Ordovician ignimbrite. Defining the distribution of such textures helps
to constrain the ignimbrite's cooling and degassing history. Suites of spherulitic and perlitic textures closely reflect variation
in cooling rates. Seven facies are defined based on the style and intensity of crystallisation: (1) a medium to coarsely crystalline,
equigranular facies; (2) an intensely spherulitic facies; (3) a sparsely spherulitic facies; (4) a pectinate facies; (5) a
microcrystalline to cryptocrystalline, equigranular facies; (6) a lithophysal facies; and (7) a transitional perlite–pectinate
facies. Textural changes from facies 1 to 5 reflect progressively higher cooling rates. Facies 1 occurs in proximal settings
in the ignimbrite's core. Facies 2 to 5 successively envelop facies 1, with facies 2 becoming the dominant fabric in the ignimbrite's
core in medial settings. Facies 5 is typically developed in the originally glassy perlitic zones at the ignimbrite's welded
margins. Crystallisation under hydrous conditions is reflected by second-boiling textures in the sporadically developed lithophysal
facies. The seventh facies reflects a subtle interplay between cooling, hydration and crystallisation which locally prevented
perlitic fracturing. The distribution of amygdales reflects patterns of volatile migration and entrapment. In the lower levels
of the ignimbrite, amygdales occur in irregular concentrations or rare subvertical pipe-like structures. Pipe-like structures
attest to fumarolic activity while the ignimbrite was in a rheomorphic state. Amygdales are widespread and evenly distributed
in the upper levels of the ignimbrite. However, the top of the welding profile is characterised by a thin, poorly vesiculated,
originally vitrophyric horizon that abruptly caps an intense concentration of amygdales. Ductile and brittle fabrics developed
during the upward migration of gas. Microscopic drag folds occur around some amygdales. Jigsaw-fit to clast-rotated breccias
originated through both late-stage pneumatic fracturing and autobrecciation. Vaporisation of water at the flow base provided
a significant source of volatiles in addition to gas released during cooling and crystallisation. Secondary alteration has
enhanced or modified some fabrics. Perlitic zones were susceptible to patchy chlorite–sericite–carbonate diagenetic alteration.
Diagenesis and metamorphism have contributed to the infilling of vesicles.
Received: 22 August 1997 / Accepted: 24 June 1998 相似文献
5.
The Kos Plateau Tuff consists of pyroclastic deposits from a major Quaternary explosive rhyolitic eruption, centred about 10 km south of the island of Kos in the eastern Aegean, Greece. Five main units are present, the first two (units A and B) were the product of a phreatoplinian eruption. The eruption style then changed to `dry' explosive style as the eruption intensity increased forming a sequence of ignimbrites and initiating caldera collapse. The final waning phase returned to phreatomagmatic eruptive conditions (unit F). The phreatomagmatic units are fine grained, poorly sorted, and dominated by blocky vitric ash, thickly ash-coated lapilli and accretionary lapilli. They are non-welded and were probably deposited at temperatures below 100°C. All existing exposures occur at distances between 10 km and 40 km from the inferred source. Unit A is a widespread (>42 km from source), thin (upwind on Kos) to very thick (downwind), internally laminated, dominantly ash bed with mantling, sheet-like form. Upwind unit A and the lower and middle part of downwind unit A are ash-rich (ash-rich facies) whereas the upper part of downwind unit A includes thin beds of well sorted fine pumice lapilli (pumice-rich facies). Unit A is interpreted to be a phreatoplinian fall deposit. Although locally the bedforms were influenced by wind, surface water and topography. The nature and position of the pumice-rich facies suggests that the eruption style alternated between `wet' phreatoplinian and `dry' plinian during the final stages of unit A deposition.Unit B is exposed 10–19 km north of the inferred source on Kos, overlying unit A. It is a thick to very thick, internally stratified bed, dominated by ash-coated, medium and fine pumice lapilli in an ash matrix. Unit B shows a decrease in thickness and grain size and variations in bedforms downcurrent that allow definition of several different facies and laterally equivalent facies associations. Unit B ranges from being very thick, coarse and massive or wavy bedded in the closest outcrops to source, to being partly massive and partly diffusely stratified or cross-bedded in medial locations. Pinch and swell, clast-supported pumice layers are also present in medial locations. In the most distal sections, unit B is stratified or massive, and thinner and finer grained than elsewhere and dominated by thickly armoured lapilli. Unit B is interpreted to have been deposited from an unsteady, density stratified, pyroclastic density current which decelerated and progressively decreased its particle load with distance from source. Condensation of steam during outflow of the current promoted the early deposition of ash and resulted in the coarser pyroclasts being thickly ash-coated. The distribution, texture and stratigraphic position of unit B suggest that the pyroclastic density current was generated from collapse of the phreatoplinian column following a period of fluctuating discharge when the eruptive activity alternated between `wet' and `dry'. The pyroclastic density current was transitional in particle concentration between a dilute pyroclastic surge and a high particle concentration pyroclastic flow. Unidirectional bedforms in unit B suggest that the depositional boundary was commonly turbulent and in this respect did not resemble conventional pyroclastic flows. However, unit B is relatively thick and poorly sorted, and was deposited more than 19 km from source, implying that the current comprised a relatively high particle concentration and in this respect, did not resemble a typical pyroclastic surge. 相似文献
6.
da Silva Richarde Marques Santos Celso Augusto Guimarães Silva Jorge Flávio Casé Braga da Costa Silva Alexandro Medeiros Brasil Neto Reginaldo Moura 《Natural Hazards》2020,104(2):1953-1954
Natural Hazards - The article “Spatial distribution and estimation of rainfall trends and erosivity in the Epitácio Pessoa reservoir catchment, Paraíba, Brazil”, written by da... 相似文献
7.
Submarine lava flow morphology is commonly used to estimate relative flow velocity, but the effects of crystallinity and viscosity are rarely considered. We use digital petrography and quantitative textural analysis techniques to determine the crystallinity of submarine basaltic lava flows, using a set of samples from previously mapped lava flow fields at the hotspot-affected Galápagos Spreading Center. Crystallinity measurements were incorporated into predictive models of suspension rheology to characterize lava flow consistency and rheology. Petrologic data were integrated to estimate bulk lava viscosity. We compared the crystallinity and viscosity of each sample with its flow morphology to determine their respective roles in submarine lava emplacement dynamics. We find no correlation between crystallinity, bulk viscosity, and lava morphology, implying that flow advance rate is the primary control on submarine lava morphology. However, we show systematic variations in crystal size and shape distribution among pillows, lobates, and sheets, suggesting that these parameters are important indicators of eruption processes. Finally, we compared the characteristics of lavas from two different sampling sites with contrasting long-term magma supply rates. Differences between lavas from each study site illustrate the significant effect of magma supply on the physical properties of the oceanic upper crust. 相似文献
8.
Chiara Lesti Massimiliano Porreca Guido Giordano Massimo Mattei Raymond A. F. Cas Heather M. N. Wright Chris B. Folkes Josè Viramonte 《Bulletin of Volcanology》2011,73(10):1535-1565
Estimates of pyroclastic flow emplacement temperatures in the Cerro Galán ignimbrite and Toconquis Group ignimbrites were
determined using thermal remanent magnetization of lithic clasts embedded within the deposits. These ignimbrites belong to
the Cerro Galán volcanic system, one of the largest calderas in the world, in the Puna plateau, NW Argentina. Temperature
estimates for the 2.08-Ma Cerro Galán ignimbrite are retrieved from 40 sites in 14 localities (176 measured clasts), distributed
at different distances from the caldera and different stratigraphic heights. Additionally, temperature estimates were obtained
from 27 sample sites (125 measured clasts) from seven ignimbrite units forming the older Toconquis Group (5.60–4.51 Ma), mainly
outcropping along a type section at Rio Las Pitas, Vega Real Grande. The paleomagnetic data obtained by progressive thermal
demagnetization show that the clasts of the Cerro Galán ignimbrite have one single magnetic component, oriented close to the
expected geomagnetic field at the time of emplacement. Results show therefore that most of the clasts acquired a new magnetization
oriented parallel to the magnetic field at the moment of the ignimbrite deposition, suggesting that the clasts were heated
up to or above the highest blocking temperature (T
b) of the magnetic minerals (T
b = 580°C for magnetite; T
b = 600–630°C for hematite). We obtained similar emplacement temperature estimations for six out of the seven volcanic units
belonging to the Toconquis Group, with the exception of one unit (Lower Merihuaca), where we found two distinct magnetic components.
The estimation of emplacement temperatures in this latter case is constrained at 580–610°C, which are lower than the other
ignimbrites. These estimations are also in agreement with the lowest pre-eruptive magma temperatures calculated for the same
unit (i.e., 790°C; hornblende–plagioclase thermometer; Folkes et al. 2011b). We conclude that the Cerro Galán ignimbrite and Toconquis Group ignimbrites were emplaced at temperatures equal to or higher
than 620°C, except for Lower Merihuaca unit emplaced at lower temperatures. The homogeneity of high temperatures from proximal
to distal facies in the Cerro Galán ignimbrite provides constraints for the emplacement model, marked by a relatively low
eruption column, low levels of turbulence, air entrainment, surface–water interaction, and a high level of topographic confinement,
all ensuring minimal heat loss. 相似文献
9.
The distribution of diamonds within individual kimberlite pipes is poorly documented in the public domain due to the proprietary
nature of the data. The study of the diamond distribution within two pipes, Fox and Koala, from the EKATI Diamond Mine, NWT,
Canada, in conjunction with detailed facies models has shown several distinct relationships of deposit type and grade distribution.
In both pipes, the lithological facies represent grade units which can be distinguished from each other in terms of relative
size and abundance of diamonds. A positive relationship between olivine grain size and abundance with diamond size and abundance
is observed, indicating that sorting of fragmental kimberlites influences diamond distribution. Though surface geological
processes do not control the diamond potential of the erupting magma, they can be responsible for concentrating diamonds into
economically significant proportions. A good understanding of the eruption, transport and depositional processes responsible
for the individual lithological units and the diamond distribution within them is important for successful resource estimation.
This may lead to recognition of areas suitable for selective mining, making a marginal deposit economic. 相似文献
10.
The Jurassic Muskox and Jericho kimberlites (Northern Slave Province, Nunavut, Canada) contain a variety of facies exhibiting
different geometries, contact relationships, internal organisation, country rock abundance and olivine shapes, although many
have similar matrix/groundmass mineralogies and textures. Five facies are examined that either have characteristics consistent
with coherent rocks in general (i.e. intrusive and extrusive non-fragmental rocks) or are mineralogically and texturally similar
to kimberlite described as coherent (or apparent coherent). Three facies are interpreted as coherent on the basis of: (1)
geological setting, (2) apparent-porphyritic texture, (3) sharp contacts with fragmental kimberlite, (4) relative abundance
of elongate and unbroken olivine crystals and (5) paucity of country rock xenoliths, while the remaining two facies are interpreted
as fragmental on the basis of: (1) the gradational contacts with demonstrably fragmental kimberlite, (2) relative abundance
and range of sizes of country rock lithic clasts and (3) numerous broken olivine crystals. Comparisons are made with coherent
and apparent-coherent kimberlite from the literature. Our three coherent facies are similar to literature reported coherent
kimberlite dykes hosted in country rock (CKd) in terms of internal organisation, low abundance of country rock xenoliths, and apparent-porphyritic texture. Conversely,
our two fragmental facies share attributes with previously described pipe-filling coherent and apparent-coherent kimberlite
(CKpf) in terms of geometry, internal organisation and abundance of country rock xenoliths. We conclude that CKd and most CKpf, although similar in matrix/groundmass mineralogy and texture, can be distinguished on the basis of internal organisation,
country rock lithic clast abundance, texture (e.g. apparent-porphyritic texture) and possibly olivine crystal shapes and suggest
that fragmental kimberlite is more common than reported. 相似文献