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1.
The May, 2008, Chaitén (southern Chile) eruption was characterized by several explosive events, each associated with plumes which reached up to about 19?km above sea level on May 6. A study of the textural and physical features of the juvenile clasts erupted during the climactic phase of the 2008 eruption of Chaitén is presented. Pumice clasts show unimodal density distribution (main mode at 600?kg/m3), average vesicularity of about 69?%, a glassy groundmass with no microcrystals, and vesicles with dimension between ～1?μm and ～2?mm. They also show a unimodal vesicle size distribution with most frequent vesicle size in the range 0.05–0.08?mm and an estimated vesicle number density of 1.3?±?0.5?×?105?mm?3 related to a rapid nucleation event produced during the late phases of magma rise. This is confirmed by the absence of microcrystals that could otherwise have delayed vesicle formation and allowed the magma to maintain a low viscosity and a supersaturation in volatiles. Vesiculation and fragmentation were triggered by a sudden decompression of the melt associated with the opening of the volcanic conduit (～10?MPa?s?1). 相似文献
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Michael E. Evans 《Studia Geophysica et Geodaetica》2012,56(3):725-734
New results from the La Lieude Formation now complete the magneto-stratigraphic coverage of the Permian redbeds preserved in the Lodève Basin of southern France. The majority of the samples yield reversed polarity, with a mean of declination D = 187.4°, inclination I = ?0.5° (Fisherian statistics k = 44.2, ??95 = 5.6°, n = 16). Together with previously published results, these data indicate that the entire basin lies within the Permo-Carboniferous Reversed Superchron (PCRS). But the stratigraphically highest sample exhibits normal polarity, suggesting that the transition marking the end of the PCRS may be close. The unusual behaviour of the geodynamo that generates superchrons prompts one to ask if there are concomitant influences on the morphology of the field. The intersecting palaeomeridian method offers a means of pursuing this question. An updated analysis suggests that there are currently no compelling reasons for adding significant higher-order terms to the geocentric axial dipole (GAD) model for the Late Permian. 相似文献
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The paper presents the results of analyzing the set of dual-polarity paleomagnetic results the Global Paleomagnetic Database
(GPMDB). The dataset was expanded by the results from the Paleomagnetic Data Catalogue for the USSR and with new data published
after 2005. Some results were rejected to avoid the influence of overprints of ancient and recent magnetization. Overall,
59 dual-polarity results for the lithospheric plates of Baltica, Laurentia, and Siberia with their immediate framing were
selected for the analysis in the interval of ages 207–359 Ma. The new data confirmed the model of the paleomagnetic field,
according to which the field contains a long-lived component corresponding to the equatorial dipole which is responsible for
the non-antipodal paleomagnetic directions in the zones of normal and reverse polarity in sedimentary and volcanic rock sequences.
Retaining its value of 5–8% of the central axial dipole, the equatorial dipole changed its polarity a few times during the
interval 359–207 Ma. The northern poles of the dipole formed two antipodal groups on the Earth’s surface, which lie within
or near the subduction zones on the periphery of the Pangaea Supercontinent. Such localization of the equatorial dipole is
suggested to be related to the ascending branches of the mantle convection and to the topography of both boundaries of the
outer Earth’s core. 相似文献
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John Alexander Stevenson Neil Charles Mitchell Fiona Mochrie Michael Cassidy Harry Pinkerton 《Bulletin of Volcanology》2012,74(1):33-46
The Nesjahraun is a basaltic lava flow erupted from a subaerial fissure, extending NE along the Tingvellir graben from the
Hengill central volcano that produced pāhoehoe lava followed by ‘a‘ā. The Nesjahraun entered Iceland’s largest lake, Tingvallavatn,
along its southern shore during both phases of the eruption and exemplifies lava flowing into water in a lacustrine environment
in the absence of powerful wave action. This study combines airborne light detection and ranging, sidescan sonar and Chirp
seismic data with field observations to investigate the behaviour of the lava as it entered the water. Pāhoehoe sheet lava
was formed during the early stages of the eruption. Along the shoreline, stacks of thin (5–20 cm thick), vesicular, flows
rest upon and surround low (<5 m) piles of coarse, unconsolidated, variably oxidised spatter. Clefts within the lava run inland
from the lake. These are 2–5 m wide, >2 m deep, ∼50 m long, spaced ∼50 m apart and have sub-horizontal striations on the walls.
They likely represent channels or collapsed tubes along which lava was delivered into the water. A circular rootless cone,
Eldborg, formed when water infiltrated a lava tube. Offshore from the pāhoehoe lavas, the gradient of the flow surface steepens,
suggesting a change in flow regime and the development of a talus ramp. Later, the flow was focused into a channel of ‘a‘ā
lava, ∼200–350 m wide. This split into individual flow lobes 20–50 m wide along the shore. ‘A‘ā clinker is exposed on the
water’s edge, as well as glassy sand and gravel, which has been locally intruded by small (<1 m), irregularly shaped, lava
bodies. The cores of the flow lobes contain coherent, but hackly fractured lava. Mounds consisting predominantly of scoria
lapilli and the large paired half-cone of Grámelur were formed in phreatomagmatic explosions. The ‘a‘ā flow can be identified
underwater over 1 km offshore, and the sidescan data suggest that the flow lobes remained coherent flowing down a gradient
of <10°. The Nesjahraun demonstrates that, even in the absence of ocean waves, phreatomagmatic explosions are ubiquitous and
that pāhoehoe flows are much more likely to break up on entering the water than ‘a‘ā flows, which, with a higher flux and
shallow underlying surface gradient, can penetrate water and remain coherent over distances of at least 1 km. 相似文献
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Janusz W. Krzyścin Piotr S. Sobolewski Janusz Jarosławski Jerzy Podgórski Bonawentura Rajewska-Więch 《Acta Geophysica》2011,59(1):155-182
Measurements of erythemal irradiance have been carried out continuously at Belsk since May 1975. We present a homogenization
procedure of the UV time series for the period of 1976–2008. Long-term oscillations discovered in the homogenized data set
agree with those extracted from the reconstructed UV data for all-sky and clear-sky conditions. The UV climatology was established
and the UV variability was determined. Positive UV trends were found for the period of 1976-2008 in the annual mean (5.6±0.9%
per decade), in the seasonal mean for the warm subperiod of the year (April–October, 5.5±1.0% per decade), and in monthly
means (∼2–9% per decade). A satisfactory agreement between the trend extracted from the homogenized ground-based data and
that found in satellite UV data for Belsk (1979–2008) supports the reliability of satellite trend analyses over wider areas
during snowless periods. 相似文献
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Tomáš Lepeška Artur Radecki-Pawlik Jakub Wojkowski Andrzej Walega 《Acta Geophysica》2017,65(6):1253-1267
Human society deals with floods, drought and water pollution. Facing those problems, the question how to prevent or at least to minimalize the adverse effects of water-related issues is asked of the landscape managers. In this way, any help given to landscape managers seems to be an additional useful tool. Within this paper, an approach leading to mitigation of water-related problems is presented that relates the retention of precipitation and the use of ecosystems as a tool for improving the quality, quantity of water resources and availability throughout the region. One approach is the determination of the landscape’s hydric potential (LHP). This study examines one example of using this method within the conditions of Poland. The results of the research show that national data are entirely appropriate for implementation of the LHP method. Further, this approach revealed the classes of the hydric potential of the Pr?dnik river basin which was selected as the experimental territory. LHP results reflect the ecosystem attributes of the model river basin; areas of average LHP cover 63.26%, areas of high and limited hydric potential cover approximately 18.3% each. The spatial distribution of LHP means the results of this study provide a baseline for management of the river basin. 相似文献
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The initial subevent of the 1994 Northridge, California, earthquake: Is earthquake size predictable? 总被引：1，自引：0，他引：1
We examine the initial subevent (ISE) of the M 6.7, 1994 Northridge, California, earthquake in order to discriminate between two end-member rupture initiation models: the preslip and cascade models. Final earthquake size may be predictable from an ISE's seismic signature in the preslip model but not in the cascade model. In the cascade model ISEs are simply small earthquakes that can be described as purely dynamic ruptures. In this model a large earthquake is triggered by smaller earthquakes; there is no size scaling between triggering and triggered events and a variety of stress transfer mechanisms are possible. Alternatively, in the preslip model, a large earthquake nucleates as an aseismically slipping patch in which the patch dimension grows and scales with the earthquake's ultimate size; the byproduct of this loading process is the ISE. In this model, the duration of the ISE signal scales with the ultimate size of the earthquake, suggesting that nucleation and earthquake size are determined by a more predictable, measurable, and organized process. To distinguish between these two end-member models we use short period seismograms recorded by the Southern California Seismic Network. We address questions regarding the similarity in hypocenter locations and focal mechanisms of the ISE and the mainshock. We also compare the ISE's waveform characteristics to those of small earthquakes and to the beginnings of earthquakes with a range of magnitudes. We find that the focal mechanisms of the ISE and mainshock are indistinguishable, and both events may have nucleated on and ruptured the same fault plane. These results satisfy the requirements for both models and thus do not discriminate between them. However, further tests show the ISE's waveform characteristics are similar to those of typical small earthquakes in the vicinity and more importantly, do not scale with the mainshock magnitude. These results are more consistent with the cascade model. 相似文献
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Mario Octavio Cotilla Rodríguez Diego Córdoba Barba 《Izvestiya Physics of the Solid Earth》2011,47(6):496-518
All available data on the January 23, 1880, earthquake near San Cristobal, Western Cuba, are compiled and presented here.
The earthquake reached a maximum intensity of eight degrees (MSK) and caused three fatalities. It was accompanied by 65 aftershocks
and was felt as far away as the Florida Keys. Twentieth century specialists has associated this event, in its day the strongest
recorded (Ms = 6.2) in the region, with the Pinar fault. The Pinar fault is well expressed topographically as the boundary
between the Guaniguanico Range in the north and an alluvial plain to the south. Most of the major damage caused by the earthquake
was located on the alluvial plain, which in consequence has been considered the epicenter area. In the study presented here,
the data compiled from the first reports of Father Benito Vines Martorell, S.J., and Pedro Salteraín y Legarra, indicate that
the seismic structure was located in the alluvial plain, and that it was the Guane fault, and not the Pinar fault, that was
responsible for the earthquake. The Guane fault, found below the alluvial sediments, extends NE-SW for over 110 km. Its eastern
extreme, near San José de las Lajas (La Habana), is linked to another active fault which represents a seismoactive knot responsible
for the earthquake of March 9, 1995 (I = 5 degrees, MSK). Seismic events of the Western Cuban region are related to the transpressive
interaction of the North American and Caribbean Plates, damped by oceanic structures. 相似文献
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A number of overflows from a large lava channel and tube system on the southwest rift zone of Mauna Loa were studied. Initial overflows were very low viscosity gas-rich phoehoe evidenced by flow-unit aspect ratios and vesicle sizes and contents. Calculated volumetric flow-rates in the channel range between 80 and 890 m3/s, and those of the overflows between 35 and 110 m3/s. After traveling tens to hundreds of meters the tops of these sheet-like overflows were disrupted into a surface composed of clinker and phoehoe fragments. After these 'a' overflows came to rest, lava from the interiors was able to break out on to the surface as phoehoe. The surface structure of a lava flow records the interaction between the differential shear rate (usually correlated with the volumetric flow-rate) and viscosity-induced resistance to flow. However, the interior of a flow, being better insulated, may react differently or record a later set of emplacement conditions. Clefts of toothpaste lava occurring within fields of clinker on proximal-type 'a' flows also record different shear rates during different times of flow emplacement. The interplay between viscosity and shear rate determines the final morphological lava type, and although no specific portion of lava ever makes a transition from 'a' back to phoehoe, parts of a flow can appear to do so. 相似文献
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Introduction As we well know, the hazard of earthquake is very wide especially in cities. The conventionalmethods to investigate the damage are difficult to meet the requirements in applications. In recentyears, with the rapid development of remote sensing, especially the successful launch and applica-tion of high-resolution commercial remote sensing satellite, it has become possible to recognize andextract damage information by using remote sensing. The researchers at home and abroad hav… 相似文献
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Taxonomic revision of Cylindroporella ? lusitanica Ramalho, 1970: In search for the origins of the Family Dasycladaceae 
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下载免费PDF全文 A study of new material of Cylindroporella ? lusitanica Ramalho, 1970, from the Upper Jurassic (Oxfordian) strata of Portugal identified features not observed previously. This includes the pattern of its laterals, including the gametophores. The algal body plan is unique and justifies the taxonomic reassessment of this species and the introduction of Barattoloporellopsis n. gen. Furthermore this species is most likely a primitive Dasycladaceae, a “missing link” between the modern ones (starting from the Cretaceous times) and their Jurassic ancestors. 相似文献
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V. O. Mikhailov K. Arora A. V. Ponomarev D. Srinagesh V. B. Smirnov R. K. Chadha 《Izvestiya Physics of the Solid Earth》2017,53(4):518-529
The state of the art in the geological and geophysical study of the region of Koyna and Warna water reservoirs is reviewed. The probable geodynamical factors of induced seismicity are discussed. The detailed geophysical surveys, satellite geodetic data, and time history of the seismicity in the region reveal a complicated pattern of the structure and recent geodynamics of the region. The existing data suggest that the induced seismicity is here most likely to be caused by the regional (intraplate) stresses driving the displacements along the orthogonal network of the faults whose strength has dropped and continues decreasing due to the reservoir impoundment and operation processes. The evolution of the seismicity which started immediately after the rapid filling of the Koyna reservoir in the region of the dam, then rapidly expanded southwards and eventually became concentrated in the region of the subsequently constructed Warna reservoir shows that seismic events can be initiated by a number of factors whose contributions may vary with time. The key ones among them include reservoir loading and its seasonal variations; water saturation of the faults which guide the propagation of the front of fracture, increased permeability, and, probably, mineral transformations (hydrolysis) under the water level fluctuations in the reservoirs; and displacement of the front of the high pore pressure down to the main source zone of the earthquakes at a depth of 6–8 km. Based on the analysis presented in the paper, we outline the directions of the future research aimed at studying the nature and dynamics of induced seismicity in the region of large water reservoirs. 相似文献
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John Alexander Stevenson Neil Charles Mitchell Michael Cassidy Harry Pinkerton 《Bulletin of Volcanology》2012,74(1):15-31
This study describes the emplacement of the Nesjahraun, a basaltic lava flow that entered the lake Tingvallavatn, SW Iceland.
High-resolution remotely sensed data were combined with fieldwork to map the flow field. Onshore, the Nesjahraun exhibits
a variety of textures related to the widespread inflation and collapse of a pāhoehoe flow field. Its emplacement is interpreted
as follows: Initially, the eruption produced sheet pāhoehoe. In the central part of the flow field, the lava has a platy-ridged
surface, which is similar to some other lava flows in Iceland and on Mars. Here, the texture is interpreted to have formed
by unsteady inflation of the brittle crust of stationary sheet pāhoehoe, causing it to break into separate plates. The ridges
of broken pāhoehoe slabs formed as the plates of crust moved vertically past each other in a process similar to the formation
of shatter rings. Upstream, fresh lava overflowed repeatedly from channels and tubes, covering the surface with shelly pāhoehoe.
Formation of a 250-m-wide open channel through the flow field allowed the inflated central part of the flow to drain rapidly.
This phase produced ‘a‘ā lava, which eroded the channel walls, carrying broken pāhoehoe slabs, lava balls and detaching large
(>200 m long) rafts of compound shelly pāhoehoe lava. Much of this channelized lava flowed into the lake, leaving a network
of drained channels and tubes in the upstream part of the flow. As in other locations, the platy-ridged texture is associated
with a low underlying slope and high eruption rate. Here, its formation was possibly enhanced by lateral confinement, hindered
entry into the lake and an elevated vent location. We suggest that formation of this type of platy-ridged lava, where the
plates are smooth and the ridges are slabs of broken pāhoehoe, can occur without significant horizontal transport, as the
surface crust is broken into plates in situ. This reconstruction of the emplacement of the Nesjahraun also demonstrates that
high-resolution aerial survey data are extremely useful in the mapping and measurement of lithofacies distributions in large
flow fields, but that fieldwork is still necessary to obtain the detailed textural information necessary to interpret them. 相似文献
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WenDi Chen WenLan Zhang RuCheng Wang ZhuYin Chu Rong Xiao Di Zhang XuDong Che 《中国科学:地球科学(英文版)》2016,59(11):2121-2141
The polymetallic Dushiling W-Cu deposit is a large, altered, skarn-type deposit, located in the northeastern part of the Miao'ershan-Yuechengling pluton, China. Two types of granite have been identified in the deposit: a medium-grained porphyritic biotite granite, and a medium- to fine-grained biotite granite. Both are spatially and temporally related to ore bodies, suggesting they may be the source of mineralization in the deposit. A medium- to fine-grained porphyritic biotite granite is exposed at the surface in the region of mineralization. U-Pb dating of zircons yielded magmatic ages of 423 Ma for the medium-grained porphyritic biotite granite and 421 Ma for the medium- to fine-grained porphyritic biotite granite, while a younger age(217 Ma) obtained for surface samples indicates later diagenesis. Thus, magmatism occurred during the Caledonian and Indosinian, respectively. The petrological and geochemical characteristics of the two Caledonian granites show that both are calc-alkaline and peraluminous.They are moderately enriched in Cs, Rb, U, and REE, and strongly depleted in Sr,Ba,P,and Ti; they show similar REE behavior,including negative Eu anomalies. These geochemical similarities suggest that the two granites were derived from the same source,although they were emplaced during different stages of the evolution of the magma. Furthermore, the granites are associated with mineralization, suggesting they were the source of mineralization in the Dushiling W-Cu deposit. Sm-Nd ages of scheelite from the Dushiling W-Cu deposit indicate that metallogenesis occurred at 417±35 Ma, while the two types of titanite, intergrown with scheelite, yield U-Pb ages of 423–425 Ma(in altered granite sample) and 218 Ma(in skarn sample). These ages place the main mineralization event in the late Caledonian, and later magmatic-hydrothermal activity occurred in the Indosinian. The ages obtained for the Dushiling W-Cu deposit in the western Nanling Range, northern Yuechengling, together with the occurrence and ages of the Niutangjie W deposit in southern Yuechengling, provide insight into the process of ore concentration during the Caledonian and Indosinian. 相似文献