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1.
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   

2.
Gas pistoning is a type of eruptive behavior described first at K??lauea volcano and characterized by the (commonly) cyclic rise and fall of the lava surface within a volcanic vent or lava lake. Though recognized for decades, its cause continues to be debated, and determining why and when it occurs has important implications for understanding vesiculation and outgassing processes at basaltic volcanoes. Here, we describe gas piston activity that occurred at the Pu??u ???????? cone, in K??lauea??s east rift zone, during June 2006. Direct, detailed measurements of lava level, made from time-lapse camera images captured at close range, show that the gas pistons during the study period lasted from 2 to 60?min, had volumes ranging from 14 to 104?m3, displayed a slowing rise rate of the lava surface, and had an average gas release duration of 49?s. Our data are inconsistent with gas pistoning models that invoke gas slug rise or a dynamic pressure balance but are compatible with models which appeal to gas accumulation and loss near the top of the lava column, possibly through the generation and collapse of a foam layer.  相似文献   

3.
The first eruptive activity at Kīlauea Volcano’s summit in 25 years began in March 2008 with the opening of a 35-m-wide vent in Halema‘uma‘u crater. The new activity has produced prominent very-long-period (VLP) signals corresponding with two new behaviors: episodic tremor bursts and small explosive events, both of which represent degassing events from the top of the lava column. Previous work has shown that VLP seismicity has long been present at Kīlauea’s summit, and is sourced approximately 1 km below Halema‘uma‘u. By integrating video observations, infrasound and seismic data, we show that the onset of the large VLP signals occurs within several seconds of the onset of the degassing events. This timing indicates that the VLP is caused by forces—sourced at or very near the lava free surface due to degassing—transmitted down the magma column and coupling to the surrounding rock at 1 km depth.  相似文献   

4.
We investigated the infrared stimulated luminescence (IRSL) and post-infrared IRSL (pIRIR) signals emitted by K-feldspars from sedimentary samples from Asia, Europe and Africa using a single-aliquot multiple elevated temperature (MET) stimulation procedure. For separate aliquots of the same sample, we show that variation among the dose response curves (DRCs), or growth curves, constructed from the regenerative dose signal (Lx), the test dose signal (Tx, an indicator of luminescence sensitivity) and the sensitivity-corrected signal (Lx/Tx) can be largely eliminated by normalising the DRCs using one of the regenerative dose signals; we call this procedure ‘regenerative-dose normalisation’ or re-normalisation. Furthermore, for the MET-pIRIR signals measured at 250 °C, we find that different samples have re-normalised DRCs that follow the same growth function, despite the samples differing significantly in terms of their geological provenance, sedimentary context, equivalent dose (De) and luminescence sensitivity. This common feature offers the potential to establish a ‘global standardised growth curve’ (gSGC) for different samples of K-feldspar, and thereby enable De values to be estimated for a large number of single aliquots by projecting the re-normalised natural signals on to the gSGC. For the 18 samples investigated in this study, we find that De estimates obtained from the gSGC are consistent with those obtained using full single-aliquot regenerative dose (SAR) procedures for doses of up to ∼1600 Gy. The establishment of a gSGC would greatly reduce the time required to date older samples using K-feldspar, as regenerative doses of several hundreds to a few thousands of Gy are typically delivered to each aliquot in each SAR cycle.  相似文献   

5.
The variation of chemical element compositions in two pyroclastic sequences (Astroni 6 and Averno 2, Phlegrean Fields, Italy) is studied. Both sequences are compositionally zoned indicating a variability of melt compositions in the magma chamber prior to eruption. A clear dichotomy between the behaviour of major vs. trace elements is also observed in both sequences, with major elements displaying nearly linear inter-elemental trends and trace elements showing a variable scattered behaviour. Together with previous petrological investigations these observations are consistent with the hypothesis that magma mixing processes played a key role in the evolution of these two magmatic systems. Recently it has been suggested that mixing processes in igneous systems may strongly influence the mobility of trace elements inducing a ‘diffusive fractionation’ phenomenon, whose extent depends on the mixing time-scale. Here we merge information from 1) numerical simulations of magma mixing, and 2) magma mixing experiments (using as end-members natural compositions from Phlegrean Fields) to derive a relationship relating the degree of ‘diffusive fractionation’ to the mixing time-scales. Application of the ‘diffusive fractionation’ model to the two studied pyroclastic sequences allowed us to apply the relationship derived by numerical simulations and experiments to estimate the mixing time-scales for these two magmatic systems. Results indicate that mixing processes in Astroni 6 and Averno 2 systems lasted for approximately 2 and 9 days, respectively, prior to eruption.  相似文献   

6.
We report investigations of the optically stimulated luminescence (OSL) signals of sedimentary quartz from different regions of Asia, Africa, Europe and North America using a single-aliquot regenerative-dose (SAR) procedure. We show that variations in the shape of dose response curves (DRCs), or growth curves, of the test dose sensitivity-corrected OSL signals among single aliquots composed of multiple grains can be greatly reduced by normalising the DRCs using one of the regenerative dose OSL signals. We refer to this regenerative-dose normalisation procedure as ‘re-normalisation’. We find a common re-normalised DRC extends to doses of ∼250 Gy for samples that differ significantly in terms of geological provenance, sedimentary context and depositional age. This feature permits the development of a ‘global standardised growth curve’ (gSGC) for OSL signals from single aliquots of quartz. The equivalent dose (De) of an aliquot can be estimated from the natural signal, one regenerative dose signal and their corresponding test dose signals. For the variety of samples investigated, we find that De estimates obtained from the gSGC are consistent with those obtained using full SAR procedures for doses of up to ∼250 Gy. Use of the gSGC for single aliquots would greatly reduce the time required to estimate the De values of older samples and for a large number of aliquots.  相似文献   

7.
Heavy rainfall and dense vegetation on tropical volcanoes produce abundant carbonized wood in pyroclastic deposits, in addition to easy contamination of this wood by root systems and soluble humic material. Because the physical nature of the charcoal varies, some samples are more prone to contamination. Two independent studies of the same volcano, Mt Liamuiga on St Kitts in the Lesser Antilles, sometimes using samples from the same carbonized tree, yielded a systematic difference in radiocarbon ages. An exchange of samples and a re-investigation of three physically distinct types of charcoal yielded the following results. Rare, hard, dense charcoal, lacking contamination, which had yielded a spurious age of 2860 years bp, was redated at 1845±58 years bp. Common soft, friable charcoal with good cellular structure proved to be susceptible to contamination. A field decontamination technique utilized by one group seems significant as it yields older ages than when only routine laboratory pre-treatment was used, indicating that the latter technique only partly removes the dried and hard residue produced by the decomposition of modern plant rootlets. A previous date of 24870 years bp obtained from powdery charcoal in a horizon beneath the Mansion Series contradicted ages older than 41000 years bp from common friable charcoal in the lower Mansion Series. The soft powdery charcoal was re-investigated using a sample collected a few centimeters from the original, although field decontamination of this sample was not possible, more extensive laboratory treatment yielded an age of ca. 43000 years bp, again proving that routine laboratory pretreatments are inadequate. A revised geochronology for the Mansion Series is described and a cautionary discussion is presented for the benefit of investigators using radiocarbon ages to date volcanic deposits.  相似文献   

8.
Casualties‘ States During Destructive Earthquakes   总被引：1，自引：0，他引：1  
The index of casualties is introduced for the trapped that is still alive after a destructive earthquake to indicate his(her)injury degree.In order to describe the injury-developing process controlled by three factors:the initial injury degree,the trap surroundings and the physique of the cornered,a function SFC( State-Function of Casualties) can be naturally constructes.Through parameter analysis from eight pieces of figures,it can be found that the trapped with weaker physique and worse initial injury degree and in adverse trap surroundings deserves sooner rescue.  相似文献   

9.
Since the last eruption (1888–1890) volcanism at Vulcano, Aeolian Archipelago, southern Tyrrhenian Sea, has taken the form of persistent fumarolic activity. The gas-vapour phases of the geothermal systems are mainly discharged within two restricted areas about 1 km apart from each other, in the northern part of the island. These areas are La Fossa crater, and the beach fumaroles of the Baia di Levante. Fluids released at the two main fumarolic fields display quite different chemical and temperature characteristics, implying different origins. The local seismicity essentially takes the form of discrete shocks of shallow origin (depth1 km) at La Fossa, usually with energy < 1013 ergs. They are thought to be related to the uprise of pressurized hot gases and vapours discharged at the crater fumaroles. The present investigation points to the existence of two principal categories of seismic events (called M-shocks and N-shocks). These are short events (normally < 10 s). M-type shocks are thought to be due to resonance vibrations within the interior of the volcano, probably driven by the excitation of shock-waves within cavities deeply affected by deposition and alteration of self-sealant hydrothermal minerals. N-type events display features that resemble those of volcano-tectonic earthquakes, but have no recognizable S-phases. Here they are tentatively attributed to microfracturing of rocks which have been extensively hydrothermally altered. Results of the present study permit a preliminary conceptual model of the local shallow seismic processes in the framework of geochemical modelling of fumarolic activity and geological inferences from geothermal drilling.  相似文献   

10.
We present a model of chemical reaction within hills to explore how evolving porosity (and by inference, permeability) affects flow pathways and weathering. The model consists of hydrologic and reactive-transport equations that describe alteration of ferrous minerals and feldspar. These reactions were chosen because previous work emphasized that oxygen- and acid-driven weathering affects porosity differently in felsic and mafic rocks. A parameter controlling the order of the fronts is presented. In the absence of erosion, the two reaction fronts move at different velocities and the relative depths depend on geochemical conditions and starting composition. In turn, the fronts and associated changes in porosity drastically affect both the vertical and lateral velocities of water flow. For these cases, estimates of weathering advance rates based on simple models that posit unidirectional constant-velocity advection do not apply. In the model hills, weathering advance rates diminish with time as the Darcy velocities decrease with depth. The system can thus attain a dynamical steady state at any erosion rate where the regolith thickness is constant in time and velocities of both fronts become equal to one another and to the erosion rate. The slower the advection velocities in a system, the faster it attains a steady state. For example, a massive rock with relatively fast-dissolving minerals such as diabase – where solute transport across the reaction front mainly occurs by diffusion – can reach a steady state more quickly than granitoid rocks in which advection contributes to solute transport. The attainment of a steady state is controlled by coupling between weathering and hydrologic processes that force water to flow horizontally above reaction fronts where permeability changes rapidly with depth and acts as a partial barrier to fluid flow. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.  相似文献   

11.
Anthony Whitworth argues for a clear practical distinction between planets and stars – and an end to substellar objects.  相似文献   

12.
Based on the solutions of the Green’s function for a saturated porous medium obtained by the authors,and using transformation of axisymmetric coordinates,Sommerfeld integrals and superposition of the influence field on a free surface,the authors have obtained displacement solutions of a saturated porous medium subjected to a torsional force in a half-space.The relationship curves of the displacement solutions and various parameters(permeability,frequency,etc.)under action of a unit of torque are also given in this paper.The results are consistent with previous Reissner’s solutions,where a two-phase medium decays to a single-phase medium.The solution is useful in solving relevant dynamic problems of a twophase saturated medium in engineering.  相似文献   

13.
Five consecutive multibeam bathymetries collected before, during, and after the 2007 Stromboli eruption, combined with visual inspections, allowed us to document the morphological evolution of an ‘a’ā lava-fed delta and to reconstruct the main processes acting during its submarine emplacement. The 2007 Stromboli delta extended down to 600-m water depth and covered an area of 420?×?103 m2, with a maximum thickness of 65 m and a total estimated volume of ≈7?×?106 m3, i.e., three times larger than its subaerial counterpart. The lava delta grew mainly through the emplacement of discrete lobes about 50–150 m in size. Lobes were fed from point sources along the paleoshoreline, and their subaqueous pathways seem to be mainly controlled by the submarine morphology, with flows mostly filling in depressions left by previous lobes. The main controlling factors on the lobe morphology and thickness are the effusion rates and the pre-eruption morphology, i.e., the geometry and gradients of the basal surface. Data also shows that sudden slope failure of portions of the submarine delta may occur simultaneously with accretion, implying that a significant part of the delta material can be transported to greater depths by submarine gravity flows. The present study is relevant for future monitoring and hazard assessment during the growth of active lava-fed deltas as well as for a better interpretation of ancient volcaniclastic successions inland.  相似文献   

14.
In view of increasing environmental awareness and biodiversity conservation, understanding the main forcing mechanism driving biogeochemical cycles in coral reefs and lagoon coastal areas is a priority. La Ni?a events cause unbalanced situations in the Equatorial Pacific and result in enhanced precipitation in South West Pacific coastal areas. We investigated the impact of heavy rainfalls during the 2008 La Ni?a event on the New Caledonia lagoon using a 3D coupled on-line hydrodynamic-biogeochemical model. Simulations and data showed that the whole lagoon was impacted by river inputs and stronger hydrodynamics, enhancing chlorophyll-a concentration by a factor between 1.7 and 1.9. The coupled model provided new insights into plume transport, highlighting that eastern plumes can be advected northwards or can reach the South West Lagoon, depending on the balance between regional, tide-induced, and wind-induced surface currents. It also provided a synoptic view of lagoon biogeochemical-hydrodynamic response, when remote sensing data are not available due to cloud coverage.  相似文献   

15.
Humans constitute one of the main geomorphological agents in modern times. As an example, post-mining regions represent a typical landscape of the Anthropocene. Strong relief modifications are particularly obvious with open pit mining. However, many existing mining areas are lacking detailed pre-mining information for the quantification of anthropogenic relief changes, which is a considerable challenge in regions with historic mining activities. Here, the Ville (Rhenish lignite district, Germany) is used to quantify surface mining induced relief changes in one of the oldest and currently largest lignite districts in Europe. Historical maps from first geodetic mapping in 1893 enabled construction of a historic digital elevation model to quantify the relief changes in comparison to elevation data from 2000 and 2015. The vertical accuracy of the historic data is remarkably high, with relief differences < 2 m in areas not affected by mining. In total, 49.2% of the investigated area (184 km2) shows a relief deficit and 14.5% has positive relief differences. Absolute changes account for more than 80 m heightening (dumpsites of overburden) and lowering of the natural relief (pits). Besides these altitudinal changes, overall steeper slopes are significant for the new topography, but levelling exists likewise. The spatial variabilities are discussed in the context of the regional geology and the mining techniques. Undoubtedly, such large-scale anthropogenic relief changes persist for a very long time and will last as a human legacy far into the future. Only the detailed reconstruction of the pre-mining relief offers the ability to clarify the dimension of humans as geomorphological agents and to understand landscape perception. Due to the fact that the impact of open pit mining has such a large vertical and horizontal extension, their consideration as part of anthropogeomorphology can significantly contribute to support future Critical Zone research in the Anthropocene.  相似文献   

16.
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17.
White Island is an active andesitic-dacitic composite volcano surrounded by sea, yet isolated from sea water by chemically sealed zones that confine a long-lived acidic hydrothermal system, within a thick sequence of fine-grained volcaniclastic sediment and ash. The rise of at least 106 m3 of basic andesite magma to shallow levels and its interaction with the hydrothermal system resulted in the longest historical eruption sequence at White Island in 1976–1982. About 107 m3 of mixed lithic and juvenile ejecta was erupted, accompanied by collapse to form two coalescing maar-like craters. Vent position within the craters changed 5 times during the eruption, but the vents were repeatedly re-established along a line linking pre-1976 vents. The eruption sequence consisted of seven alternating phases of phreatomagmatic and Strombolian volcanism. Strombolian eruptions were preceded and followed by mildly explosive degassing and production of incandescent, blocky juvenile ash from the margins of the magma body. Phreatomagmatic phases contained two styles of activity: (a) near-continuous emission of gas and ash and (b) discrete explosions followed by prolonged quiescence. The near-continuous activity reculted from streaming of magmatic volatiles and phreatic steam through open conduits, frittering juvennile shards from the margins of the magma and eroding loose lithic particles from the unconsolidated wall rock. The larger discrete explosions produced ballistic block aprons, downwind lobes of fall tephra, and cohesive wet surge deposits confined to the main crater. The key features of the larger explosions were their shallow focus, random occurrence and lack of precursors, and the thermal heterogeneity of the ejecta. This White Island eruption was unusual because of the low discharge rate of magma over an extended time period and because of the influence of a unique physical and hydrological setting. The low rate of magma rise led to very effective separation of magmatic volatiles and high fluxes of magmatic gas even during phreatic phases of the eruption. While true Strombolian phases did occur, more frequently the decoupled magmatic gas rose to interact with the conduit walls and hydrothermal system, producing phreatomagmatic eruptions. The form of these wet explosions was governed by a delicate balance between erosion and collapse of the weak conduit walls. If the walls were relatively stable, fine ash was slowly eroded and erupted in weak, near-continous phreatomagmatic events. When the walls were unstable, wall collapse triggered larger discrete phreatomagmatic explosions.  相似文献   

18.
ＴｈｅｒｅｓｅａｒｃｈｏｆａｓｏｌｕｔｉｏｎｏｎｌｏｃａｔｉｎｇｏｐｔｉｍａｌｌｙａｓｔａｔｉｏｎｆｏｒｓｅｉｓｍｉｃｄｉｓａｓｔｅｒｓｒｅｓｃｕｅｉｎａｃｉｔｙＱｉｎｇ－ＬｉｎＹＡＯ（姚清林）（ＩｎｓｔｉｔｕｔｅｏｆＧｅｏｌｏｇｙ，ＳｔａｔｅＳｅｉ...  相似文献   

19.
王海涛 《内陆地震》1989,3(2):176-179
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20.
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