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1.
Anisotropy of magnetic susceptibility (AMS) data analysis is a convenient method used to investigate strain and flow during lava flow emplacement. In order to make a sound interpretation, the origin of the AMS signal must be verified. Two questions must be answered: 1) what phase, or phases carry the AMS signal and 2) when was the AMS fabric acquired? The verification steps themselves can provide extra data for interpreting lava flow conditions. Here, we present a methodology to answer the two questions in a 6 km-long Chaîne des Puys trachybasaltic lava flow that descended into the future site of Clermont Ferrand (France) 45,000 years ago. Knowledge of lava flow emplacement will be useful specifically to this site, if a reactivation of the volcanic chain occurs. The results are also of more general interest to understand lava flow emplacement dynamics.  相似文献   

2.
A long-standing question in lava flow studies has been how to infer emplacement conditions from information preserved in solidified flows. From a hazards perspective, volumetric flux (effusion rate) is the parameter of most interest for open-channel lava flows, as the effusion rate is important for estimating the final flow length, the rate of flow advance, and the eruption duration. The relationship between effusion rate, flow length, and flow advance rate is fairly well constrained for basaltic lava flows, where there are abundant recent examples for calibration. Less is known about flows of intermediate compositions (basaltic andesite to andesite), which are less frequent and where field measurements are limited by the large block sizes and the topographic relief of the flows. Here, we demonstrate ways in which high-resolution digital topography obtained using Light Detection and Ranging (LiDAR) systems can provide access to terrains where field measurements are difficult or impossible to collect. We map blocky lava flow units using LiDAR-generated bare earth digital terrain models (DTMs) of the Collier Cone lava flow in the central Oregon Cascades. We also develop methods using geographic information systems to extract and quantify morphologic features such as channel width, flow width, flow thickness, and slope. Morphometric data are then analyzed to estimate both effusion rates and emplacement times for the lava flow field. Our data indicate that most of the flow outline (which comprises the earliest, and most voluminous, flow unit) can be well explained by an average volumetric flux ～14–18?m3/s; channel data suggest an average flux ～3?m3/s for a later, channel-filling, flow unit. When combined with estimates of flow volume, these data suggest that the Collier Cone lava flow was most likely emplaced over a time scale of several months. This example illustrates ways in which high-resolution DTMs can be used to extract and analyze morphologic measurements and how these measurements can be analyzed to estimate emplacement conditions for inaccessible, heavily vegetated, or extraterrestrial lava flows.  相似文献   

3.
Summary Mean magnetisation directions from 6 lava flows of questionable historical age have been determined. The results, which are based upon very stringent linearity criteria during demagnetisation, point out erroneous dating and the corresponding lavas can be ascribed to medieval activity of Mount Etna, as already suggested for some of the investigated flows, covering a time interval of some centuries around AD 1000. The site mean direction of AD 1536 historically dated lava flow agrees with a previous study (Tanguy et al., 1985), while the site mean directions of AD 1566 and AD 1169/812 differ significantly from those reported earlier. Neither AD 812 nor AD 1169 dating is consistent with the site mean direction of the related flow being compatible with an emplacement age possibly a few decades before AD 1000. The 3 lava flows historically dated AD 1595 exhibit different within site mean directions indicating that they cannot be referred to coeval activity of the volcano; the site mean direction of one of the flows is compatible with an emplacement occurring very close to AD 1000. The resulting SVC is significantly shifted eastwards with respect to that of previous study (Tanguy et al., 1985). However, its older part nearly overlaps the medieval Vesuvius SVC relocated to Etna. This indicates that the strict linearity analysis carried out during demagnetisation has allowed to clearly define the characteristic magnetisations of the investigated lava flows. The adopted procedure is appropriate for performing successful magnetic stratigraphy surveys in volcanic areas.  相似文献   

4.
We studied the anisotropy of magnetic susceptibility (AMS) of 22 basaltic flow units, including S-type pahoehoe, P-type pahoehoe, toothpaste lava and 'a' emplaced over different slopes in two Hawaiian islands. Systematic differences occur in several aspects of AMS (mean susceptibility, degree of anisotropy, magnetic fabric and orientation of the principal susceptibilities) among the morphological types that can be related to different modes of lava emplacement. AMS also detects systematic changes in the rate of shear with position in a unit, allowing us to infer local flow direction and some other aspects of the velocity field of each unit. 'A' flows are subject to stronger deformation than pahoehoe, and also their internal parts behave more like a unit. According to AMS, the central part of pahoehoe commonly reveals a different deformation history than the upper and lower extremes, probably resulting from endogenous growth.  相似文献   

5.
In an attempt to model the effect of slope on the dynamics of lava flow emplacement, four distinct morphologies were repeatedly produced in a series of laboratory simulations where polyethylene glycol (PEG) was extruded at a constant rate beneath cold sucrose solution onto a uniform slope which could be varied from 1° through 60°. The lowest extrusion rates and slopes, and highest cooling rates, produced flows that rapidly crusted over and advanced through bulbous toes, or pillows (similar to subaerial “toey” pahoehoe flows and to submarine pillowed flows). As extrusion rate and slope increased, and cooling rate decreased, pillowed flows gave way to rifted flows (linear zones of liquid wax separated by plates of solid crust, similar to what is observed on the surface of convecting lava lakes), then to folded flows with surface crusts buckled transversely to the flow direction, and, at the highest extrusion rates and slopes, and lowest cooling rates, to leveed flows, which solidified only at their margins. A dimensionless parameter, Ψ, primarily controlled by effusion rate, cooling rate and flow viscosity, quantifies these flow types. Increasing the underlying slope up to 30° allows the liquid wax to advance further before solidifying, with an effect similar to that of increasing the effusion rate. For example, conditions that produce rifted flows on a 10° slope result in folded flows on a 30° slope. For underlying slopes of 40°, however, this trend reverses, slightly owing to increased gravitational forces relative to the strength of the solid wax. Because of its significant influence on heat advection and the disruption of a solid crust, slope must be incorporated into any quantitative attempt to correlate eruption parameters and lava flow morphologies. These experiments and subsequent scaling incorporate key physical parameters of both an extrusion and its environment, allowing their results to be used to interpret lava flow morphologies on land, on the sea floor, and on other planets.  相似文献   

6.
The 1614–1624 lava flow of Mt. Etna was formed during a long-duration flank eruption involving predominantly pahoehoe flows which produced unusual surface features including mega-tumuli (here defined) and terraces. Detailed mapping of the flow units, surface features, and associated tubes reveals a complex sequence of emplacement for the field. The stair-stepped terraces appear to have been formed as a consequence of self-damming of tube-fed flows which developed «perched» ponds of lava. Surges of lava through tubes elevated sections of crusted lava at the distal ends of the flow to generate tumuli, some as high as 130 m, as a consequence of pressure via «hydrostatic head» conditions within the tube. Although pahoehoe lavas and the related features described here are atypical of Mt. Etna, they may reflect styles of eruption and lava emplacement found on volcanoes elsewhere.  相似文献   

7.
The anisotropy of magnetic susceptibility (AMS) of lava flows is an innovative method which has been proved to be directly related to the shear history of lava. One of the advantages of this method is that it can be used in the absence of other morphological features commonly employed to study the mechanism of emplacement of lava flows. This feature of the AMS method makes it very attractive to gain insight into the mechanism of emplacement of massive, relatively featureless, long lava flows such as those forming flood basalt provinces. In this work, we report the results of the measurement of AMS as a function of vertical position within the Birkett lava flow, one of the Columbia River Basalt Group flows. The observed variation of AMS allows us to identify at least 16 discrete events of lava injection and to estimate the thickness of individual injection events. The AMS-estimated thickness of each injection event (in the range of 0.5-4.0 m) coincides with the range inferred for injected lava pulses in modern Hawaiian lava flows. Thus, the evidence provided by the AMS method supports the notion that at least some flood basalt lava flows were emplaced by the same mechanism as many present-day inflated pahoehoe flows. Regarding the orientation of the principal susceptibilities, in the central part of the flow they define a preferred orientation along an E-W trend, whereas in the outer parts of the flow they have a NNE-SSW trend. This difference in the orientation of the principal susceptibilities is interpreted as the result of a change of flow direction of the lava as emplacement progressed. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00445-002-0203-8.  相似文献   

8.
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   

9.
Basaltic 'a'ā lava flows often demonstrate compound morphology, consisting of many juxtaposed and superposed flow units. Following observations made during the 2001 eruption of Mt. Etna, Sicily, we examine the processes that can result from the superposition of flow units when the underlying units are sufficiently young to have immature crusts and deformable cores. During this eruption, we observed that the emplacement of new surface flow units may reactivate older, underlying units by squeezing the still-hot flow core away from the site of loading. Here, we illustrate three different styles of reactivation that depend on the time elapsed between the emplacement of the two flow units, hence the rheological contrast between them. For relatively long time intervals (2 to 15 days), and consequently significant rheological contrasts, superposition can pressurise the underlying flow unit, leading to crustal rupture and the subsequent extrusion of a small volume of high yield strength lava. Following shorter intervals (1 to 2 days), the increased pressure caused by superposition can result in renewed, slow advance of the underlying immature flow unit front. On timescales of < 1 day, where there is little rheological contrast between the two units, the thin intervening crust can be disrupted during superposition, allowing mixing of the flow cores, large-scale reactivation of both units, and widespread channel drainage. This mechanism may explain the presence of drained channels in flows that are known to have been cooling-limited, contrary to the usual interpretation of drainage as an indicator of volume-limited behaviour. Because the remobilisation of previously stagnant lava can occur swiftly and unexpectedly, it may pose a significant hazard during the emplacement of compound flows. Constant monitoring of flow development to identify areas where superposition is occurring is therefore recommended, as this may allow potentially hazardous rapid drainage events to be forecast. Reactivation processes should also be borne in mind when reconstructing the emplacement of old lava flow fields, as failure to recognise their effects may result in the misinterpretation of features such as drained channels.  相似文献   

10.
In laboratory experiments designed to model lava flow processes, liquid polyethylene glycol wax is forced through either a small hole or a long narrow slit onto the base of a tank of cold water, where it spreads laterally while cooling and solidifying at its surface. We observe the surface structure of the flow, and its dependence on the flow rate, thermal conditions and basal roughness. In each case, solidification of a crust during spreading gives rise to a number of different surface morphologies, each of which forms under a restricted range of conditions. The dominant morphologies, referred to as “pillows”, “rifts”, transverse folds, and marginal levees, correspond to features observed on natural lava flows. Results for radial spreading over a rough base and for spreading in two directions from the line source over both smooth and rough bases complement those reported earlier for radial spreading of solidifying wax from a point source on a smooth base. Together they indicate a robust dependence of morphology on the distance from the vent at which solid crust begins to form. This distance is, in turn, determined by the extrusion rate, the rheology of the liquid wax (or magma), the reduced gravity, the magnitude of the surface heat flux, and the amount of cooling required to solidify the flow surface. The results also indicate factors influencing the distribution of crust and its deformation, and may provide a means by which observations of surface morphology can be used to place constraints on the emplacement conditions of lava flows.  相似文献   

11.
 Lineated sheet flows are flat-lying, glassy lava flows characterized by a regular surface pattern of parallel grooves or furrows aligned with the flow direction. They are unique to the submarine environment. We propose that the lineations are developed within the collapsed interiors of partially ponded lobate sheet flows that initially inflate and then drain out during emplacement. During lava drainout, the original lobate crust founders and a new crust begins to grow on the subsiding lava surface. Lineated flow texture is created where molten lava emerges laterally from beneath a growing crust. The lineations are formed by raking of the emerging lava surface by irregularities on the bottom edge of the crust and are preserved owing to rapid chilling by seawater. Therefore, lineated sheet flows are the product of a specific sequence of events over a short period of time during the course of a deep submarine eruption. Received: 23 November 1998 / Accepted: 22 February 1999  相似文献   

12.
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   

13.
The morphological evolution of the Sciara del Fuoco, Stromboli, is described from a time series dataset formed by Digital Elevation Models and orthophotos derived by digitising historical contour maps compiled in 1868 and 1937 and by processing data from aerial surveys carried out between 1954 and 2009. All maps were co-registered in the same reference system and used to build a quantitative reconstruction of the morphological changes of the Sciara del Fuoco slope. The changes mainly relate to the emplacement of many lava flows and their successive erosion. A comparative quantitative analysis yields estimates of areas and volumes of the lava fields formed on the sub-aerial part of the Sciara del Fuoco during a number of effusive events between 1937 and 2001, some of them never assessed before. The results of the analysis constrain the interpretation of the evolution and the magnitude of the recent effusive activity at the Stromboli volcano. Despite some uncertainties due to widely spaced observation periods, the results integrate all available topographic knowledge and contribute to an understanding of the main characteristics of the recent effusive eruptive styles at Stromboli volcano.  相似文献   

14.
Summary The magnetic anisotropy of lava flows has been simulated using plaster of Paris containing about 2% of nickel filings. The ellipsoid of magnetic anisotropy was oriented with its long axis close to but tilted with respect to the flow direction and its intermediate axis in the flow plane perpendicular to the flow direction; the same alignment was obtained with four different experimental conditions. This result is at variance with observations on basalt flows in which the long axis of the magnetic ellipsoid is perpendicular to the flow direction. It appears likely that the favoured alignment of elongated grains in a fluid or semi-fluid flow is sensitive to physical parameters which have not yet been examined adequately. Reliable magnetic determinations of flow directions in lavas must await clarification of this problem.  相似文献   

15.
Summary Evidence for flow structure within hexagonal columns of basalt has been sought, using measurements of the anisotropy of magnetic susceptibility. The observed magnetic grain alignment indicates that the flow was essentially uniform and horizontal across the hexagons which were sampled. The flow probably occurred during emplacement of the lava; there is no evidence of convection within the hexagons, either from the degree of grain alignment or its orientation.  相似文献   

16.
Long-lived basaltic eruptions often produce structurally complex, compound `a`ā flow fields. Here we reconstruct the development of a compound flow field emplaced during the 2001 eruption of Mt. Etna (Italy). Following an initial phase of cooling-limited advance, the reactivation of stationary flows by superposition of new units caused significant channel drainage. Later, blockages in the channel and effusion rate variations resulted in breaching events that produced two new major flow branches. We also examined small-scale, late-stage ‘squeeze-up’ extrusions that were widespread in the flow field. We classified these as ‘flows’, ‘tumuli’ or ‘spines’ on the basis of their morphology, which depended on the rheology, extrusion rate and cooling history of the lava. Squeeze-up flows were produced when the lava was fluid enough to drain away from the source bocca, but fragmented to produce blade-like features that differed markedly from `a`ā clinker. As activity waned, increased cooling and degassing led to lava arriving at boccas with a higher yield strength. In many cases this was unable to flow after extrusion, and laterally extensive, near-vertical sheets of lava developed. These are considered to be exogenous forms of tumuli. In the highest yield strength cases, near-solid lava was extruded from the flow core as a result of ramping, forming spines. The morphology and location of the squeeze-ups provides insight into the flow rheology at the time of their formation. Because they represent the final stages of activity of the flow, they may also help to refine estimates of the most advanced rheological states in which lava can be considered to flow. Our observations suggest that real-time monitoring of compound flow field evolution may allow complex processes such as channel breaching and bocca formation to be forecast. In addition, documenting the occurrence and morphology of squeeze-ups may allow us to determine whether there is any risk of a stalled flow front being reactivated. This will therefore enhance our ability to track and assess hazard posed by lava flow emplacement.  相似文献   

17.
A petrological study of the eruptive products of El Reventador allowed us to infer the magmatic processes related to the 2002 and 2004–05 eruptions of this andesitic stratovolcano. On November 3, 2002, El Reventador experienced a highly explosive event, which was followed by emplacement of two lava flows in November–December 2002. Silica contents range from 62 to 58 wt.% SiO2 for the November 3 pyroclastic deposits to 58–56 and 54–53 wt.% SiO2 for the successive lava flows. In November 2004 eruptive activity resumed supplying four new lava flows (56–54 wt.% SiO2) between November 2004 and August 2005.  相似文献   

18.
The present study is probably the first of its kind in the Deccan Volcanic Province (DVP) that deals in detail with the morphology and emplacement of the Deccan Trap flows, and employs modern terminology and concepts of flow emplacement. We describe in detail the two major types of flows that occur in this province. Compound pahoehoe flows, similar to those in Hawaii and the Columbia River Basalts (CRB) constitute the older stratigraphic Formations. These are thick flows, displaying the entire range of pahoehoe morphology including inflated sheets, hummocky flows, and tumuli. In general, they show the same three-part structure associated with pahoehoe flows from other provinces. However, in contrast to the CRB, pahoehoe lobes in the DVP are smaller, and hummocky flows are quite common. 'Simple' flows occur in the younger Formations and form extensive sheets capped by highly vesicular, weathered crusts, or flow-top breccias. These flows have few analogues in other provinces. Although considered to be a'a flows by previous workers, the present study clearly reveals that the simple flows differ considerably from typical a'a flows, especially those of the proximal variety. This is very significant in the context of models of flood basalt emplacement. At the same time, they do not display direct evidence of endogenous growth. Understanding the emplacement of these flows will go a long way in determining whether all extensive flows are indeed inflated flows, as has recently been postulated.Most of the studies relating to the emplacement of Continental Flood Basalt (CFB) lavas have relied on observations of flows from the CRB. Much of the current controversy surrounding the emplacement of CFB flows centers around the comparison of Hawaiian lava flows to those from the CRB. We demonstrate that the DVP displays a variety of lava features that are similar to those from the CRB as well as those from Hawaii. This suggests that there may have been more than one mechanism or style for the emplacement of CFB flows. These need to be taken into account before arriving at any general model for flood basalt emplacement.Editorial responsibility: T. Druitt  相似文献   

19.
Historic and recent (last 2,000?years) eruptions on the active volcanic island of Tenerife have been predominantly effusive, indicating that this is the most probable type of activity to be expected in the near future. In the past, lava flow invasion caused major damage on the island, and as the population and infrastructure have increased dramatically since the last eruption, lava flows are the most important short-term volcanic risk on Tenerife. Hence, an understanding of lava flow behaviour is vital to manage risks from lava flows and minimise future losses on the island. This paper focuses on the lava flows from the historic eruptions in Tenerife, providing new data on the volumes emitted, advance rates and the timing of the emplacement of flows. The studies show three main stages in the development of unconfined flow fields: the first stage, corresponding to the fast advance of the initial fronts during the first 24?C36?h of eruption (reaching calculated velocities of up to 1.1?m/s); the second stage, in which fronts stagnate; and a third stage, in which secondary lava flows develop from breakouts 4?C7?days after the initial eruption and farther extend the flow field (velocities of up to 0.02?m/s have been calculated for this stage). The breakouts identified originated at sites both proximal and distal to the vent and, in both cases, caused damage through lengthening and widening the original flow field. Hence, the probability of damage from lavas to land and property is highest during stages 1 and 3, and this should be accounted for when planning the response to a future effusive eruption. Tenerife??s lava flows display a similar behaviour to that of lava flows on volcanoes characterised by basaltic effusive activity (such as Etna or Kilauea), indicating the possibility of applying forecasting models developed at those frequently active volcanoes to Tenerife.  相似文献   

20.
Volcanic geomorphologists have investigated various relationships between eruption rate and morphologic parameters of lava flows, particularly with regard to preferred statistical correlations and the conditions under which they are valid. Here we employ two simple models for lava flow heat loss by Stefan-Boltzmann radiation to derive eruption rate versus planimetric area relationships. Both of these models predict a linear relationship between eruption rate and planimetric area, modulated by distinct prefactors potentially sensitive to compositional and temperature differences among different flows. Regardless of any theoretical considerations, we show that eruption rate is highly correlated with planimetric area for the Hawaiian basaltic flows analyzed in this work. Moreover, this observed correlation is superior to those from other obvious combinations of eruption rate and flow dimensions. On the basis of the theoretical models for lava flow heat loss, the correlations obtained here suggest that the surfaces of Hawaiian flows radiate at an effective temperature much less than the inner parts of the flowing lava in agreement with numerous field observations. This work also indicates that eruption rate versus planimetric area correlations can be markedly degraded when data from different vents, volcanoes and epochs are combined. These previously unrecognized sensitivities identified by the thermal loss modeling may have contributed to past unresolved debates on relationships between eruption rates and morphologic dimensions.  相似文献   

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