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1.
Rock shear strength plays a fundamental role in volcano flank collapse, yet pertinent data from modern collapse surfaces are rare. Using samples collected from the inferred failure surface of the massive 1980 collapse of Mount St. Helens (MSH), we determined rock shear strength via laboratory tests designed to mimic conditions in the pre-collapse edifice. We observed that the 1980 failure shear surfaces formed primarily in pervasively shattered older dome rocks; failure was not localized in sloping volcanic strata or in weak, hydrothermally altered rocks. Our test results show that rock shear strength under large confining stresses is reduced ∼20% as a result of large quasi-static shear strain, as preceded the 1980 collapse of MSH. Using quasi-3D slope-stability modeling, we demonstrate that this mechanical weakening could have provoked edifice collapse, even in the absence of transiently elevated pore-fluid pressures or earthquake ground shaking. Progressive strength reduction could promote collapses at other volcanic edifices.  相似文献   

2.
 On 30 March 1956 a catastrophic directed blast took place at Bezymianny volcano. It was caused by the failure of 0.5 km3 portion of the volcanic edifice. The blast was generated by decompression of intra-crater dome and cryptodome that had formed during the preclimactic stage of the eruption. A violent pyroclastic surge formed as a result of the blast and spread in an easterly direction effecting an area of 500 km2 on the lower flank of the volcano. The thickness of the deposits, although variable, decreases with distance from the volcano from 2.5 m to 4 cm. The volume of the deposit is calculated to be 0.2–0.4 km3. On average, the deposits are 84% juvenile material (andesite), of which 55% is dense andesite and 29% vesicular andesite. On a plot of sorting vs median diameter (Inman coefficients) the deposits occupy the area between the fall and flow fields. In the proximal zone (less than 19 km from the volcano) three layers can be distinguished in the deposits. The lower one (layer A) is distributed all over the proximal area, is very poorly sorted, enriched in fragments of dense juvenile andesite and contains an admixture of soil and uncharred plant remains. The middle layer (layer B) is distributed in patches tens to hundreds of metres across on the surface of layer A. Layer B is relatively well sorted as a result of a very low content of fine fractions, and it contains rare charred plant remains. The uppermost layer (layer C) forms still smaller patches on the surface of layer B. Layer C is characterized by intermediate sorting, is enriched in vesicular juvenile andesitic fragments, and contains a high percentage of the fine fraction and very rare plant remains which are thoroughly charred. Maximum clast size decreases from layer A to layer C. The absence of internal cross bedding is a characteristic of all three layers. In the distal zone (more than 19 km from the volcano) stratigraphy changes abruptly. Deposit here consists of one layer 26 to 4 cm in thickness, is composed of wavy laminated sand with a touch of gravel, is well sorted and contains uncharred plant remains. The Bezymianny blast deposits are not analogous with known types of pyroclastic surges, with the exception of the directed blast deposits of the Mount St.Helens eruption of 18 May 1980. The peculiarities of deposits from these two eruptions allow them to be separated into a special type: blast surge. This type of surge is formed when failure of volcanic edifice relieves the pressure from an inter-crater dome and/or cryptodome. A model is proposed to explain the peculiarities of the formation, transportation and emplacement of the Bezymianny blast surge deposits. Received: 19 December 1994 / Accepted: 12 December 1995  相似文献   

3.
The explosion of a cryptodome at Mount St. Helens in 1980 produced two juvenile rock types that are derived from the same source magma. Their differences-color, texture and density-are due only to vesicularity differences. The vesicular gray dacite comprises bout 72% of the juvenile material; the black dacite comprises the other 28%. The density of juvenile dacite is bimodally distributed, with peaks at 1.6 g cm-3 (gray dacite) and 2.3 g cm-3 (black dacite). Water contents, deuterium abundances, and the relationship of petrographic structures to vapor-phase crystals indicate both rock types underwent pre-explosion subsurface vesiculation and degassing. The gray dacite underwent a second vesiculation event, probably during the 18 May explosion. In the subsurface, gases probably escaped through interconnected vesicles into the permeable volcanic edifice. We suggest that nonuniform degassing of an initially homogeneous magma produced volatile gradients in the cryptodome and that these gradients were responsible for the density bimodality. That is, water contents less than about 0.2–0.4 wt% produced vesicle growth rates that were slow in comparison to the pyroclast cooling rates; greater water contents produced vesicle growth rates that were fast in comparison to cooling rates. In this scheme, the dacite densities are bimodally distributed simply because, following decompression on 18 May 1980, one clast population vesiculated while the other did not. For clasts that did vesiculate, vesicle growth continued until it was arrested by fragmentation.  相似文献   

4.
El Chichón volcano is an andesite stratovolcano in southern México. It erupted in March 1982, after about 550 years of quiescence. The 1982 eruption of El Chichón has not been followed by the growth of a lava dome within the newly formed crater. This is rather anomalous since the construction of a new dome after the destruction of an old one is a common process during the eruptions at andesite and dacite volcanoes. To discuss this anomalous aspect of the El Chichón eruption, some regularity in the process of re-awakening of dormant (here defined as a period of quiescence of more than 100 years) andesite and dacite volcanoes are studied based on the seismic activity recorded at the volcanoes Bezymianny, Mount St. Helens, El Chichón, Unzen, Pinatubo and Soufrière Hills. Three stages were identified in the re-awakening activity of these volcanoes: (1) preliminary seismic activity, leading up to the first phreatic explosion; (2) activity between the first and the largest explosions; (3) post-explosion dome-building process. The eruptions were divided into two groups: low-VEI (Volcanic Explosivity Index) and the long duration stage-1 events (Unzen, 1991 and Soufrière Hills volcano, 1995) and high-VEI and the short duration stage-1 events (Bezymianny, 1956; Mount St. Helens, 1980; El Chichón, 1982 and Pinatubo, 1992). The comparative analysis of the seismo-eruptive activity of two eruptions of the second group, the 1980 of Mt. St. Helens and the 1982 of El Chichón, produced an explanation the absence of new dome building during the 1982 eruption of El Chichón volcano. It may be explained in terms of the unusually rapid emission of gas and water from the magmatic and hydrothermal system beneath the volcano during a relatively short sequence of large explosions that could have sharply increased the viscosity of the magma making impossible its exit to the surface.  相似文献   

5.
Major slope failures are a significant degradational process at volcanoes. Slope failures and associated explosive eruptions have resulted in more than 20 000 fatalities in the past 400 years; the historic record provides evidence for at least six of these events in the past century. Several historic debris avalanches exceed 1 km3 in volume. Holocene avalanches an order of magnitude larger have traveled 50–100 km from the source volcano and affected areas of 500–1500 km2. Historic eruptions associated with major slope failures include those with a magmatic component (Bezymianny type) and those solely phreatic (Bandai type). The associated gravitational failures remove major segments of the volcanoes, creating massive horseshoe-shaped depressions commonly of caldera size. The paroxysmal phase of a Bezymianny-type eruption may include powerful lateral explosions and pumiceous pyroclastic flows; it is often followed by construction of lava dome or pyroclastic cone in the new crater. Bandai-type eruptions begin and end with the paroxysmal phase, during which slope failure removes a portion of the edifice. Massive volcanic landslides can also occur without related explosive eruptions, as at the Unzen volcano in 1792.The main potential hazards from these events derive from lateral blasts, the debris avalanche itself, and avalanche-induced tsunamis. Lateral blasts produced by sudden decompression of hydrothermal and/or magmatic systems can devastate areas in excess of 500km2 at velocities exceeding 100 m s–1. The ratio of area covered to distance traveled for the Mount St. Helens and Bezymianny lateral blasts exceeds that of many pyroclastic flows or surges of comparable volume. The potential for large-scale lateral blasts is likely related to the location of magma at the time of slope failure and appears highest when magma has intruded into the upper edifice, as at Mount St. Helens and Bezymianny.Debris avalanches can move faster than 100 ms–1 and travel tens of kilometers. When not confined by valley walls, avalanches can affect wide areas beyond the volcano's flanks. Tsunamis from debris avalanches at coastal volcanoes have caused more fatalities than have the landslides themselves or associated eruptions. The probable travel distance (L) of avalanches can be estimated by considering the potential vertical drop (H). Data from a catalog of around 200 debris avalanches indicates that the H/L rations for avalanches with volumes of 0.1–1 km3 average 0.13 and range 0.09–0.18; for avalanches exceeding 1 km3, H/L ratios average 0.09 and range 0.5–0.13.Large-scale deformation of the volcanic edefice and intense local seismicity precede many slope failures and can indicate the likely failure direction and orientation of potential lateral blasts. The nature and duration of precursory activity vary widely, and the timing of slope faliure greatly affects the type of associated eruption. Bandai-type eruptions are particularly difficult to anticipate because they typically climax suddenly without precursory eruptions and may be preceded by only short periods of seismicity.  相似文献   

6.
 Physical properties of cryptodome and remelted samples of the Mount St. Helens grey dacite have been measured in the laboratory. The viscosity of cryptodome dacite measured by parallel–plate viscometry ranges from 10.82 to 9.94 log10 η (Pa s) (T=900–982  °C), and shrinkage effects were dilatometrically observed at T>900  °C. The viscosity of remelted dacite samples measured by the micropenetration method is 10.60–9.25 log10 η (Pa s) (T=736–802  °C) and viscosities measured by rotational viscometry are 3.22–1.66 log10 η (Pa s) (T=1298–1594  °C). Comparison of the measured viscosity of cryptodome dacitic samples with the calculated viscosity of corresponding water-bearing melt demonstrates significant deviations between measured and calculated values. This difference reflects a combination of the effect of crystals and vesicles on the viscosity of dacite as well as the insufficient experimental basis for the calculation of crystal-bearing vesicular melt viscosities at low temperature. Assuming that the cryptodome magma of the 18 May 1980 Mount St. Helens eruption was residing at 900  °C with a phenocryst content of 30 vol.%, a vesicularity of 36 vol.% and a bulk water content of 0.6 wt.%, we estimate the magma viscosity to be 1010.8 Pa s. Received: 25 August 1996 / Accepted: 19 July 1997  相似文献   

7.
The dispersal of an ash plume as it propagates downwind with the ash settling under gravity is presented. It is shown that wind shear, the initial plume height and the plume grain size distribution have an important role in the dispersal of the ash, in particular in determining the evolution of the upper surface and the leading edge of the ash cloud. Once the ash has thermally equilibrated with the atmosphere, the temperature of the upper surface of the ash plume is directly related to its altitude. As a result we can use the model to interpret satellite images of the temperature of the upper surface of ash plumes. These calculations are compared with new analyses of satellite data from the 18 May 1980 eruptions of Mount St Helens in which both thermal infra-red and visible GOES satellite data were examined. In accord with the data, the model is able to predict the rate at which the Mount St Helens ash plume propagated downwind through a combination of the wind shear and gravitational settling of different size particles. The model is also able to explain the observed thermal structure of the upper surface of the ash plume, in which the temperature initially decreases, but then increases with downwind distance as the ash falls through the tropopause.  相似文献   

8.
The edifice of Yate volcano, a dissected stratocone in the Andean Southern Volcanic Zone, has experienced multiple summit collapses throughout postglacial time restricted to sectors NE and SW of the summit. The largest such historic event occurred on 19th February 1965 when ～6.1–10?×?106 m3 of rock and ice detached from 2,000-m elevation to the SW of the summit and transformed into a debris flow. In the upper part of the flow path, velocities are estimated to have reached 40 m s?1. After travelling 7,500 m and descending 1,490 m, the flow entered an intermontane lake, Lago Cabrera. A wavemaker of estimated volume 9?±?3?×?106 m3 generated a tsunami with an estimated amplitude of 25 m and a run-up of ～60 m at the west end of the lake where a settlement disappeared with the loss of 27 lives. The landslide followed 15 days of unusually heavy summer rain, which may have caused failure by increasing pore water pressure in rock mechanically weathered through glacial action. The preferential collapse directions at Yate result from the volcano’s construction on the dextral strike-slip Liquiñe-Ofqui fault zone. Movement on the fault during the lifetime of the volcano is thought to have generated internal instabilities in the observed failure orientations, at ～10° to the fault zone in the Riedel shear direction. This mechanically weakened rock may have led to preferentially orientated glacial valleys, generating a feedback mechanism with collapse followed by rapid glacial erosion, accelerating the rate of incision into the edifice through repeated landslides. Debris flows with magnitudes similar to the 1965 event are likely to recur at Yate, with repeat times of the order of 102 years. With a warming climate, increased glacial meltwater due to snowline retreat and increasing rain, at the expense of snow, may accelerate rates of edifice collapse, with implications for landslide hazard and risk at glaciated volcanoes, in particular those in strike-slip tectonic settings where orientated structural instabilities may exist.  相似文献   

9.
Stratovolcanoes and lava domes are particularly susceptible to sector collapse resulting from wholesale rock failure as a consequence of decreasing rock strength. Here, we provide insights into the influence of thermal and cyclic stressing on the strength and mechanical properties of volcanic rocks. Specifically, this laboratory study examines the properties of samples from Mount St. Helens; chosen because its strength and stability have played a key role in its history, influencing the character of the infamous 1980 eruption. We find that thermal stressing exerts different effects on the strengths of different volcanic units; increasing the heterogeneity of rocks in situ. Increasing the uniaxial compressive stress generates cracking, the timing and magnitude of which was monitored via acoustic emission (AE) output during our experiments. AEs accelerated in the approach to failure, sometimes following the pattern predicted by the failure forecast method (Kilburn 2003). Crack damage during the experiments was tracked using the evolving static Young’s modulus and Poisson’s ratio, which represent the quasi-static deformation in volcanic edifices more accurately than dynamic elastic moduli which are usually implemented in volcanic models. Cyclic loading of these rocks resulted in a lower failure strength, confirming that volcanic rocks may be weakened by repeated inflation and deflation of the volcanic edifice. Additionally, volcanic rocks in this study undergo significant elastic hysteresis; in some instances, a material may fail at a stress lower than the peak stress which has previously been endured. Thus, a volcanic dome repeatedly inflated and deflated may progressively weaken, possibly inducing failure without necessarily exceeding earlier conditions.  相似文献   

10.
During the large explosions of the Bezymianny (1956), Shiveluch (1964) and Mount St. Helens (1980) volcanoes, 4.8·1012, 3.0·1012 and 8.2·1012 kg of resurgent and magmatic material were ejected respectively. The eruptions were preceded and accompanied by significant crustal deformations and by a great number of volcanic earthquakes. In all three cases, earthquakes with an energy of E = 109 J occurred 8–11 days before the eruption; their foci were at a distance of less than 5 km from the floor of the active crater and the power of earthquake swarms increased continuously and monotonously until the beginning of the eruption. The data obtained on deformations, earthquakes and volcanic activity may be used for the prediction of the place, time, energy and hazards of large explosions of andesitic volcanoes.  相似文献   

11.
—Catastrophic edifice and sector failure occur commonly on stratovolcanoes worldwide and in some cases leave telltale horseshoe-shaped calderas. Many of these failures are now recognised as having resulted from large-scale landsliding. These slides often transform into debris avalanches and lahars that can devastate populations downstream of the volcano. Research on these phenomena has been directed mainly at understanding avalanche mechanics and travel distances and related socioeconomic impacts. Few investigations have examined volcanic avalanche source characteristics. The focus of this paper is to 1) describe a methodology for obtaining rock strengths that control initial failure and 2) report results of rock mass strength testing from Mount Rainier and Mount Hood. Rock mass and shear strength for fresh and hydrothermally altered rocks were obtained by 1) utilizing rock strength and structural information obtained from field studies and 2) applying rock mechanics techniques common in mining and civil engineering to the edifice region. Rock mass and intact rock strength differences greatly in excess of one order of magnitude were obtained when comparing strength behavior of fresh and completely altered volcanic rock. The recognition and determination of marked strength differences existing on the volcano edifice and flank, when combined with detailed geologic mapping, can be used to quantify volcano stability assessment and improve hazard mitigation efforts.  相似文献   

12.
Eruptions of Mount St Helens (Washington, USA) decreased in intensity and explosivity after the main May 18, 1980 eruption. As the post-May 18 eruptions progressed, albitic plagioclase microlites began to appear in the matrix glass, although the bulk composition of erupted products, the phenocryst compositions and magmatic temperatures remained fairly constant. Equilibrium experiments on a Mount St Helens white pumice show that at 160 MPa water pressure and 900°C, conditions deduced for the 8 km deep magma storage zone, the stable plagioclase is An47. The microlites in the natural samples, which are more albitic, had to grow at lower water pressures during ascent. Isothermal decompression experiments reported here demonstrate that a decrease in water pressure from 160 to 2 MPa over four to eight days is capable of producing the albitic groundmass plagioclase and evolved melt compositions observed in post-May 18 1980 dacites. Because groundmass crystallization occurs over a period of days during and after decreases in pressure, microlite crystallization in the Mount St Helens dacites must have occurred during the ascent of each magma batch from a deep reservoir rather than continuously in a shallow holding chamber. This is consistent with data on the kinetics of amphibole breakdown, which require that a significant portion of magma vented in each eruption ascended from a depth of at least 6.5 km (160 MPa water pressure) in a matter of days. The size and shape of the microlite population have not been studied because of the small size of the experimental samples; it is possible that the texture continues to mature long after chemical equilibrium is approached. As the temperature, composition, crystal content and water content of magma in the deep reservoir remained approximately constant from May 1980 to at least March 1982, the spectacular decrease in eruption intensity during this period cannot be attributed to changes in viscosity or density of the magma. Simple fluld mechanical considerations indicate, however, that the observed changes in mass flux of magma can be modelled by a five-fold decrease in conduit radius from 35 to 7 m, produced perhaps by plating of magma along the conduit walls. The decreased ascent rates which accompanied the decrease in conduit radius can explain the change from closed-system to open-system degassing and the shift from explosive to effusive eruptions during 1980.  相似文献   

13.
Many volcanic edifices have a remarkably symmetric geometrical form. An example is Mount Fuji in Japan. We model this form assuming that the surface of the volcano is a surface of uniform hydraulic potential; that an erupting magma will follow the path of minimum resistance to the surface. In order to model the resistance to fluid flow we assume the volcanic edifice is a uniform porous medium. The vertical flow of magma is also resisted by the gravitational body force. If the volcano becomes too tall flank eruptions will widen it; if the volcano becomes too wide summit eruptions will increase its elevation. Using the Dupuit approximation for an unconfined aquifer it is shown that the percolation equation is applicable. As magma reaches the surface it is assumed to extend the solid, porous matrix. A similarity solution is obtained to this moving boundary problem. The solution predicts a uniform shape for all volcanoes. This shape is shown to be in excellent agreement with the geometrical form of Mount Fuji.  相似文献   

14.
 Mount Rainier is one of the most seismically active volcanoes in the Cascade Range, with an average of one to two high-frequency volcano-tectonic (or VT) earthquakes occurring directly beneath the summit in a given month. Despite this level of seismicity, little is known about its cause. The VT earthquakes occur at a steady rate in several clusters below the inferred base of the Quaternary volcanic edifice. More than half of 18 focal mechanisms determined for these events are normal, and most stress axes deviate significantly from the regional stress field. We argue that these characteristics are most consistent with earthquakes in response to processes associated with circulation of fluids and magmatic gases within and below the base of the edifice.Circulation of these fluids and gases has weakened rock and reduced effective stress to the point that gravity-induced brittle fracture, due to the weight of the overlying edifice, can occur. Results from seismic tomography and rock, water, and gas geochemistry studies support this interpretation. We combine constraints from these studies into a model for the magmatic system that includes a large volume of hot rock (temperatures greater than the brittle–ductile transition) with small pockets of melt and/or hot fluids at depths of 8–18 km below the summit. We infer that fluids and heat from this volume reach the edifice via a narrow conduit, resulting in fumarolic activity at the summit, hydrothermal alteration of the edifice, and seismicity. Received: 10 February 1999 / Accepted: 26 June 1999  相似文献   

15.
The 8-10 May 1997 eruption of Bezymianny volcano began with extrusion of a crystallized plug from the vent in the upper part of the dome. Progressive gravitational collapses of the plug caused decompression of highly crystalline magma in the upper conduit, leading at 13:12 local time on 9 May to a powerful, vertical Vulcanian explosion. The dense pyroclastic mixture collapsed in boil-over style to generate a pyroclastic surge which was focused toward the southeast by the steep-walled, 1956 horseshoe-shaped crater. This surge, with a temperature <200 °C, covered an elliptical area >30 km2 with deposits as much as 30 cm thick and extending 7 km from the vent. The surge deposits comprised massive to vaguely laminated, gravelly sand (Md -1.2 to 3.7J sorting 1.2 to 3J) of poorly vesiculated andesite (mean density 1.82 g cm-3; vesicularity 30 vol%; SiO2 content ~58.0 wt%). The deposits, with a volume of 5-15᎒6 m3, became finer grained and better sorted with distance; the maximal diameter of juvenile clasts decreased from 46 to 4 cm. The transport and deposition of the surge over a snowy landscape generated extensive lahars which traveled >30 km. Immediately following the surge, semi-vesiculated block-and-ash flows were emplaced as far as 4.7 km from the vent. Over time the juvenile lava in clasts of these flows became progressively less crystallized, apparently more silicic (59.0 to 59.9 wt% SiO2) and more vesiculated (density 1.64 to 1.12 g cm-3; vesicularity 37 to 57 vol%). At this stage the eruption showed transitional behavior, with mass divided between collapsing fountain and buoyant column. The youngest pumice-and-ash flows were accompanied by a sustained sub-Plinian eruption column ~14 km high, from which platy fallout clasts were deposited (~59.7% SiO2; density 1.09 g cm-3; vesicularity 58 vol%). The explosive activity lasted about 37 min and produced a total of ~0.026 km3 dense rock equivalent of magma, with an average discharge of ~1.2᎒4 m3 s-1. A lava flow ~200 m long terminated the eruption. The evolutionary succession of different eruptive styles during the explosive eruption was caused by vertical gradients in crystallization and volatile content of the conduit magma, which produced significant changes in the properties of the erupting mixture.  相似文献   

16.
In order to evaluate and further constrain models for volatile movement and vapor enrichment of magma stored at shallow levels, amphibole phenocrysts from 2004–2005 Mount St. Helens dacite were analyzed for major and selected trace elements (Li, Cu, Zn, Mn, and REE) and Li isotopes. Several recent studies have examined fluid-mobile trace element abundances in phencryst phases and melt inclusions as a means of tracking volatile movement within subvolcanic magmatic systems, and high Li contents in plagioclase phenocrysts from 1980 and 2004 Mount St. Helens dacites have been interpreted as evidence that shallow magma was fluxed by a Li-bearing vapor phase prior to eruption.  相似文献   

17.
Public statements about volcanic activity at Mount St. Helens include factual statements, forecasts, and predictions. A factual statement describes current conditions but does not anticipate future events. A forecast is a comparatively imprecise statement of the time, place, and nature of expected activity. A prediction is a comparatively precise statement of the time, place, and ideally, the nature and size of impending activity. A prediction usually covers a shorter time period than a forecast and is generally based dominantly on interpretations and measurements of ongoing processes and secondarily on a projection of past history. The three types of statements grade from one to another, and distinctions are sometimes arbitrary.Forecasts and predictions at Mount St. Helens became increasingly precise from 1975 to 1982. Stratigraphic studies led to a long-range forecast in 1975 of renewed eruptive activity at Mount St. Helens, possibly before the end of the century. On the basis of seismic, geodetic and geologic data, general forecasts for a landslide and eruption were issued in April 1980, before the catastrophic blast and landslide on 18 May 1980. All extrusions except two from June 1980 to the end of 1984 were predicted on the basis of integrated geophysical, geochemical, and geologic monitoring. The two extrusions that were not predicted were preceded by explosions that removed a substantial part of the dome, reducing confining pressure and essentially short-circuiting the normal precursors.  相似文献   

18.
There have been no substantial changes in the thermal patterns at the summit of Mount Rainier in the period September 1964–September 1966, within the detection limits of the infrared instrumentation. Some differences in radiance are attributed to differences in snow cover. The highest apparent temperature is at a snow-free area on the west flank of the summit cone, several hundred feet below the west crater rim. An anomaly at this site was recorded on both infrared surveys, but no prior reports of thermal activity here have been made by ground parties. Other anomalous thermal zones at the summit are on the northern quadrants of both crater rims. A very small, low-temperature fumarole reported on Mount Adams was not detected, nor were any other thermal manifestations recorded. One anomaly consisting of a close-spaced cluster of thermal spots was detected at The Boot on Mount St. Helens and corresponds to a known fumarole area. The only thermal feature seen on Mount Shasta is near the summit at a thermal spring that has been observed by many climbers. Two anomalies were found on the north flank of Lassen Peak. Thermal activity had not been previously reported at either site, though one is in a known solfatarized area. No ground investigation has been made at the other location. Much of the other thermal activity in the Lassen Peak area is in the northeast quadrant of Brokeoff Caldera. Most of these features are well documented in the literature; others not previously described are in fairly accessible areas and doubtless result from springs and fumaroles related to Brokeoff Caldera.  相似文献   

19.
The influence of magma expansion due to volatile exsolution and gas dilation on dyke propagation is studied using a new numerical code. Many natural magmas contain sufficient amounts of volatiles for fragmentation to occur well below Earth's surface. Magma fragmentation has been studied for volcanic flows through open conduits but it should also occur within dykes that rise towards Earth's surface. The characteristics of volatile-rich magma flow within a hydraulic fracture are studied numerically. The mixture of melt and gas is treated as a compressible viscous fluid below the fragmentation level and as a gas phase carrying melt droplets above it. The numerical code solves for elastic deformation of host rocks, the flow of the magmatic mixture and fracturing at the dyke tip. With volatile-free magma, a dyke fed at a constant rate in a uniform medium adopts a constant shape and width and rises at a constant velocity. With volatiles involved, magma expands and hence the volume flux of magma increases. With no fragmentation, this enhanced flux leads to acceleration and thinning of the dyke. Simple scaling laws allow accurate predictions of dyke width and ascent rate for a wide range of conditions. With fragmentation, dyke behaviour is markedly different. Due to the sharp drop of head loss that occurs in gas-rich fragmented material, large internal overpressures develop below the dyke tip and induce swelling of the nose region, leading to deceleration of the dyke. These results are applied to the two-month long period of volcanic unrest that preceded the May 1980 eruption of Mount St Helens. An initial phase of rapid earthquake migration from the 7–8 km deep reservoir to shallow levels was followed by very slow progression of magma within the edifice. Such behaviour can be accounted for by magma fragmentation at the top of a dyke.  相似文献   

20.
Three techniques of digital photogrammetry have been applied successfully to laboratory analogue models to study surface displacements caused by various volcano deformation types. Firstly, side-perspective videos are used to differentiate profile displacements between cryptodome intrusion models and models deforming by ductile inner-core viscous flow. Both models show similar morphologic features including a bulged flank and an asymmetric upper graben. However, differences in displacement trajectories of the bulge crest reflect upward intrusion push contrasting with essentially downward displacement vectors of weak core models. The other two techniques use vertical views correlated automatically either as time-sequence monoscopic views or as coeval stereoscopic pairs. This exploits to a maximum the method’s potential by imaging surface displacements over the whole model. Successive monoscopic photograms, because they suffer only moderate numerical processing for topographic effect removal, can detect very small displacements occurring early in deformation processes. As illustrated by analysis of intrusion models, the monoscopic method allows prediction of fault locations and main displacement locations. It can also anticipate the principal strain directions, and separate different deformation stages. On the other hand, the stereo-photogrammetry technique, although more complicated, provides topography and volume changes, as well as pictures of surface displacements in three dimensions. Results are presented for the spreading of volcano models on a ductile substratum and viscous cored cones. We have found digital photogrammetry to be a useful tool for analogue modelling, because it provides quantitative data on surface displacements, including movement invisible to the eye, as well as topographic changes. It is a good method for investigating and comparing different deformation mechanisms. It is especially useful for interpretation of displacement patterns obtained from monitoring of natural active volcanoes. In fact, results of the methods used in the laboratory can be directly compared with field data from geodetic or photogrammetric surveys, as at Mount St. Helens in 1980.  相似文献   

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