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1.
The largest natrocarbonatite lava flow eruption ever documented at Oldoinyo Lengai, NW Tanzania, occurred from March 25 to April 5, 2006, in two main phases. It was associated with hornito collapse, rapid extrusion of lava covering a third of the crater and emplacement of a 3-km long compound rubbly pahoehoe to blocky aa-like flow on the W flank. The eruption was followed by rapid enlargement of a pit crater. The erupted natrocarbonatite lava has high silica content (3% SiO2). The eruption chronology is reconstructed from eyewitness and news media reports and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data, which provide the most reliable evidence to constrain the eruption’s onset and variations in activity. The eruption products were mapped in the field and the total erupted lava volume estimated at 9.2 ± 3.0 × 105 m3. The event chronology and field evidence are consistent with vent construct instability causing magma mixing and rapid extrusion from shallow reservoirs. It provides new insights into and highlights the evolution of the shallow magmatic system at this unique natrocarbonatite volcano.  相似文献   

2.
We present thermal measurements made by high spatial resolution ground-based (a hand-held thermal camera) and low spatial resolution space-based (MODIS) instruments for a lava flow field active during the last phase of the May–July 2003 eruption at Piton de la Fournaise (La Réunion). Multiple oblique ground-based thermal images were merged to provide full coverage of the flow-field. These were then corrected for path length attenuation and orthorectified, allowing the at-surface radiance emitted by the flow-field to be estimated. Comparison with the radiance recorded by the MODIS sensors during the eruption reveals that, for clear-sky conditions and moderate-to-low viewing angles (satellite zenith <40°), the satellite measurements represent ∼90% of the at-surface radiance, and thus represent valuable data for quantifying volcanic thermal anomalies. Nevertheless, extreme viewing geometries and the presence of clouds strongly affect the radiance reaching the sensor and affected data from 94% of the overpasses. Ground-based thermal data were used to investigate an empirical relationship between the radiant heat flux and lava discharge rate during the emplacement of pahoehoe flows. While the average radiation temperature for flow surface that were 6–24 h old ranged between 500 K and 625 K, the ratio between radiative heat flux and Time-Averaged lava Discharge Rate (TADR) ranged between 1.5 × 108 J m−3 and 3.5 × 108 J m−3. This relationship was used to estimate TADR values from optimal MODIS data and produced results in line with those obtained from GPS surveys (Coppola et al. 2005). Our results underscore the importance of ground-based thermal analysis for the interpretation of satellite measurements, particularly in terms of calculating discharge rate trends.  相似文献   

3.
The eruption of the Pelagatos scoria cone in the Sierra Chichinautzin monogenetic field near the southern suburbs of Mexico City occurred less than 14,000 years ago. The eruption initiated at a fissure with an effusive phase that formed a 7-km-long lava flow, and continued with a phase of alternating and/or simultaneous explosive and effusive activity that built a 50-m-high scoria cone on the western end of the fissure and formed a compound lava flow-field near the vent. The eruption ended with the emplacement of a short lava flow that breached the cone and was accompanied by weak explosions at the crater. Products consist of a microlite-rich high-Mg basaltic andesite. Samples were analyzed to determine the magma’s initial properties as well as the effects of degassing-induced crystallization on eruptive style. Although distal ash fallout deposits from this eruption are not preserved, a recent quarry exposes a large section of the scoria cone. Detailed study of exposed layers allows us to elucidate the mode of cone-building activity. Petrological and textural data, combined with models calibrated by experimental work and melt-inclusion analyses of similar magmas elsewhere, indicate that the magma was initially hot (>1,200°C), gas-rich (up to 5 wt.% H2O), crystal-poor (~10 vol.% Fo90 olivine phenocrysts) and thus poorly viscous (40–80 Pa s). During the early phase, low magma ascent velocity at the fissure vent allowed low-viscosity magma to degas and crystallize during ascent, producing lava flows with elevated crystal contents at T < 1,100°C, and blocky surfaces. Later, the closure of the fissure by cooling dikes focused the magma flow at a narrow section of the fissure. This led to an increased magma ascent velocity. Rapid and shallow degassing (<3 km deep) triggered ~40 vol.% microlite crystallization. Limited times for gas-escape and higher magma viscosity (6 × 105–4 × 106 Pa s) drove strong explosions of highly (60–80 vol.%) and finely vesicular magma. Coarse clasts broke on landing, which implies brittle behavior due to complete solidification. This requires sufficient time to cool and in turn implies ejection heights of over 1 km, which is much higher than “normal” Strombolian activity. Hence, magma viscosity significantly impacts eruption style at monogenetic volcanoes because it affects the kinetics of shallow degassing. The long-lasting eruptions of Jorullo and Paricutin, which produced similar magmas in western México, were more explosive. This can be related to higher magma fluxes and total erupted volumes. Implications of this study are important because basaltic andesites are commonly erupted to form monogenetic scoria cones of the Trans-Mexican Volcanic Belt.  相似文献   

4.
Petrological, volcanological and geochronological data collected at Mathews Tuya together provide constraints on paleoclimate conditions during formation of the edifice. The basaltic tuya was produced via Pleistocene glaciovolcanism in northern British Columbia, Canada, and is located within the Tuya volcanic field (59.195°N/130.434°W), which is part of the northern Cordilleran volcanic province (NCVP). The edifice comprises a variety of lithofacies, including columnar-jointed lava, pillow lava, massive dikes, and volcaniclastic rocks. Collectively these deposits record the transition from an explosive subaqueous to an effusive subaerial eruption environment dominated by Pleistocene ice. As is typical for tuyas, the volcaniclastic facies record multiple fragmentation processes including explosive, quench and mechanical fragmentation. All samples from Mathews Tuya are olivine-plagioclase porphyritic alkali olivine basalts. They are mineralogically and geochemically similar to nearby glaciovolcanic centers from the southeastern part of the Tuya volcanic field (e.g., Ash Mountain, South Tuya, Tuya Butte) as well as the dominant NCVP rock type. Crystallization scenarios calculated with MELTS account for variations between whole rock and glass compositions via low pressure fractionation. The presence of olivine microphenocrysts and the absence of pyroxene phenocrysts constrain initial crystallization pressures to less than 0.6 GPa. The eruption of Mathews Tuya occurred between 0.718 ± 0.054 Ma and 0.742 ± 0.081 Ma based on 40Ar/39Ar geochronology (weighted mean age of 0.730 Ma). The age determinations provide the first firm documentation for large (>700 m thick), pre-Fraser/Wisconsin glaciers in north-central British Columbia ~0.730 Ma, and correlate in age with glaciovolcanic deposits in Russia (e.g., Komatsu et al. Geomorph 88: 352-366, 2007) and with marine isotopic evidence for large global ice volumes ~0.730 Ma.  相似文献   

5.
The August 1991 eruptions of Hudson volcano produced ~2.7 km3 (dense rock equivalent, DRE) of basaltic to trachyandesitic pyroclastic deposits, making it one of the largest historical eruptions in South America. Phase 1 of the eruption (P1, April 8) involved both lava flows and a phreatomagmatic eruption from a fissure located in the NW corner of the caldera. The paroxysmal phase (P2) began several days later (April 12) with a Plinian-style eruption from a different vent 4 km to the south-southeast. Tephra from the 1991 eruption ranges in composition from basalt (phase 1) to trachyandesite (phase 2), with a distinct gap between the two erupted phases from 54–60 wt% SiO2. A trend of decreasing SiO2 is evident from the earliest part of the phase 2 eruption (unit A, 63–65 wt% SiO2) to the end (unit D, 60–63 wt% SiO2). Melt inclusion data and textures suggest that mixing occurred in magmas from both eruptive phases. The basaltic and trachyandesitic magmas can be genetically related through both magma mixing and fractional crystallization processes. A combination of observed phase assemblages, inferred water content, crystallinity, and geothermometry estimates suggest pre-eruptive storage of the phase 2 trachyandesite at pressures between ~50–100 megapascal (MPa) at 972 ± 26°C under water-saturated conditions (log fO2 –10.33 (±0.2)). It is proposed that rising P1 basaltic magma intersected the lower part of the P2 magma storage region between 2 and 3 km depth. Subsequent mixing between the two magmas preferentially hybridized the lower part of the chamber. Basaltic magma continued advancing towards the surface as a dyke to eventually be erupted in the northwestern part of the Hudson caldera. The presence of tachylite in the P1 products suggests that some of the magma was stalled close to the surface (<0.5 km) prior to eruption. Seismicity related to magma movement and the P1 eruption, combined with chamber overpressure associated with basalt injection, may have created a pathway to the surface for the trachyandesite magma and subsequent P2 eruption at a different vent 4 km to the south-southeast. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

6.
Forward-Looking Infrared (FLIR) nighttime thermal images were used to extract the thermal and morphological properties for the surface of a blocky-to-rubbley lava mass active within the summit crater of the Caliente vent at Santiaguito lava dome (Guatemala). Thermally the crater was characterized by three concentric regions: a hot outer annulus of loose fine material at 150–400°C, an inner cold annulus of blocky lava at 40–80°C, and a warm central core at 100–200°C comprising younger, hotter lava. Intermittent explosions resulted in thermal renewal of some surfaces, mostly across the outer annulus where loose, fine, fill material was ejected to expose hotter, underlying, material. Surface heat flux densities (radiative + free convection) were dominated by losses from the outer annulus (0.3–1.5 × 104  s−1m−2), followed by the hot central core (0.1–0.4 × 104 J s−1m−2) and cold annulus (0.04–0.1 × 104 J s−1m−2). Overall surface power output was also dominated by the outer annulus region (31–176 MJ s−1), but the cold annulus contributed equal power (2.41–7.07 MJ s−1) as the hot central core (2.68–6.92 MJ s−1) due to its greater area. Cooled surfaces (i.e. the upper thermal boundary layer separating surface temperatures from underlying material at magmatic temperatures) across the central core and cold annulus had estimated thicknesses, based on simple conductive model, of 0.3–2.2 and 1.5–4.3 m. The stability of the thermal structure through time and between explosions indicates that it is linked to a deeper structural control likely comprising a central massive plug, feeding lava flow from the SW rim of the crater, surrounded by an arcuate, marginal fracture zone through which heat and mass can preferentially flow.  相似文献   

7.
 Samples collected from a lava channel active at Kīlauea Volcano during May 1997 are used to constrain rates of lava cooling and crystallization during early stages of flow. Lava erupted at near-liquidus temperatures (∼1150  °C) cooled and crystallized rapidly in upper parts of the channel. Glass geothermometry indicates cooling by 12–14  °C over the first 2 km of transport. At flow velocities of 1–2 m/s, this translates to cooling rates of 22–50  °C/h. Cooling rates this high can be explained by radiative cooling of a well-stirred flow, consistent with observations of non-steady flow in proximal regions of the channel. Crystallization of plagioclase and pyroxene microlites occurred in response to cooling, with crystallization rates of 20–50% per hour. Crystallization proceeded primarily by nucleation of new crystals, and nucleation rates of ∼104/cm3s are similar to those measured in the 1984 open channel flow from Mauna Loa Volcano. There is no evidence for the large nucleation delays commonly assumed for plagioclase crystallization in basaltic melts, possibly a reflection of enhanced nucleation due to stirring of the flow. The transition of the flow surface morphology from pāhoehoe to 'a'ā occurred at a distance of 1.9 km from the vent. At this point, the flow was thermally stratified, with an interior temperature of ∼1137  °C and crystallinity of ∼15%, and a flow surface temperature of ∼1100  °C and crystallinity of ∼45%. 'A'ā formation initiated along channel margins, where crust was continuously disrupted, and involved tearing and clotting of the flow surface. Both observations suggest that the transition involved crossing of a rheological threshold. We suggest this threshold to be the development of a lava yield strength sufficient to prevent viscous flow of lava at the channel margin. We use this concept to propose that 'a'ā formation in open channels requires both sufficiently high strain rates for continued disruption of surface crusts and sufficient groundmass crystallinity to generate a yield strength equivalent to the imposed stress. In Hawai'i, where lava is typically microlite poor on eruption, these combined requirements help to explain two common observations on 'a'ā formation: (a) 'a'ā flow fields are generated when effusion rates are high (thus promoting crustal disruption); and (b) under most eruption conditions, lava issues from the vent as pāhoehoe and changes to 'a'ā only after flowing some distance, thus permitting sufficient crystallization. Received: 3 September 1998 / Accepted: 12 April 1999  相似文献   

8.
The ~4-ka trachytic Rungwe Pumice (RP) deposit from Rungwe Volcano in South-Western Tanzania is the first Plinian-style deposit from an African volcano to be closely documented focusing on its physical characterization. The RP is a mostly massive fall deposit with an inversely graded base. Empirical models suggest a maximum eruption column height H T of 30.5–35 km with an associated peak mass discharge rate of 2.8–4.8 × 108 kg/s. Analytical calculations result in H T values of 33 ± 4 km (inversion of TEPHRA2 model on grain size data) corresponding to mass discharge ranging from 2.3 to 6.0 × 108 kg/s. Lake-core data allow extrapolation of the deposit thinning trend far beyond onland exposures. Empirical fitting of thickness data yields volume estimates between 3.2 and 5.8 km3 (corresponding to an erupted mass of 1.1–2.0 × 1012 kg), whereas analytical derivation yields an erupted mass of 1.1 × 1012 kg (inversion of TEPHRA2 model). Modelling and dispersal maps are consistent with nearly no-wind conditions during the eruption. The plume corner is estimated to have been ca. 11–12 km from the vent. After an opening phase with gradually increasing intensity, a high discharge rate was maintained throughout the eruption, without fountain collapse as is evidenced by a lack of pyroclastic density current deposits.  相似文献   

9.
 Results are presented from 11 microgravity surveys on Mt. Etna between 1987 and 1993, a period including the major 1989 and 1991–1993 flank eruptions and subordinate 1990 activity. Measurements were made with LaCoste and Romberg D-62 and D-157 gravity meters along a network around the volcano between 1000 and 1900 m a.s.l. and, since 1992, a N–S summit profile. Gravity changes of as much as 200 μGal were observed at scales from the size of the summit region to that of the volcano. None was associated with significant changes in ground elevation. The data show an increase in gravity for 2 years before the 1989 eruption. The increase is attributed to the accumulation of magma (0.25–1.7×109 m3) in an elongate zone, oriented NNW–SSE, between 2.5 and 6 km below sea level. Part of this magma was injected into the volcanic pile to supply the 1989 and 1990 eruptions. It also probably fed the start of the 1991–1993 eruption, since this event was not preceded by significant gravity changes. A large gravity increase (up to 140 μGal) detected across the volcano between June and September 1992 is consistent with the arrival in the accumulation zone of 0.32–2.2×109 m3 of new magma, thus favoring continued flank effusion until 1993. A large gravity decrease (200 μGal) in the summit region marked the closing stages of the 1991–1993 event and is associated with magma drainage from the upper levels of Etna's central feeding system. Received: 15 July 1995 / Accepted: 27 October 1997  相似文献   

10.
 High-resolution bathymetric mapping has shown that submarine flat-topped volcanic cones, morphologically similar to ones on the deep sea floor and near mid-ocean ridges, are common on or near submarine rift zones of Kilauea, Kohala (or Mauna Kea), Mahukona, and Haleakala volcanoes. Four flat-topped cones on Kohala were explored and sampled with the Pisces V submersible in October 1998. Samples show that flat-topped cones on rift zones are constructed of tholeiitic basalt erupted during the shield stage. Similarly shaped flat-topped cones on the northwest submarine flank of Ni'ihau are apparently formed of alkalic basalt erupted during the rejuvenated stage. Submarine postshield-stage eruptions on Hilo Ridge, Mahukona, Hana Ridge, and offshore Ni'ihau form pointed cones of alkalic basalt and hawaiite. The shield stage flat-topped cones have steep (∼25°) sides, remarkably flat horizontal tops, basal diameters of 1–3 km, and heights <300 m. The flat tops commonly have either a low mound or a deep crater in the center. The rejuvenated-stage flat-topped cones have the same shape with steep sides and flat horizontal tops, but are much larger with basal diameters up to 5.5 km and heights commonly greater than 200 m. The flat tops have a central low mound, shallow crater, or levees that surrounded lava ponds as large as 1 km across. Most of the rejuvenated-stage flat-topped cones formed on slopes <10° and formed adjacent semicircular steps down the flank of Ni'ihau, rather than circular structures. All the flat-topped cones appear to be monogenetic and formed during steady effusive eruptions lasting years to decades. These, and other submarine volcanic cones of similar size and shape, apparently form as continuously overflowing submarine lava ponds. A lava pond surrounded by a levee forms above a sea-floor vent. As lava continues to flow into the pond, the lava flow surface rises and overflows the lowest point on the levee, forming elongate pillow lava flows that simultaneously build the rim outward and upward, but also dam and fill in the low point on the rim. The process repeats at the new lowest point, forming a circular structure with a flat horizontal top and steep pillowed margins. There is a delicate balance between lava (heat) supply to the pond and cooling and thickening of the floating crust. Factors that facilitate construction of such landforms include effusive eruption of lava with low volatile contents, moderate to high confining pressure at moderate to great ocean depth, long-lived steady eruption (years to decades), moderate effusion rates (probably ca. 0.1 km3/year), and low, but not necessarily flat, slopes. With higher effusion rates, sheet flows flood the slope. With lower effusion rates, pillow mounds form. Hawaiian shield-stage eruptions begin as fissure eruptions. If the eruption is too brief, it will not consolidate activity at a point, and fissure-fed flows will form a pond with irregular levees. The pond will solidify between eruptive pulses if the eruption is not steady. Lava that is too volatile rich or that is erupted in too shallow water will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones, as occurs during the post-shield stage. The steady effusion of lava on land constructs lava shields, which are probably the subaerial analogs to submarine flat-topped cones but formed under different cooling conditions. Received: 30 September 1999 / Accepted: 9 March 2000  相似文献   

11.
The November 2002 eruption of Piton de la Fournaise in the Indian Ocean was typical of the activity of the volcano from 1999 to 2006 in terms of duration and volume of magma ejected. The first magma erupted was a basaltic liquid with a small proportion of olivine phenocrysts (Fo81) that contain small numbers of melt inclusions. In subsequent flows, olivine crystals were more abundant and richer in Mg (Fo83–84). These crystals contain numerous melt and fluid inclusions, healed fractures, and dislocation features such as kink bands. The major element composition of melt inclusions in this later olivine (Fo83–84) is out of equilibrium with that of its host as a result of extensive post-entrapment crystallization and Fe2+ loss by diffusion during cooling. Melt inclusions in Fo81 olivine are also chemically out of equilibrium with their hosts but to a lesser degree. Using olivine–melt geothermometry, we determined that melt inclusions in Fo81 olivine were trapped at lower temperature (1,182 ± 1°C) than inclusions in Fo83–84 olivine (1,199–1,227°C). This methodology was also used to estimate eruption temperatures. The November 2002 melt inclusion compositions suggest that they were at temperatures between 1,070°C and 1,133°C immediately before eruption and quenching. This relatively wide temperature range may reflect the fact that most of the melt inclusions were from olivine in lava samples and therefore likely underwent minor but variable amounts of post-eruptive crystallization and Fe2+ loss by diffusion due to their relatively slow cooling on the surface. In contrast, melt inclusions in tephra samples from past major eruptions yielded a narrower range of higher eruption temperatures (1,163–1,181°C). The melt inclusion data presented here and in earlier publications are consistent with a model of magma recharge from depth during major eruptions, followed by storage, cooling, and crystallization at shallow levels prior to expulsion during events similar in magnitude to the relatively small November 2002 eruption.  相似文献   

12.
Lava flowing into a pit crater will become entrapped to form an inactive lava lake. At Masaya volcano (Nicaragua) pit filling lavas are exposed in the walls of Nindiri, Santiago and San Pedro pits. Mapping of these lavas shows that fill can involve emplacement of both ’a’a and pahoehoe, with single fill units ranging in thickness from 2 to 22 m. Thick units with columnar joints were emplaced as simple inactive lava lakes during high effusion rate episodes. Sequences of thinner units, which can form pit floor shields or compound lakes, were emplaced at lower effusion rates. Lava withdrawal caused unsupported sections of three 20-m-thick units to subside, resulting in unit flexure and faulting, and viscous peeling features reveal that subsidence occurred while at least one unit was still partially molten. Where withdrawal has not occurred, fill sequences are flat lying and symmetrically distributed around the feeder structures (cinder cones and dykes). The filled Nindiri pit holds 5 × 107 m3 of lava in a 215-m-thick sequence. Partial fill of Santiago pit with 1 × 107 m3 of lava has filled the pit with a 110-m-thick lava sequence, of which ∼50% has been consumed by formation of a secondary pit. Altogether, 6.4 × 107 m3 of lava was erupted into Nindiri and Santiago during 1525–1965, with 94% of this volume remaining pit-contained; the remainder forms a north flank lava flow field. Pit development and filling is a dynamic and ephemeral process, having short-lived effects on volcano morphology, where pits develop and fill over hours-to-centuries. However, pits play an important role in shaping an edifice, representing lava sinks and controlling whether lavas are trapped or able to spread onto the flanks.  相似文献   

13.
The most voluminous eruption of natrocarbonatite lava hitherto recorded on Earth occurred at Oldoinyo Lengai in March–April 2006. The lava flows produced in this eruption range from blocky 'a'a type to smooth-surfaced inflated pahoehoe. We measured lava inflation features (i.e. one tumulus and three pressure ridges) that formed in the various pahoehoe flows emplaced in this event. The inflation features within the main crater of Oldoinyo Lengai are relatively small-scale, measuring 1-5 m in width, 2.5–24.4 m in length and with inflation clefts less than 0.4 m deep. Their small sizes are in contrast to a tumulus that formed on the northwestern slope of the volcano (situated ~1140 m below the crater floor). The tumulus is roughly circular, measures 17.5 × 16.0 m, and is cut by a 4.4 m deep axial inflation cleft exposing two separate flow units. We measured the elastic properties (i.e. shear- and bulk moduli) of natrocarbonatitic crust and find that these are similar to those reported for basaltic crust, and that there is no direct correlation between magmastatic head and pressure required to form tumuli. All inflated flows in the 2006 event were confined by lateral barriers (main crater, erosional channel or erosional gully) suggesting that the two most important factors for endogenous growth in natrocarbonatitic lava flows are (1) lateral barriers that prevent widening of the flow, and (2) influx of new material beneath the viscoelastic and brittle crust.  相似文献   

14.
Imaging short period variations in lava flux   总被引：2，自引：1，他引：1  
Short period (e.g. <1 h) variations in lava effusion rate have been detected previously on Mount Etna, Sicily, but the causes and effects of such changes are poorly understood because of difficulties in obtaining suitably high frequency measurements over long periods. Here, we report short period flux variations in active lava flows, recorded in dense time series imagery over a 7-night period using modified remote trail cameras. The sequences of night-time images show significant pulses of enhanced incandescence, interpreted as short period increases in lava flux, travelling down-channel at velocities of ∼10–20 m min−1. Pulse generation decreased from an average of one pulse per hour on the first night to approximately one per night within a few nights. Effusion rate changes on these timescales are considered to reflect instabilities in magma ascent and, consequently, could provide insight into subsurface flow processes.  相似文献   

15.
Degassing during magma ascent in the Mule Creek vent (USA)   总被引：1，自引：1，他引：1  
 The structures and textures of the rhyolite in the Mule Creek vent (New Mexico, USA) indicate mechanisms by which volatiles escape from silicic magma during eruption. The vent outcrop is a 300-m-high canyon wall comprising a section through the top of a feeder conduit, vent and the base of an extrusive lava dome. Field relations show that eruption began with an explosive phase and ended with lava extrusion. Analyses of glass inclusions in quartz phenocrysts from the lava indicate that the magma had a pre-eruptive dissolved water content of 2.5–3.0 wt% and, during eruption, the magma would have been water-saturated over the vertical extent of the present outcrop. However, the vesicularity of the rhyolite is substantially lower than that predicted from closed-system models of vesiculation under equilibrium conditions. At a given elevation in the vent, the volume fraction of primary vesicles in the rhyolite increases from zero close to the vent margin to values of 20–40 vol.% in the central part. In the centre the vesicularity increases upward from approximately 20 vol.% at 300 m below the canyon rim to approximately 40 vol.% at 200 m, above which it shows little increase. To account for the discrepancy between observed vesicularity and measured water content, we conclude that gas escaped during ascent, probably beginning at depths greater than exposed, by flow through the vesicular magma. Gas escape was most efficient near the vent margin, and we postulate that this is due both to the slow ascent of magma there, giving the most time for gas to escape, and to shear, favouring bubble coalescence. Such shear-related permeability in erupting magma is supported by the preserved distribution of textures and vesicularity in the rhyolite: Vesicles are flattened and overlapping near the dense margins and become progressively more isolated and less deformed toward the porous centre. Local zones have textures which suggest the coalescence of bubbles to form permeable, collapsing foams, implying the former existence of channels for gas migration. Local channelling of gas into the country rocks is suggested by the presence of sub-horizontal syn-eruptive rhyolitic tuffisite veins which depart from the vent margin and invade the adjacent country rock. In the central part of the vent, similar local channelling of gas is indicated by steep syn-eruption tuffisite veins which cut the rhyolite itself. We conclude that the suppression of explosive eruption resulted from gas separation from the ascending magma and vent structure by shear-related porous flow and channelling of gas through tuffisite veins. These mechanisms of gas loss may be responsible for the commonly observed transition from explosive to effusive behaviour during the eruption of silicic magma. Received: 24 May 1995 / Accepted: 13 March 1996  相似文献   

16.
After 33 years of repose, one of the most active volcanoes of the Kurile island arc—Sarychev Peak on Matua Island in the Central Kuriles—erupted violently on June 11, 2009. The eruption lasted 9 days and stands among the largest of recent historical eruptions in the Kurile Island chain. Satellite monitoring of the eruption, using Moderate Resolution Imaging Spectroradiometer, Meteorological Agency Multifunctional Transport Satellite, and Advanced Very High Resolution Radiometer data, indicated at least 23 separate explosions between 11 and 16 June 2009. Eruptive clouds reached altitudes of generally 8–16 km above sea level (ASL) and in some cases up to 21 km asl. Clouds of volcanic ash and gas stretched to the north and northwest up to 1,500 km and to the southeast for more than 3,000 km. For the first time in recorded history, ash fall occurred on Sakhalin Island and in the northeast sector of the Khabarovsky Region, Russia. Based on satellite image analysis and reconnaissance field studies in the summer of 2009, the eruption produced explosive tephra deposits with an estimated bulk volume of 0.4 km3. The eruption is considered to have a Volcanic Explosivity Index of 4. Because the volcano is remote, there was minimal risk to people or infrastructure on the ground. Aviation transport, however, was significantly disrupted because of the proximity of air routes to the volcano.  相似文献   

17.
The vesiculation of magma during the 1983 eruption of Miyakejima Volcano, Japan, is discussed based on systematic investigations of water content, vesicularity, and bubble size distribution for the products. The eruption is characterized by simultaneous lava effusion and explosive sub-plinian (‘dry’) eruptions with phreatomagmatic (‘wet’) explosions. The magmas are homogeneous in composition (basaltic andesite) and in initial water content (H2O = 3.9±0.9 wt%), and residual groundmass water contents for all eruption styles are low (H2O <0.4 wt%) suggestive of extensive dehydration of magma. For the scoria erupted during simultaneous ‘dry’ and ‘wet’ explosive eruptions, inverse correlation was observed between vesicularity and residual water content. This relation can be explained by equilibrium exsolution and expansion of ca. 0.3 wt% H2O at shallow level with different times of quenching, and suggests that each scoria with different vesicularity, which was quenched at a different time, provides a snapshot of the vesiculation process near the point of fragmentation. The bubble size distribution (BSD) varies systematically with vesicularity, and total bubble number density reaches a maximum value at vesicularity Φ ∼ 0.5. At Φ  ∼ 0.5, a large number of bubbles are connected with each other, and the average thickness of bubble walls reaches the minimum value below which they would rupture. These facts suggest that vesiculation advanced by nucleation and growth of bubbles when Φ < 0.5, and then by expansion of large bubbles with coalescence of small ones for Φ > 0.5, when bubble connection becomes effective. Low vesicularity and low residual water content of lava and spatter (Φ  < 0.1, H2O  < 0.1 wt%), and systematic decrease in bubble number density from scoria through spatter to lava with decrease in vesicularity suggest that effusive eruption is a consequence of complete degassing by bubble coalescence and separation from magma at shallow levels when magma ascent rate is slow.
T. ShimanoEmail:
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18.
 During the 1944 eruption of Vesuvius a sudden change occurred in the dynamics of the eruptive events, linked to variations in magma composition. K-phonotephritic magmas were erupted during the effusive phase and the first lava fountain, whereas the emission of strongly porphyritic K-tephrites took place during the more intense fountain. Melt inclusion compositions (major and volatile elements) highlight that the magmas feeding the eruption underwent differentiation at different pressures. The K-tephritic volatile-rich melts (up to 3 wt.% H2O, 3000 ppm CO2, and 0.55 wt.% Cl) evolved to reach K-phonotephritic compositions by crystallization of diopside and forsteritic olivine at total fluid pressure higher than 300 MPa. These magmas fed a very shallow reservoir. The low-pressure differentiation of the volatile-poor K-phonotephritic magmas (H2O<1 wt.%) involved mixing, open-system degassing, and crystallization of leucite, salite, and plagioclase. The eruption was triggered by intrusion of a volatile-rich magma batch that rose from a depth of 11–22 km into the shallow magma chamber. The first phase of the eruption represents the partial emptying of the shallow reservoir, the top of which is within the volcanic edifice. The newly arrived magma mixed with that resident in the shallow reservoir and forced the transition from the effusive to the lava fountain phase of the eruption. Received: 14 September 1998 / Accepted: 10 January 1999  相似文献   

19.
 The Kupaianaha vent, the source of the 48th episode of the 1983-to-present Pu'u 'O'o–Kupaianaha eruption, erupted nearly continuously from July 1986 until February 1992. This investigation documents the geophysical and geologic monitoring of the final 10 months of activity at the Kupaianaha vent. Detailed very low frequency (VLF) electromagnetic profiles across the single lava tube transporting lava from the vent were used to determine the cross-sectional area of the molten lava within the tube. Combined with measurements of lava velocity, these data provide an estimate of the lava output of Kupaianaha. In addition, lava temperatures (calculated from analysis of quenched glass) and bulk-rock chemistry were obtained for samples taken from the tube at the same site. The combined data set shows the lava flux from Kupaianaha vent declining linearly from 250 000 m3/day in April 1991 to 54 000 m3/day by November 1991. During that time surface breakouts of lava from weak points along the tube occurred progressively closer to the vent, consistent with declining efficiency in lava transport. There were no significant changes in lava temperature or in bulk MgO content during this period. Another eruptive episode (the 49th) began uprift of Kupaianaha on 8 November 1991 and erupted lava concurrently with Kupaianaha for 18 days. Lava flux from Kupaianaha decreased in response to this new episode, but the response was delayed by approximately 1 day. After 14 November 1991, lava velocities were no longer measurable in the tube because the lava stream beneath the skylight had crusted over; however, the VLF-derived electrical conductances documented the decreasing flux of molten lava through the tube. Kupaianaha remained active, but output continued to decrease until early February 1992 when the last active surface flows were seen. In November 1991 we used the linearly decreasing effusion rate to accurately predict the date for the death of the Kupaianaha vent. The linear nature of the decline in lava tube conductance and the delayed and slow response of the Waha'ula tube conductances to the 49th eruptive episode led us to speculate that (a) the Kupaianaha vent shut down because of a decrease in driving pressure and not because of a freeze-up of the vent, and (b) that Pu'u 'O'o, episode 49, and Kupaianaha were fed nearly vertically from a source deep within the rift zone. Received: 29 September 1995 / Accepted: 21 November 1995  相似文献   

20.
 The tube-fed pāhoehoe lava flows covering much of the northeast flank of Kīlauea Volcano are named the 'Ailā'au flows. Their eruption age, based on published and six new radiocarbon dates, is approximately AD 1445. The flows have distinctive paleomagnetic directions with steep inclinations (40°–50°) and easterly declinations (0°–10°E). The lava was transported ∼40 km from the vent to the coast in long, large-diameter lava tubes; the longest tube (Kazumura Cave) reaches from near the summit to within several kilometers of the coast near Kaloli Point. The estimated volume of the 'Ailā'au flow field is 5.2±0.8 km3, and the eruption that formed it probably lasted for approximately 50 years. Summit overflows from Kīlauea may have been nearly continuous between approximately AD 1290 and 1470, during which time a series of shields formed at and around the summit. The 'Ailā'au shield was either the youngest or the next to youngest in this series of shields. Site-mean paleomagnetic directions for lava flows underlying the 'Ailā'au flows form only six groups. These older pāhoehoe flows range in age from 2750 to <18,000 BP, and the region was inundated by lava flows only three times in the past 5000 years. The known intervals between eruptive events average ∼1600 years and range from ∼1250 years to >2200 years. Lava flows from most of these summit eruptions also reached the coast, but none appears as extensive as the 'Ailā'au flow field. The chemistry of the melts erupted during each of these summit overflow events is remarkably similar, averaging approximately 6.3 wt.% MgO near the coast and 6.8 wt.% MgO near the summit. The present-day caldera probably formed more recently than the eruption that formed the 'Ailā'au flows (estimated termination ca. AD 1470). The earliest explosive eruptions that formed the Keanakāko'i Ash, which is stratigraphically above the 'Ailā'au flows, cannot be older than this age. Received: 10 October 1998 / Accepted: 12 May 1999  相似文献   

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