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1.
Oldoinyo Lengai is the world's only active carbonatite volcano and considerable debate still surrounds the genesis of its magmas. Gases were collected from two fumaroles discharging close to the then active vent in October 2003. Measured fumarole temperatures were ≤ 195°C, despite the nearby, vigorous eruptive activity. Gases were sampled and analysed for noble gas isotopes. Freshly erupted natrocarbonatite lavas, a 1917 nephelinite and a sub-recent wollastonite bearing rock were also collected and analysed for noble gas isotopes using vacuum crushing techniques. In all the lava samples the neon, argon, krypton and xenon isotope ratio data are indistinguishable from air implying atmospheric contamination of the hygroscopic rocks. In the fumaroles, measured 3He/4He ratios are between 4 and 7 R/Ra. This range is similar to published values for silicate xenoliths of the East African Rift implying a local lithospheric mantle source for the volatile component of the Lengai magmas. It is still unclear if the natrocarbonatites themselves come from this region, or if fractionation and/or liquid immiscibility generate the carbonate magmas from a silicate melt within the crust itself.  相似文献   

2.
The composition of carbonate, sulphate, halide and other phases trapped as fluid inclusions in apatite crystals in ijolite and urtite from sub-volcanic complexes in East Africa have been determined using the electron micro-probe. The bulk composition of these inclusions is alkali-rich and closely comparable to the natrocarbonatite lava from the active volcano of Oldoinyo Lengai in N. Tanzania. The inclusions are interpreted as having been originally immiscible alkali carbonate-rich droplets within a nephelinitic melt. The evidence suggests that magma of the composition of natrocarbonatite lava is that most likely for the parent magma during carbonatite petrogenesis.  相似文献   

3.
Natrocarbonatitic magmas are characterized by their extremely low viscosities and fast elemental diffusion, and as a consequence of this, their chemistry and crystallinity can change significantly during residence in shallow reservoirs or even due to cooling during lava flow emplacement. Here, we present the results of a series of crystallization experiments conducted at 1-atm confining pressure and in a temperature range between 630°C and 300°C. The experiments were set up to characterize the chemistry and growth processes of the phenocryst phases present in natrocarbonatites. The results are applicable to (1) processes occurring during residence in shallow magma reservoirs and/or (2) during lava flow emplacement. We show that during crystallization of natrocarbonatites at atmospheric pressure, gregoryite is the first mineral to crystallize at 630°C, followed by nyerereite at 595°C. Crystal size distributions of the gregoryites show that the crystals grow rapidly by textural coarsening (i.e., Ostwald ripening). As the crystallization is a continuous process at this pressure, the composition of the residual melt changes in response to the crystallization. However, the experiments also show that individual crystals completely reequilibrate with the changes in melt composition in as little time as <11 min. We therefore conclude that crystallization and diffusion are extremely fast processes in the natrocarbonatitic system and that the measured chemical variations in phenocrysts from Oldoinyo Lengai can be explained by different cooling histories. Finally, we model the rheological control on the emplacement of highly crystallized natrocarbonatitic lavas at Oldoinyo Lengai.  相似文献   

4.
Oldoinyo Lengai in the Northern Tanzania rift is the only active nephelinite–carbonatite stratovolcano. We report the discovery of thermonatrite, aphthitalite, halite and sylvite fumarole deposits on recent natrocarbonatite lava flows erupted in the summit crater during the wet season. These salt deposits occur as delicate, concave fringes or tubes that line the cooling cracks in the lava flows and consist of intergrowths of euhedral crystals. The presence of a dark altered zone, depleted in halides and alkalies, adjacent to cooling cracks and observations of steam fumaroles emanating from the fractures suggest that the salts are formed by sublimation from saturated vapours generated by the extrusion of lavas over meteoric water. The crystallisation sequence recorded in the salts suggests that mixing between meteoric steam and magmatic CO2 and H2S occurs at high temperatures resulting in the sublimation of carbonates and sulphates. At lower temperatures the vapours are dominated by meteoric steam and sublimate halides. The high solubility of the fumarole salts within meteoric water and their formation only during the wet season implies that these are ephemeral deposits that are unlikely to be preserved in the geological record.  相似文献   

5.
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   

6.
With a paroxysmal ash eruption on 4 September 2007 and the highly explosive activity continuing in 2008, Oldoinyo Lengai (OL) has dramatically changed its behavior, crater morphology, and magma composition after 25 years of quiet extrusion of fluid natrocarbonatite lava. This explosive activity resembles the explosive phases of 1917, 1940–1941, and 1966–1967, which were characterized by mixed ashes with dominantly nephelinitic and natrocarbonatitic components. Ash and lapilli from the 2007–2008 explosive phase were collected on the slopes of OL as well as on the active cinder cone, which now occupies the entire north crater having buried completely all earlier natrocarbonatite features. The lapilli and ash samples comprise nepheline, wollastonite, combeite, Na-åkermanite, Ti-andradite, resorbed pyroxene and Fe–Ti oxides, and a Na–Ca carbonate phase with high but varying phosphorus contents which is similar, but not identical, to the common gregoryite phenocrysts in natrocarbonatite. Lapilli from the active cone best characterize the erupted material as carbonated combeite–wollastonite–melilite nephelinite. The juvenile components represent a fundamentally new magma composition for OL, containing 25–30 wt.% SiO2, with 7–11 wt.% CO2, high alkalies (Na2O 15–19%, K2O 4–5%), and trace-element signatures reminiscent of natrocarbonatite enrichments. These data define an intermediate composition between natrocarbonatite and nephelinite, with about one third natrocarbonatite and two thirds nephelinite component. The data are consistent with a model in which the carbonated silicate magma has evolved from the common combeite–wollastonite nephelinite (CWN) of OL by enrichment of CO2 and alkalies and is close to the liquid immiscible separation of natrocarbonatite from carbonated nephelinite. Material ejected in April/May 2008 indicates reversion to a more common CWN composition.  相似文献   

7.
Fumarolic encrustations and natrocarbonatite lava from the active crater of Oldoinyo Lengai volcano, Tanzania, were sampled and analysed. Two types of encrustation were distinguished on the basis of their REE content, enriched (~ 2800–5600 × [REEchondrite]) and depleted (~ 100–200 × [REEchondrite]) relative to natrocarbonatite (1700–1900 × [REEchondrite]. REE-enriched encrustations line the walls of actively degassing fumaroles, whereas REE-depleted encrustations occur mainly along cracks in and as crusts on cooling natrocarbonatite lava flows; one of the low REE encrustation samples was a stalactite from the wall of a possible fumarole. The encrustations are interpreted to have different origins, the former precipitating from volcanic gas and the latter from meteoric/ground water converted to steam by the heat of the overlying lava flow(s). REE-profiles of encrustations and natrocarbonatite are parallel, suggesting that there was no preferential mobilization of specific REE by either volcanic vapour or meteoric water vapour. The elevated REE-content of the first group of encrustations suggests that direct REE-transport from natrocarbonatite to volcanic vapour is possible. The REE trends observed in samples precipitating directly from the volcanic vapour cannot be explained by dry volatility based on the available data as there is no evidence in the encrustation compositions of the greatly enhanced volatility predicted for Yb and Eu. The observed extreme REE-fractionation with steep La/Sm slopes parallel to those of the natrocarbonatite reflects solvation and complexation reactions in the vapour phase that did not discriminate amongst the different REE or similar transport of REE in both the natrocarbonatite magma and its exsolving vapour. The low concentrations of REE in the encrustations produced by meteoric vapour suggest that the temperature was too low or that this vapour did not contain the ligands necessary to permit significant mobilization of the REE.  相似文献   

8.
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   

9.
On September 4, 2007, after 25 years of effusive natrocarbonatite eruptions, the eruptive activity of Oldoinyo Lengai (OL), N Tanzania, changed abruptly to episodic explosive eruptions. This transition was preceded by a voluminous lava eruption in March 2006, a year of quiescence, resumption of natrocarbonatite eruptions in June 2007, and a volcano-tectonic earthquake swarm in July 2007. Despite the lack of ground-based monitoring, the evolution in OL eruption dynamics is documented based on the available field observations, ASTER and MODIS satellite images, and almost-daily photos provided by local pilots. Satellite data enabled identification of a phase of voluminous lava effusion in the 2 weeks prior to the onset of explosive eruptions. After the onset, the activity varied from 100 m high ash jets to 2–15 km high violent, steady or unsteady, eruption columns dispersing ash to 100 km distance. The explosive eruptions built up a ∼400 m wide, ∼75 m high intra-crater pyroclastic cone. Time series data for eruption column height show distinct peaks at the end of September 2007 and February 2008, the latter being associated with the first pyroclastic flows to be documented at OL. Chemical analyses of the erupted products, presented in a companion paper (Keller et al. 2010), show that the 2007–2008 explosive eruptions are associated with an undersaturated carbonated silicate melt. This new phase of explosive eruptions provides constraints on the factors causing the transition from natrocarbonatite effusive eruptions to explosive eruptions of carbonated nephelinite magma, observed repetitively in the last 100 years at OL.  相似文献   

10.
The active volcano, Oldoinyo Lengai in the Eastern Rift in northern Tanganyika consists mainly of yellow ijolitic pyroclasts with interbedded relatively thin phonolite and nephelinite lavas, overlain by nephelinitic pyroclasts and younger ashes with marked unconformity. Ejected blocks in the pyroclasts are of rocks of the urtite-jacupirangite series with or without wollastonite, wollastonitite, fenite, carbonite, biotite pyroxenite and various lavas. Observations were made of minor activity during September–October 1960 when it was noted that soda-rich carbonate lava was extruded on the crater-floor in addition to minor emissions of ash. From examination of the rock sequence it appears that the volcano is waning and there has been a change from earlier dominantly gas eruptions to the more recent minor emissions of lava.  相似文献   

11.
The Monticchio Lakes Formation MLF is a newly identified carbonatite-melilitite tuff sequence which is exposed in the southwestern sector of the Vulture volcano. It is the youngest example ca. 0.13 m.y. of this type of volcanism in Italy, although other carbonatites of smaller volume, but with similar characteristics, have been discovered recently. This volcanic event occurred in isolation after a 0.35 m.y. period of inactivity at Vulture. The eruption produced two maar-type vents and formed tuff aprons mainly composed of dune beds of lapilli. Depositional features suggest that a dry surge mechanism, possibly triggered by CO2 expansion, was dominant during tuff emplacement. The MLF event involved a mixture of carbonatite and melilitite liquids which were physically separated before the eruption. Abundant mantle xenoliths are direct evidence of the deep-seated origin of the parental magma and its high velocity of propagation towards the surface. Often, these nodules form the core of lapilli composed of concentric shells of melilitite andror porphyritic carbonatite. Coarse-ash beds alternate with lapilli beds and consist of abundant lumps and spherulae of very fine-grained calcite immersed in a welded, highly compacted carbonatite matrix. Porphyritic carbonatite shells of the lapilli and fine-grained spherulae of calcite in the tuff matrix suggest incipient crystallisation of a carbonatite liquid in subvolcanic conditions and eruption of carbonatite-spray droplets. Dark coloured juvenile fragments mainly consist of melilite, phlogopite, calcite, apatite, perovskite, and häuyne crystals in a carbonatite or melilitite matrix. The rocks have an extremely primitive, ultramafic composition with very high Mga) 85. and Cr and Ni content 1500 ppm-. The calcite contains high SrO, BaO and REE of up to 1.5 wt.%. Similar compositions are typical of primary, magmatic carbonates which are found in both intrusive and extrusive carbonatites. The high modal Sr-Ba-REE-rich calcite, the typical mineralogy, and the high amount of Sr-group elements identify the carbonate component as a carbonatite. The very high Mga, mantle debris and C, O, He isotope ratios in the range of mantle values indicate a near-primary character for the carbonatite which is distinctive of a restricted group of extrusive carbonatites only found in continental rift areas.  相似文献   

12.
The continuous measurement of molecular hydrogen (H2) emissions from passively degassing volcanoes has recently been made possible using a new generation of low-cost electrochemical sensors. We have used such sensors to measure H2, along with SO2, H2O and CO2, in the gas and aerosol plume emitted from the phonolite lava lake at Erebus volcano, Antarctica. The measurements were made at the crater rim between December 2010 and January 2011. Combined with measurements of the long-term SO2 emission rate for Erebus, they indicate a characteristic H2 flux of 0.03?kg s–1 (2.8?Mg? day–1). The observed H2 content in the plume is consistent with previous estimates of redox conditions in the lava lake inferred from mineral compositions and the observed CO2/CO ratio in the gas plume (～0.9 log units below the quartz–fayalite–magnetite buffer). These measurements suggest that H2 does not combust at the surface of the lake, and that H2 is kinetically inert in the gas/aerosol plume, retaining the signature of the high-temperature chemical equilibrium reached in the lava lake. We also observe a cyclical variation in the H2/SO2 ratio with a period of ～10?min. These cycles correspond to oscillatory patterns of surface motion of the lava lake that have been interpreted as signs of a pulsatory magma supply at the top of the magmatic conduit.  相似文献   

13.
The late Pleistocene San Venanzo maar and nearby Pian di Celle tuff ring in the San Venanzo area of Umbria, central Italy, appear to represent different aspects of an eruptive cycle accompanied by diatreme formation. Approximately 6x106 m3 of mostly lapillisized, juvenile ejecta with lesser amounts of lithics and 1x106 m3 of lava were erupted. The stratigraphy indicates intense explosive activity followed by lava flows and subvolcanic intrusions. The pyroclastic material includes lithic breccia derived from vent and diatreme wall erosion, roughly stratified lapilli tuff deposited by concentrated pyroclastic surge, chaotic scoriaceous pyroclastic flow and inverse graded grain-flow deposits. The key feature of the pyroclastics is the presence of concentric-shelled lapilli generated by accretion around the lithics during magma ascent in the diatreme conduits. The rock types range from kalsilite leucite olivine melilitite lavas and subvolcanic intrusions to carbonatite, phonolite and calcitic melilitite pyroclasts. Juvenile ejecta contain essential calcite whose composition and texture indicate a magmatic origin. Pyroclastic carbonatite activity is also indicated by the presence of carbonatite ash beds. The San Venanzo maar-forming event is believed to have been trigered by fluid-rich carbonatite-phonolite magma. The eruptive centre the moved to the Pian di Celle tuff ring, where the eruption of degassed olivine melilititic magma and late intrusions ended magmatic activity in the area. In both volcanoes the absence of phreatomagmatic features together with the presence of large amounts of primary calcite suggests carbonatite segregation and violent exsolution of CO2 which, flowing through the diatremes, produced the peculiar intrusive pyroclastic facies and triggered explosions.  相似文献   

14.
Heat and mass transfer rates were studied at the Niragongo lava lake during two expeditions directed by H. Tazieff in 1959 and 1972. The results of this study are as follows:Heat is transferred to the surface of the lake by the movement of lava; gas discharge is a result and not the cause of convection. The chemical composition of the gases and magma has changed very little between 1959 and 1972, whereas the mass and energy outputs differ by an order of magnitude. In 1977 a catastrophic explosion seems to have been caused by tectonic factors, stopping the slow convection of magma under the volcano and hence reducing surface manifestations in the form of the lava lake and escaping fumarolic and magmatic gases. The gas discharge was, in tons day−1, 5000 for H2O, 11,000 for CO2, 1000 for SO2 in 1959, and in 1972 7700 for H2O, 180,000 for CO2 and 23,000 for SO2. These values correspond to an energy transfer of 0.9 × 109 W in 1959 and 16 × 109 W in 1972.  相似文献   

15.
An earthquake swarm struck the North Tanzania Divergence, East African Rift over a 2 month period between July and September 2007. It produced approximately 70 M > 4 earthquakes (peak magnitude Mw 5.9), and extensive surface deformation, concurrent with eruptions at the nearby Oldoinyo Lengai volcano. The spatial and temporal evolution of the entire deformation event was resolved by Interferometric Synthetic Aperture Radar (InSAR) observations, owing to a particularly favorable acquisition programming of the Envisat and ALOS satellites, and was verified by detailed ground observations. Elastic modeling based on the InSAR measurements clearly distinguishes between normal faulting, which dominated during the first week of the event, and intermittent episodes of dike propagation, oblique dike opening and dike-induced faulting during the following month. A gradual decline in the intensity of deformation occurred over the final weeks. Our observations and modeling suggest that the sequence of events was initiated by pressurization of a deep-seated magma chamber below Oldoinyo Lengai which opened the way to lateral dike injection, and dike-induced faulting and seismicity. As dike intrusion terminated, silicate magma ascended the volcano conduit, reacted with the carbonatitic magma, and set off a major episode of explosive ash eruptions producing mixed silicate-carbonatitic ejecta. The rise of the silicate magma within the volcano conduit is attributed to bubble growth and buoyancy increase in the magma chamber either due to a temporary pressure drop after the termination of the diking event, or due to the dynamic effects of seismic wave passage from the earthquake swarm. Similar temporal associations between earthquake swarms and major explosive ash eruptions were observed at Oldoinyo Lengai over the past half century.  相似文献   

16.
Erta Ale volcano, Ethiopia, erupted in November 2010, emplacing new lava flows on the main crater floor, the first such eruption from the southern pit into the main crater since 1973, and the first eruption at this remote volcano in the modern satellite age. For many decades, Erta Ale has contained a persistently active lava lake which is ordinarily confined, several tens of metres below the level of the main crater, within the southern pit. We combine on-the-ground field observations with multispectral imaging from the SEVIRI satellite to reconstruct the entire eruptive episode beginning on 11 November and ending prior to 14 December 2010. A period of quiescence occurred between 14 and 19 November. The main eruptive activity developed between 19 and 22 November, finally subsiding to pre-eruptive levels between 8 and 15 December. The estimated total volume of lava erupted is ??0.006?km3. The mineralogy of the 2010 lava is plagioclase?+?clinopyroxene?+?olivine. Geochemically, the lava is slightly more mafic than previously erupted lava lining the caldera floor, but lies within the range of historical lavas from Erta Ale. SIMS analysis of olivine-hosted melt inclusions shows the Erta Ale lavas to be relatively volatile-poor, with H2O contents ??1,300?ppm and CO2 contents of ??200?ppm. Incompatible trace and volatile element systematics of melt inclusions show, however, that the November 2010 lavas were volatile-saturated, and that degassing and crystallisation occurred concomitantly. Volatile saturation pressures are in the range 7?C42?MPa, indicating shallow crystallisation. Calculated pre-eruption and melt inclusion entrapment temperatures from mineral/liquid thermometers are ??1,150?°C, consistent with previously published field measurements.  相似文献   

17.
At Cotopaxi volcano, Ecuador, rhyolitic and andesitic bimodal magmatism has occurred periodically during the past 0.5 Ma. The sequential eruption of rhyolitic (70–75% SiO2) and andesitic (56–62% SiO2) magmas from the same volcanic vent over short time spans and without significant intermingling is characteristic of Cotopaxi’s Holocene behavior. This study documents the eruptive history of Cotopaxi volcano, presenting its stratigraphy and geologic field relations, along with the relevant mineralogical and chemical nature of the eruptive products, in order to determine the temporal and spatial relations of this bimodal alternation. Cotopaxi’s history begins with the Barrancas rhyolite series, dominated by pumiceous ash flows and regional ash falls between 0.4 and 0.5 Ma, which was followed by occasional andesitic activity, the most important being the ample andesitic lava flows (∼4.1 km3) that descended the N and NW sides of the edifice. Following a ∼400 ka long repose without silicic activity, Cotopaxi began a new eruptive phase about 13 ka ago that consisted of seven rhyolitic episodes belonging to the Holocene F and Colorado Canyon series; the onset of each episode occurred at intervals of 300–3,600 years and each produced ash flows and regional tephra falls with DRE volumes of 0.2–3.6 km3. Andesitic tephras and lavas are interbedded in the rhyolite sequence. The Colorado Canyon episode (4,500 years BP) also witnessed dome and sector collapses on Cotopaxi’s NE flank which, with associated ash flows, generated one of the largest cohesive debris flows on record, the Chillos Valley lahar. A thin pumice lapilli fall represents the final rhyolitic outburst which occurred at 2,100 years BP. The pumices of these Holocene rhyolitic eruptions are chemically similar to those of older rhyolites of the Barrancas series, with the exception of the initial eruptive products of the Colorado Canyon series whose chemistry is similar to that of the 211 ka ignimbrite of neighboring Chalupas volcano. Since the Colorado Canyon episode, andesitic magmatism has dominated Cotopaxi’s last 4,400 years, characterized by scoria bomb and lithic-rich pyroclastic flows, infrequent lava flows that reached the base of the cone, andesitic lapilli and ash falls that were carried chiefly to the W, and large debris flows. Andesitic magma emission rates are estimated at 1.65 km3 (DRE)/ka for the period from 4,200 to 2,100 years BP and 1.85 km3 (DRE)/ka for the past 2,100 years, resulting in the present large stratocone.  相似文献   

18.
In 1874 and 1875 the fissure swarm of Askja central volcano was activated during a major rifting episode. This rifting resulted in a fissure eruption of 0.3 km3 basaltic magma in Sveinagja graben, 50 to 70 km north of Askja and subsequent caldera collapse forming the Oskjuvatn caldera within the main Askja caldera. Five weeks after initial collapse, an explosive mixed magma eruption took place in Askja. On the basis of matching chemistry, synchronous activity and parallels with other rifted central volcanoes, the events in Askja and its lissure swarm are attributed to rise of basaltic magma into a high-level reservoir in the central volcano, subsequent rifting of the reservoir and lateral flow magma within the fissure swarm to emerge in the Sveinagja eruption. This lateral draining of the Askja reservoir is the most plausible cause for caldera collpse. The Sveinagja basalt belong to the group of evolved tholejites characteristie of several Icelandic central volcanoes and associated fissure swarms. Such tholeiites, with Mgvalues in the 40 to 50 tange, represent magmas which have suffered extensive fractional crystallization within the crust. The 12% porphyritic Sveinagja basalt contains phenocrysts of olivine (Fo62–67), plagioclase (An57–62), clinopyroxene (Wo38En46Wo16) and titanomagnetite. Extrusion temperature of the lava, calculated on the basis of olivine and plagioclase geothermometry, is found to be close to 1150°C.  相似文献   

19.
On January 16, 2002, short-term unrest occurred at San Miguel volcano. A gas-and-steamash plume rose a few hundred meters above the summit crater. An anomalous microseismicity pattern, about 75 events between 7:30 and 10:30 hours, was also observed. Continuous monitoring of CO2 efflux on the volcano started on November 24, 2001, in the attempt to provide a multidisciplinary approach for its volcanic surveillance. The background mean of the diffuse CO2 emission is about 16 g m-2 d-1, but a 17- fold increase, up to 270 g m-2 d-1, was detected on January 7, nine days before the January 2002 short-term unrest at San Miguel volcano. These observed anomalous changes on diffuse CO2 degassing could be related to either a sharp increase of CO2 pressure within the volcanic-hydrothermal system or degassing from an uprising fresh gas-rich magma within the shallow plumbing system of the volcano since meteorological fluctuations cannot explain this observed increase of diffuse CO2 emission.  相似文献   

20.
Santiaguito volcano has shown a continuous slow extrusion of dacite lava since 1922. In the 50 years of activity there have been four periods of abnormally high extrusion rates, interspersed by periods of little magma production. The type of activity shown by the volcano has been varied and crudely cyclic. Dome extrusion periods are accompanied by pyroclastic activity and followed by lava flows. There are now 16 time stratigraphic units delineated on the dome. Activity since 1967 has been especially closely observed. Dome extrusion at the west end of the complex has been accompanied by pyroclastic cruptions and plug dome extrusion at the east end. The eurrent extrusion rate has remained essentially constant since 1967 at about 5×106 m3/yr, far below Santiaguito’s 1922–71 average of 14×106 m3/yr. The active vent at the east end of the volcano (Caliente vent) has been the principal vent of the volcano since the creation of the explosion crater in 1902. After its initial period of dome extrusion (1922–25), the Caliente vent has chiefly produced pyroclastic eruptions as well as at least 95% of the dome’s lumarolic activity, while lateral vents have continued to give rise to lavas. Lava flows at Santiaguito have effective viscosity values of about 106 poises, while dome lavas are significantly more viscous. The differences in viscosity are in part related to volatile content of the lava when it reaches the surface. During dome extrusion, lavas lose their volatiles through pyroclastic activity before they reach the surface. Lava flows at Santiaguito occur when lava reaches the surface with higher volatile content. Obstruction of either the central (pyroclastic) vent or the lateral (dome extrusion) vent or both vents has an important influence on succeeding activity. In June 1972, at the time of this writing, the outbreak of new lava flows at both the Caliente and lateral El Brujo vents has just occurred, resulting from obstruction of pyroclastic activity by a large plug dome at the Caliente vent.  相似文献   

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