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1.
 The evolution of the Somma-Vesuvius caldera has been reconstructed based on geomorphic observations, detailed stratigraphic studies, and the distribution and facies variations of pyroclastic and epiclastic deposits produced by the past 20,000 years of volcanic activity. The present caldera is a multicyclic, nested structure related to the emptying of large, shallow reservoirs during Plinian eruptions. The caldera cuts a stratovolcano whose original summit was at 1600–1900 m elevation, approximately 500 m north of the present crater. Four caldera-forming events have been recognized, each occurring during major Plinian eruptions (18,300 BP "Pomici di Base", 8000 BP "Mercato Pumice", 3400 BP "Avellino Pumice" and AD 79 "Pompeii Pumice"). The timing of each caldera collapse is defined by peculiar "collapse-marking" deposits, characterized by large amounts of lithic clasts from the outer margins of the magma chamber and its apophysis as well as from the shallow volcanic and sedimentary units. In proximal sites the deposits consist of coarse breccias resulting from emplacement of either dense pyroclastic flows (Pomici di Base and Pompeii eruptions) or fall layers (Avellino eruption). During each caldera collapse, the destabilization of the shallow magmatic system induced decompression of hydrothermal–magmatic and hydrothermal fluids hosted in the wall rocks. This process, and the magma–ground water interaction triggered by the fracturing of the thick Mesozoic carbonate basement hosting the aquifer system, strongly enhanced the explosivity of the eruptions. Received: 24 November 1997 / Accepted: 23 March 1999  相似文献   

2.
 The ca. 10,500 years B.P. eruptions at Ruapehu volcano deposited 0.2–0.3 km3 of tephra on the flanks of Ruapehu and the surrounding ring plain and generated the only known pyroclastic flows from this volcano in the late Quaternary. Evidence of the eruptions is recorded in the stratigraphy of the volcanic ring plain and cone, where pyroclastic flow deposits and several lithologically similar tephra deposits are identified. These deposits are grouped into the newly defined Taurewa Formation and two members, Okupata Member (tephra-fall deposits) and Pourahu Member (pyroclastic flow deposits). These eruptions identify a brief (<ca. 2000-year) but explosive period of volcanism at Ruapehu, which we define as the Taurewa Eruptive Episode. This Episode represents the largest event within Ruapehu's ca. 22,500-year eruptive history and also marks its culmination in activity ca. 10,000 years B.P. Following this episode, Ruapehu volcano entered a ca. 8000-year period of relative quiescence. We propose that the episode began with the eruption of small-volume pyroclastic flows triggered by a magma-mingling event. Flows from this event travelled down valleys east and west of Ruapehu onto the upper volcanic ring plain, where their distal remnants are preserved. The genesis of these deposits is inferred from the remanent magnetisation of pumice and lithic clasts. We envisage contemporaneous eruption and emplacement of distal pumice-rich tephras and proximal welded tuff deposits. The potential for generation of pyroclastic flows during plinian eruptions at Ruapehu has not been previously considered in hazard assessments at this volcano. Recognition of these events in the volcanological record is thus an important new factor in future risk assessments and mitigation of volcanic risk at Tongariro Volcanic Centre. Received: 5 July 1998 / Accepted: 12 March 1999  相似文献   

3.
New studies of the deposits from the latest caldera-forming eruption (the “Dk” event) at Dakataua Volcano, New Britain Island, Papua New Guinea, help identify an intense space-time concentration of large-scale volcanism during the 7th century AD on New Britain. Radiocarbon dating of charcoal from the Dk deposits yields an age of 1,383 ± 28 BP. Calibration of this result gives an age in the range AD 635–670 (at 1 s. d.). At about the same time, two other volcanoes on New Britain, Rabaul and Witori, also produced very large eruptions. Very high acidity levels in ice cores from Antarctica and Greenland at AD 639 and AD 640 respectively may be linked to either or both of the Dakataua and Rabaul eruptions. Another ice core high acidity level, at AD 692, may be associated with the Witori eruption. Significant volcanic risk within the New Britain region is indicated by its Late Cenozoic history of relatively frequent large-scale eruptions from as many as 8 caldera systems within an arc-parallel zone about 380 km long. Over the last 20 ka the return period for major (VEI 5+) eruptions in this region was about 1.0 ka and individually high frequencies of major eruptive activity were experienced at Witori and Rabaul. The relatively short return period for major eruptions in the region would tend to increase the chance that such events could cluster in time.  相似文献   

4.
We consider the identification and diagnostics of active and potentially active volcanic features (regional zones of cinder cones, fields sheet volcanism, fields of concentrated multivent extrusive volcanism, calderas, and underwater eruption centers in the sea) in the Kuril-Kamchatka island arc and in the Commander Islands link of the Aleutian island arc, as well as the condition of this region as of late 2007. We have identified and examined three periods in the research of active and potentially active volcanic features in the region: the early (1697–1934), the new (1935–1962), and the most recent, still in progress (1963 until today). We provide a new definition of the term “active volcano,” which is scientifically well-grounded, for the first time here. We present modified (compared with those available until now) catalogs of active and potentially active volcanic forms in Kamchatka and the Kuril Islands. For typical multieruption volcanoes now in phase I (the active) and II (the passive) of their evolution, we provide long-term forecasts of the character and parameters of future eruptions and the associated volcanic hazard.  相似文献   

5.
 Two methods were used to quantify the flux of volcanic sulphur (as the equivalent mass of SO2) to the stratosphere over different timescales during the Holocene. A combination of satellite-based measurements of sulphur yields from recent explosive volcanic eruptions with an appropriate rate of explosive volcanism for the past 200 years constrains the medium-term (∼102 years) flux of volcanic sulphur to the stratosphere to be ∼1 Mt a–1, with lower and upper bounds of 0.3 and 3 Mt a–1. The short-term (∼10- to 20-year) flux due to small magnitude (1010–1012 kg) eruptions is of the order of 0.4 Mt a–1. At any time the instantaneous levels of sulphur in the stratosphere are dominated by the most recent (0–3 years) volcanic events. The flux calculations do not attempt to address this very short timescale variability. Although there are significant errors associated with the raw sulphur emission data on which this analysis is based, the approach presented is general and may be readily modified as the quantity and quality of the data improve. Data from a Greenland ice core support these conclusions. Integration of the sulphate signals from presumed volcanic sources recorded in the GISP2 core provides a minimum estimate of the 103–year volcanic SO2 flux to the stratosphere of 0.5–1 Mt a–1 over the past 9000 years. The short-term flux calculations do not account for the impact of rare, large events. The ice-core record does not fully account for the contribution from small, frequent events. Received: 27 September 1995 / Accepted: 13 December 1995  相似文献   

6.
 Shiveluch Volcano, located in the Central Kamchatka Depression, has experienced multiple flank failures during its lifetime, most recently in 1964. The overlapping deposits of at least 13 large Holocene debris avalanches cover an area of approximately 200 km2 of the southern sector of the volcano. Deposits of two debris avalanches associated with flank extrusive domes are, in addition, located on its western slope. The maximum travel distance of individual Holocene avalanches exceeds 20 km, and their volumes reach ∼3 km3. The deposits of most avalanches typically have a hummocky surface, are poorly sorted and graded, and contain angular heterogeneous rock fragments of various sizes surrounded by coarse to fine matrix. The deposits differ in color, indicating different sources on the edifice. Tephrochronological and radiocarbon dating of the avalanches shows that the first large Holocene avalanches were emplaced approximately 4530–4350 BC. From ∼2490 BC at least 13 avalanches occurred after intervals of 30–900 years. Six large avalanches were emplaced between 120 and 970 AD, with recurrence intervals of 30–340 years. All the debris avalanches were followed by eruptions that produced various types of pyroclastic deposits. Features of some surge deposits suggest that they might have originated as a result of directed blasts triggered by rockslides. Most avalanche deposits are composed of fresh andesitic rocks of extrusive domes, so the avalanches might have resulted from the high magma supply rate and the repetitive formation of the domes. No trace of the 1854 summit failure mentioned in historical records has been found beyond 8 km from the crater; perhaps witnesses exaggerated or misinterpreted the events. Received: 18 August 1997 / Accepted: 19 December 1997  相似文献   

7.
Victims from volcanic eruptions: a revised database   总被引：2，自引：1，他引：1  
 The number of victims from volcanism and the primary cause(s) of death reported in the literature show considerable uncertainty. We present the results of investigations carried out either in contemporary accounts or in specific studies of eruptions that occurred since A.D. 1783. More than 220 000 people died because of volcanic activity during this period, which includes approximately 90% of the recorded deaths throughout history. Most of the fatalities resulted from post-eruption famine and epidemic disease (30.3%), nuées ardentes or pyroclastic flows and surges (26.8%), mudflows or lahars (17.1%), and volcanogenic tsunamis (16.9%). At present, however, international relief efforts might reduce the effects of post-eruption crop failure and disease, and at least some of the lahars could be anticipated in time by adequate scientific and social response. Thus, mitigation of hazards from pyroclastic flows and tsunamis will become of paramount importance to volcanologists and civil authorities. Received: 3 August 1997 / Accepted: 10 April 1998  相似文献   

8.
A new pyroclastic stratigraphy is presented for the island of Ischia, Italy, for the period ∼75–50 ka BP. The data indicate that this period bore witness to the largest eruptions recorded on the island and that it was considerably more volcanically active than previously thought. Numerous vents were probably active during this period. The deposits of at least 10 explosive phonolite to basaltic-trachyandesite eruptions are described and interpreted. They record a diverse range of explosive volcanic activity including voluminous fountain-fed ignimbrite eruptions, fallout from sustained eruption columns, block-and-ash flows, and phreatomagmatic eruptions. Previously unknown eruptions have been recognised for the first time on the island. Several of the eruptions produced pyroclastic density currents that covered the whole island as well as the neighbouring island of Procida and parts of the mainland. The morphology of Ischia was significantly different to that seen today, with edifices to the south and west and a submerged depression in the centre. The largest volcanic event, the Monte Epomeo Green Tuff (MEGT) resulted in caldera collapse across all or part of the island. It is shown to comprise at least two thick intracaldera ignimbrite flow-units, separated by volcaniclastic sediments that were deposited during a pause in the eruption. Extracaldera deposits of the MEGT include a pumice fall deposit emplaced during the opening phases of the eruption, a widespread lithic lag breccia outcropping across much of Ischia and Procida, and a distal ignimbrite in south-west Campi Flegrei. During this period the style and magnitude of volcanism was dictated by the dynamics of a large differentiated magma chamber, which was partially destroyed during the MEGT eruption. This contrasts with the small-volume Holocene and historical effusive and explosive activity on Ischia, the timing and distribution of which has been controlled by the resurgence of the Monte Epomeo block. The new data contribute to a clearer understanding of the long-term volcanic and magmatic evolution of Ischia.  相似文献   

9.
 The 1783–1784 Laki tholeiitic basalt fissure eruption in Iceland was one of the greatest atmospheric pollution events of the past 250 years, with widespread effects in the northern hemisphere. The degassing history and volatile budget of this event are determined by measurements of pre-eruption and residual contents of sulfur, chlorine, and fluorine in the products of all phases of the eruption. In fissure eruptions such as Laki, degassing occurs in two stages: by explosive activity or lava fountaining at the vents, and from the lava as it flows away from the vents. Using the measured sulfur concentrations in glass inclusions in phenocrysts and in groundmass glasses of quenched eruption products, we calculate that the total accumulative atmospheric mass loading of sulfur dioxide was 122 Mt over a period of 8 months. This volatile release is sufficient to have generated ∼250 Mt of H2SO4 aerosols, an amount which agrees with an independent estimate of the Laki aerosol yield based on atmospheric turbidity measurements. Most of this volatile mass (∼60 wt.%) was released during the first 1.5 months of activity. The measured chlorine and fluorine concentrations in the samples indicate that the atmospheric loading of hydrochloric acid and hydrofluoric acid was ∼7.0 and 15.0 Mt, respectively. Furthermore, ∼75% of the volatile mass dissolved by the Laki magma was released at the vents and carried by eruption columns to altitudes between 6 and 13 km. The high degree of degassing at the vents is attributed to development of a separated two-phase flow in the upper magma conduit, and implies that high-discharge basaltic eruptions such as Laki are able to loft huge quantities of gas to altitudes where the resulting aerosols can reside for months or even 1–2 years. The atmospheric volatile contribution due to subsequent degassing of the Laki lava flow is only 18 wt.% of the total dissolved in the magma, and these emissions were confined to the lowest regions of the troposphere and therefore important only over Iceland. This study indicates that determination of the amount of sulfur degassed from the Laki magma batch by measurements of sulfur in the volcanic products (the petrologic method) yields a result which is sufficient to account for the mass of aerosols estimated by other methods. Received: 30 May 1995 / Accepted: 19 April 1996  相似文献   

10.
 Experimental studies have been performed to evaluate pre-explosive water–melt mixes with respect to explosive volcanic molten–fuel–coolant interaction (MFCI), i.e., phreatomagmatic explosion. Remolten ultrabasic volcanic rock was used as a magma simulant. Measurement of the explosion intensity was used to determine optimal premixing conditions. A well-defined optimal range was found for the hydrodynamic mixing energy (differential flow speed of 4.2 m/s), as well as for the water/melt mass ratio (0.03 to 0.04) under experimental conditions. The mass flux of water had a minor influence on the explosion intensity. Additionally, transparent mixing experiments with silicon oil and inked water were carried out. They indicate a direct dependence of the pre-explosive water-melt interface area on the explosion intensity. The experimental results show that the contact conditions of water and melt required for explosive MFCI may easily be established in natural volcanic systems. Thus, explosive MFCI is a probable mechanism of explosive volcanism. Received: 23 July 1996 / Accepted: 16 December 1996  相似文献   

11.
 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   

12.
 Dike propagation and dilation increases the compression of adjacent rocks. On volcanoes, especially oceanic shields, dikes are accordingly thought to be structurally destabilizing. As compression is incremented, volcanic flanks are driven outward or downslope and thus increase their susceptibility to destructive earthquakes and giant landslides. We show, however, that the 2-m-thick dike emplaced along the east rift zone of Kilauea in 1983 actually stabilized that volcano's flank. Specifically, production of flank earthquakes dropped more than twofold after 1983 as maximum downslope motion slowed to 6 cm·year–1 from approximately 40 cm·year–1 during 1980–1982. As much as 65 cm of deflationary subsidence above Kilauea's summit and upper rift zones accompanied the dike intrusion. According to recent estimates, this deflation corresponds to a reduction in magma-reservoir pressure of approximately 4 MPa, probably about as much as the driving pressure of the 1983 dike. The volume of the dike, approximately 0.10–0.15 km3, is orders of magnitude less than the estimated 200- to 250-km3 volume of Kilauea's reservoir of magma and nearby hot, mushy rock. Thus, deflation of that reservoir reduces the compressional load on the flank over a much larger area than intrusion of the dike adds to it, particularly at the dominant depth of seismicity, 8–9 km. A Coulomb block model for flank motion during intervals between major earthquakes requires the low-angle fault beneath Kilauea's flank to exhibit slip weakening, conducive to earthquake instability. Accordingly, the triggering mechanism of destructive earthquakes, several of which have struck Hawaii during the past 150 years, need not require stresses accumulated by dike intrusions. Received: 27 October 1998 / Accepted: 24 May 1999  相似文献   

13.
The dynamics and thermodynamics of large ash flows   总被引：6，自引：6，他引：0  
 Ash flow deposits, containing up to 1000 km3 of material, have been produced by some of the largest volcanic eruptions known. Ash flows propagate several tens of kilometres from their source vents, produce extensive blankets of ash and are able to surmount topographic barriers hundreds of metres high. We present and test a new model of the motion of such flows as they propagate over a near horizontal surface from a collapsing fountain above a volcanic vent. The model predicts that for a given eruption rate, either a slow (10–100 m/s) and deep (1000–3000 m) subcritical flow or a fast (100–200 m/s) and shallow (500–1000 m) supercritical flow may develop. Subcritical ash flows propagate with a nearly constant volume flux, whereas supercritical flows entrain air and become progressively more voluminous. The run-out distance of such ash flows is controlled largely by the mass of air mixed into the collapsing fountain, the degree of fragmentation and the associated rate of loss of material into an underlying concentrated depositional system, and the mass eruption rate. However, in supercritical flows, the continued entrainment of air exerts a further important control on the flow evolution. Model predictions show that the run-out distance decreases with the mass of air entrained into the flow. Also, the mass of ash which may ascend from the flow into a buoyant coignimbrite cloud increases as more air is entrained into the flow. As a result, supercritical ash flows typically have shorter runout distances and more ash is elutriated into the associated coignimbrite eruption columns. We also show that one-dimensional, channellized ash flows typically propagate further than their radially spreading counterparts. As a Plinian eruption proceeds, the erupted mass flux often increases, leading to column collapse and the formation of pumiceous ash flows. Near the critical conditions for eruption column collapse, the flows are shed from high fountains which entrain large quantities of air per unit mass. Our model suggests that this will lead to relatively short ash flows with much of the erupted material being elutriated into the coignimbrite column. However, if the mass flux subseqently increases, then less air per unit mass is entrained into the collapsing fountain, and progressively larger flows, which propagate further from the vent, will develop. Our model is consistent with observations of a number of pyroclastic flow deposits, including the 1912 eruption of Katmai and the 1991 eruption of Pinatubo. The model suggests that many extensive flow sheets were emplaced from eruptions with mass fluxes of 109–1010 kg/s over periods of 103–105 s, and that some indicators of flow "mobility" may need to be reinterpreted. Furthermore, in accordance with observations, the model predicts that the coignimbrite eruption columns produced from such ash flows rose between 20 and 40 km. Received: 25 August 1995 / Accepted: 3 April 1996  相似文献   

14.
Lightning and electrification at volcanoes are important because they represent a hazard in their own right, they are a component of the global electrical circuit, and because they contribute to ash particle aggregation and modification within ash plumes. The role of water substance (water in all forms) in particular has not been well studied. Here data are presented from a comprehensive global database of volcanic lightning. Lightning has been documented at 80 volcanoes in association with 212 eruptions. The Volcanic Explosivity Index (VEI) could be determined for 177 eruptions. Eight percent of VEI = 3–5 eruptions have reported lightning, and 10% of VEI = 6, but less than 2% of those with VEI = 1–2. These findings suggest consistent reporting for larger eruptions but either less lightning or possible under-reporting for small eruptions. Ash plume heights (142 observations) show a bimodal distribution with main peaks at 7–12 km and 1–4 km. The former are similar to heights of typical thunderstorms and suggest involvement of water substance, whereas the latter suggest other factors contributing to electrical behavior closer to the vent. Reporting of lightning is more common at night (56%) and less common in daylight (44%). Reporting also varied substantially from year to year, suggesting that a more systematic observational strategy is needed. Several weak trends in lightning occurrence based on magma composition were found. The bimodal ash plume heights are obvious only for andesite to dacite; basalt and basaltic-andesite evenly span the range of heights; and rhyolites are poorly represented. The distributions of the latitudes of volcanoes with lightning and eruptions with lightning roughly mimic the distribution of all volcanoes, which is generally flat with latitude. Meteorological lightning, on the other hand, is common in the tropics and decreases markedly with increasing latitude as the ability of the atmosphere to hold water decreases poleward. This finding supports the idea that if lightning in large (deep) eruptions depends on water substance, then the origin of the water is primarily magma and not entrainment from the surrounding atmosphere. Seasonal effects show that more eruptions with lightning were reported in winter (bounded by the respective autumnal and vernal equinoxes) than in summer. This result also runs counter to the expectations based on entrainment of local water vapor.  相似文献   

15.
2 and approximately 85% SO2 of the total sulfur gas. Relative amounts of He, Ar, and N2 show a distinct hot-spot signature ( ). The δ13C–CO2 is approximately −3.6‰ and δ34ST is approximately +3.3‰. The δD/δ18O of fumarole H2O indicates steam separation from local meteoric waters whose estimated minimum mean residence time from 3H analyses is ≤40 years. Fumarolic activity at Alcedo is controlled by a caldera-margin fault containing at least seven hydrothermal explosion craters, and by an intracaldera rhyolite vent. Two explosion craters which formed in 1993–1994 produce approximately 15 m3/s of steam, yet discharge temperatures are ≤97°C. Water content of the total gas is 95–97 mol.%, noncondensible gas is 92–98 mol.% CO2, and sulfur gas is dominated by H2S. Relative amounts of He, Ar, and N2 show extensive mixing between hot spot and air or air-saturated meteoric water components but the average . The δ13C–CO2 is approximately −3.5‰ and δ34ST is approximately −0.8‰. The δD/δ18O of fumarole steam indicates separation from a homogeneous reservoir that is enriched 3–5‰ in 18O compared with local meteoric water. 3H indicates that this reservoir water has a maximum mean residence time of approximately 400 years and empirical gas geothermometry indicates a reservoir temperature of 260–320°C. The intracaldera hydrothermal reservoir in Alcedo is probably capable of producing up to 150 MW; however, environmental concerns as well as lack of infrastructure and power users will limit the development of this resource. Received: 19 April 1999 / Accepted: 23 October 1999  相似文献   

16.
The eruptions of the Soufrière Hills volcano on Montserrat (Lesser Antilles) from 1995 to present have draped parts of the island in fresh volcaniclastic deposits. Volcanic islands such as Montserrat are an important component of global weathering fluxes, due to high relief and runoff and high chemical and physical weathering rates of fresh volcaniclastic material. We examine the impact of the recent volcanism on the geochemistry of pre-existing hydrological systems and demonstrate that the initial chemical weathering yield of fresh volcanic material is higher than that from older deposits within the Lesser Antilles arc. The silicate weathering may have consumed 1.3% of the early CO2 emissions from the Soufrière Hills volcano. In contrast, extinct volcanic edifices such as the Centre Hills in central Montserrat are a net sink for atmospheric CO2 due to continued elevated weathering rates relative to continental silicate rock weathering. The role of an arc volcano as a source or sink for atmospheric CO2 is therefore critically dependent on the stage it occupies in its life cycle, changing from a net source to a net sink as the eruptive activity wanes. While the onset of the eruption has had a profound effect on the groundwater around the Soufrière Hills center, the geochemistry of springs in the Centre Hills 5 km to the north appear unaffected by the recent volcanism. This has implications for the potential risk, or lack thereof, of contamination of potable water supplies for the island’s inhabitants.  相似文献   

17.
Tofua Island is the largest emergent mafic volcano within the Tofua arc, Tonga, southwest Pacific. The volcano is dominated by a distinctive caldera averaging 4 km in diameter, containing a freshwater lake in the south and east. The latest paroxysmal (VEI 5–6) explosive volcanism includes two phases of activity, each emplacing a high-grade ignimbrite. The products are basaltic andesites with between 52 wt.% and 57 wt.% SiO2. The first and largest eruption caused the inward collapse of a stratovolcano and produced the ‘Tofua’ ignimbrite and a sub-circular caldera located slightly northwest of the island’s centre. This ignimbrite was deposited in a radial fashion over the entire island, with associated Plinian fall deposits up to 0.5 m thick on islands >40 km away. Common sub-rounded and frequently cauliform scoria bombs throughout the ignimbrite attest to a small degree of marginal magma–water interaction. The common intense welding of the coarse-grained eruptive products, however, suggests that the majority of the erupted magma was hot, water-undersaturated and supplied at high rates with moderately low fragmentation efficiency and low levels of interaction with external water. We propose that the development of a water-saturated dacite body at shallow (<6 km) depth resulted in failure of the chamber roof to cause sudden evacuation of material, producing a Plinian eruption column. Following a brief period of quiescence, large-scale faulting in the southeast of the island produced a second explosive phase believed to result from recharge of a chemically distinct magma depleted in incompatible elements. This similar, but smaller eruption, emplaced the ‘Hokula’ Ignimbrite sheet in the northeast of the island. A maximum total volume of 8 km3 of juvenile material was erupted by these events. The main eruption column is estimated to have reached a height of ∼12 km, and to have produced a major atmospheric injection of gas, and tephra recorded in the widespread series of fall deposits found on coral islands 40–80 km to the east (in the direction of regional upper-tropospheric winds). Radiocarbon dating of charcoal below the Tofua ignimbrite and organic material below the related fall units imply this eruption sequence occurred post 1,000 years BP. We estimate an eruption magnitude of 2.24 × 1013 kg, sulphur release of 12 Tg and tentatively assign this eruption to the AD 1030 volcanic sulphate spike recorded in Antarctic ice sheet records.  相似文献   

18.
Mount Morning is a Cenozoic, alkaline eruptive center in the south-west Ross Sea, Antarctica. New ages on 17 Mount Morning volcanic rocks (combined with 34 existing ages) allows division of Mount Morning volcanism into two phases, erupted between at least 18.7 Ma and 11.4 Ma, and 6.13 and 0.02 Ma. The position of Mount Morning on the active West Antarctic Rift System within the stationary Antarctic plate is a key factor in the eruptive center’s longevity. The earliest, mildly alkaline, Phase I volcanism comprises predominantly trachytic rocks produced by combined assimilation and fractional crystallization processes over 7.3 m.y. Strongly alkaline Phase II volcanism is dominated by a basanite – phonolite lineage, with the youngest (post last glacial maximum) activity dominated by small volume primitive basanite eruptions. The evolution from mildly to strongly alkaline chemistry between phases reflects magma residence time in the crust, the degree of mantle melting, or the degree of magma—country-rock interaction. Phase I magmatism occurred over a comparable area to the present-day, Phase II shield. The 5.2 m.y. volcanic hiatus separating Phase I and II coincides with a cycle of eruption and glacial erosion at the nearby Minna Bluff eruptive center. Mount Morning is the likely source of volcanic detritus in Cape Roberts drill-core (about 24.1 to 18.4 Ma) and in ANDRILL drill-hole 1B (about 13.6 Ma), located 170 km north and 105 km north-east respectively, of Mount Morning. Based upon the timing of eruptions and high heat-flow, Mount Morning should be considered a dormant volcano.  相似文献   

19.
 The Alban Hills, a Quaternary volcanic center lying west of the central Apennines, 15–25 km southeast of Rome, last erupted 19 ka and has produced approximately 290 km3 of eruptive deposits since the inception of volcanism at 580 ka. Earthquakes of moderate intensity have been generated there at least since the Roman age. Modern observations show that intermittent periods of swarm activity originate primarily beneath the youngest features, the phreatomagmatic craters on the west side of the volcano. Results from seismic tomography allow identification of a low-velocity region, perhaps still hot or partially molten, more than 6 km beneath the youngest craters and a high-velocity region, probably a solidified magma body, beneath the older central volcanic construct. Thirty centimeters of uplift measured by releveling supports the contention that high levels of seismicity during the 1980s and 1990s resulted from accumulation of magma beneath these craters. The volume of magma accumulation and the amount of maximum uplift was probably at least 40×106 m3 and 40 cm, respectively. Comparison of newer levelings with those completed in 1891 and 1927 suggests earlier episodes of uplift. The magma chamber beneath the western Alban Hills is probably responsible for much of the past 200 ka of eruptive activity, is still receiving intermittent batches of magma, and is, therefore, continuing to generate modest levels of volcanic unrest. Bending of overburden is the most likely cause of the persistent earthquakes, which generally have hypocenters above the 6-km-deep top of the magma reservoir. In this view, the most recent uplift and seismicity are probably characteristic and not precursors of more intense activity. Received: 15 April 1997 / Accepted: 9 August 1997  相似文献   

20.
 Virtually all the seismicity within Ruapehu Volcano recorded during a 2-month deployment in early 1994, with 14 broadband seismographs around the Tongariro National Park volcanoes in the North Island of New Zealand, was associated with the active vent and occurred within approximately 1 km of Ruapehu Crater Lake. High-frequency volcano-tectonic earthquakes and low-frequency events (similar to bursts of 2 Hz volcanic tremor) were both found to have sources in this region. The high-frequency events, which often consisted of a smaller precursor event followed approximately 2 s later by the main event, had sharp onsets and were locatable using standard techniques. The depth of these events ranged from the surface down to approximately 1500 m below Crater Lake. The low-frequency events did not have sharp onsets and were located by phase-correlation methods. Nearly all occurred under a small region on the east side of Crater Lake, at depths from 200 to 1000 m below the surface. This low-frequency earthquake source region, in which no high-frequency events occurred, may be the steam zone within the actual vent of Ruapehu Volcano. Received: 30 June 1996 / Accepted: 16 February 1998  相似文献   

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