The 1984 to 1996 cyclic activity of Lascar Volcano, northern Chile: cycles of dome growth, dome subsidence, degassing and explosive eruptions

 Lascar Volcano (5592 m; 23°22'S, 67°44'W) entered a new period of vigorous activity in 1984, culminating in a major explosive eruption in April 1993. Activity since 1984 has been characterised by cyclic behaviour with recognition of four cycles up to the end of 1993. In each cycle a lava dome is extruded in the active crater, accompanied by vigorous degassing through high-temperature, high-velocity fumaroles distributed on and around the dome. The fumaroles are the source of a sustained steam plume above the volcano. The dome then subsides back into the conduit. During the subsidence phase the velocity and gas output of the fumaroles decrease, and the cycle is completed by violent explosive activity. Subsidence of both the dome and the crater floor is accommodated by movement on concentric, cylindrical or inward-dipping conical fractures. The observations are consistent with a model in which gas loss from the dome is progressively inhibited during a cycle and gas pressure increases within and below the lava dome, triggering a large explosive eruption. Factors that can lead to a decrease in gas loss include a decrease in magma permeability by foam collapse, reduction in permeability due to precipitation of hydrothermal minerals in the pores and fractures within the dome and in country rock surrounding the conduit, and closure of open fractures during subsidence of the dome and crater floor. Dome subsidence may be a consequence of reduction in magma porosity (foam collapse) as degassing occurs and pressurisation develops as the permeability of the dome and conduit system decreases. Superimposed upon this activity are small explosive events of shallow origin. These we interpret as subsidence events on the concentric fractures leading to short-term pressure increases just below the crater floor. Received: 12 December 1996 / Accepted: 6 May 1997     

Transient Coronal Sigmoids and Rotating Erupting Flux Ropes

To determine the relationship between transient coronal (soft X-ray or EUV) sigmoids and erupting flux ropes, we analyse four events in which a transient sigmoid could be associated with a filament whose apex rotates upon eruption and two further events in which the two phenomena were spatially but not temporally coincident. We find the helicity sign of the erupting field and the direction of filament rotation to be consistent with the conversion of twist into writhe under the ideal MHD constraint of helicity conservation, thus supporting our assumption of flux rope topology for the rising filament. For positive (negative) helicity the filament apex rotates clockwise (counterclockwise), consistent with the flux rope taking on a reverse (forward) S shape, which is opposite to that observed for the sigmoid. This result is incompatible with two models for sigmoid formation: one identifying sigmoids with upward arching kink-unstable flux ropes and one identifying sigmoids with a current layer between two oppositely sheared arcades. We find instead that the observations agree well with the model by Titov and Démoulin (Astron. Astrophys. 351, 707, 1999), which identifies transient sigmoids with steepened current layers below rising flux ropes.     

琼北地区晚更新世射气岩浆喷发初步研究

琼北地区晚更新世射气岩浆喷发形成众多的低平火山口 ,出露典型的基浪堆积物 ,在火口垣露头上可清晰地观察到大型低角度交错层理、板状层理和波状层理以及远源相的球粒状增生火山砾。玄武质岩浆在上升过程中遇水爆炸形成低平火山口及基浪堆积 ,为认识琼北地区新生代以来的火山活动规律和琼北 -雷南地区的构造环境 ,以及未来火山灾害预测提供了重要的依据     

Volcanic impediments in the progressive development of pre-Columbian civilizations in the Ecuadorian Andes

Archaeological investigations in Ecuador have proposed that there appear to be hiatus or anomalous jumps in the progressive development of pre-Columbian indigenous cultures, based upon the fact that their ceramics and tools demonstrate abrupt advances in their sophistication at several horizons in the soil profile. Because some of these horizons are clearly associated with volcanic ash layers, archaeologists have sought a causal relation with volcanism, that is, the eruptive events or their products severely interfered with the early inhabitants, resulting in their abandonment of certain areas.     

Acoustic source characterization of impulsive Strombolian eruptions from the Mount Erebus lava lake

We invert for acoustic source volume outflux and momentum imparted to the atmosphere using an infrasonic network distributed about the erupting lava lake at Mount Erebus, Ross Island, Antarctica. By modeling these relatively simple eruptions as monopole point sources we estimate explosively ejected gas volumes that range from 1,000 m³ to 24,000 m³ for 312 lava lake eruptions recorded between January 6 and April 13, 2006. Though these volumes are compatible with bubble volumes at rupture (as estimated from explosion video records), departures from isotropic radiation are evident in the recorded acoustic wavefield for many eruptions. A point-source acoustic dipole component with arbitrary axis orientation and strength provides precise fit to the recorded infrasound. This dipole source axis, corresponding to the axis of inferred short-duration material jetting, varies significantly between events. Physical interpretation of dipole orientation as being indicative of eruptive directivity is corroborated by directional emissions of ejecta observed in Erebus eruption video footage. Although three azimuthally distributed stations are insufficient to fully characterize the eruptive acoustic source we speculate that a monopole with a minor amount of oriented dipole radiation may reasonably model the primary features of the recorded infrasound for these eruptions.     

Moment tensor inversion of very long period seismic signals from Strombolian eruptions of Erebus Volcano

Strombolian eruptions from the long-lived lava lake of Erebus volcano, Ross Island, Antarctica, generate repeating Very Long Period (VLP) signals, containing energy between approximately 30 and 5 s, that persist for several minutes and through the post-eruptive refilling of the lava lake. The initial approximately 10 s of this signal is moderately variable, particularly with respect to its initial polarity, while the following VLP coda has been observed to be stable since the earliest VLP observations were made (1996). To estimate forces and force couples consistent with the Erebus VLP signature, we perform moment tensor inversions for point sources using high signal-to-noise data stacks from the six-station, 18-component broadband seismographic network and Green's function forward calculations that incorporate topography. We infer a shallow (approximate depth of less than 400 m below the lava lake surface) source centroid that underlies the center to the northwestern rim of the main crater, east and north of the lava lake. Integrated M_ii functions over the predominant (180 s) signal duration of VLP events show that the net scalar moments for these events are on the order of 4 × 10¹³ N m (corresponding to a moment magnitude mw ≈ 3) for typical sized VLP events. Moment rate tensors which characterize force couple components are dominated (85–97% of variance) by dilatational components. Approximately 25% of the data variance is attributable to single forces that are attributable to oscillatory reaction forces caused by fluid transport, however, the relative contributions of vertical forces and couples with this sparse network is poorly resolved for these shallow sources. The generally high degree of repeatability in the VLP signal across thousands of eruptions over the past decade indicates that the response of the conduit system to gas slug ascent and subsequent gravitational disequilibrium is stable, consistent with the generally unchanging surface manifestation of the convecting lava lake system, and arguing for a thermally and dynamically stable conduit system beneath the lava lake.     

Explosive eruptive record in the Katmai region,Alaska Peninsula: an overview

At least 15 explosive eruptions from the Katmai cluster of volcanoes and another nine from other volcanoes on the Alaska Peninsula are preserved as tephra layers in syn- and post-glacial (Last Glacial Maximum) loess and soil sections in Katmai National Park, AK. About 400 tephra samples from 150 measured sections have been collected between Kaguyak volcano and Mount Martin and from Shelikof Strait to Bristol Bay (∼8,500 km²). Five tephra layers are distinctive and widespread enough to be used as marker horizons in the Valley of Ten Thousand Smokes area, and 140 radiocarbon dates on enclosing soils have established a time framework for entire soil–tephra sections to 10 ka; the white rhyolitic ash from the 1912 plinian eruption of Novarupta caps almost all sections. Stratigraphy, distribution and tephra characteristics have been combined with microprobe analyses of glass and Fe–Ti oxide minerals to correlate ash layers with their source vents. Microprobe analyses (typically 20–50 analyses per glass or oxide sample) commonly show oxide compositions to be more definitive than glass in distinguishing one tephra from another; oxides from the Kaguyak caldera-forming event are so compositionally coherent that they have been used as internal standards throughout this study. Other than the Novarupta and Trident eruptions of the last century, the youngest locally derived tephra is associated with emplacement of the Snowy Mountain summit dome (<250 ¹⁴C years B.P.). East Mageik has erupted most frequently during Holocene time with seven explosive events (9,400 to 2,400 ¹⁴C years B.P.) preserved as tephra layers. Mount Martin erupted entirely during the Holocene, with lava coulees (>6 ka), two tephras (∼3,700 and ∼2,700 ¹⁴C years B.P.), and a summit scoria cone with a crater still steaming today. Mount Katmai has three times produced very large explosive plinian to sub-plinian events (in 1912; 12–16 ka; and 23 ka) and many smaller pyroclastic deposits show that explosive activity has long been common there. Mount Griggs, fumarolically active and moderately productive during postglacial time (mostly andesitic lavas), has three nested summit craters, two of which are on top of a Holocene central cone. Only one ash has been found that is (tentatively) correlated with the most recent eruptive activity on Griggs (<3,460 ¹⁴C years B.P.). Eruptions from other volcanoes NE and SW beyond the Katmai cluster represented in this area include: (1) coignimbrite ash from Kaguyak’s caldera-forming event (5,800 ¹⁴C years B.P.); (2) the climactic event from Fisher caldera (∼9,100 ¹⁴C years B.P.—tentatively correlated); (3) at least three eruptions most likely from Mount Peulik (∼700, ∼7,700 and ∼8,500 ¹⁴C years B.P.); and (4) a phreatic fallout most likely from the Gas Rocks (∼2,300 ¹⁴C years B.P.). Most of the radiocarbon dating has been done on loess, soil and peat enclosing this tephra. Ash correlations supported by stratigraphy and microprobe data are combined with radiocarbon dating to show that variably organics-bearing substrates can provide reliable limiting ages for ash layers, especially when data for several sites is available.     

The potential of cryptotephra and OSL dating for refining the chronology of open-air archaeological windblown sand sites: A case study from Mirkowice 33, northwest Poland

The discovery of a cryptotephra (nonvisible volcanic horizon) in a windblown sand archaeological site in Poland highlights how luminescence and tephrostratigraphy may combine to better refine the chronology of such sites. In this study we identify a cryptotephra horizon which on the basis of major and minor element geochemistry and an OSL age of 2.3 ± 0.1 ka is correlated to the Glen Garry tephra. The different methodological strengths of OSL and tephrostratigraphy may be harnessed to counter the limitations of a single approach to produce a more secure chronology. Although in this study the tephra deposition event is shown to post-date the archaeological activity, the methodological approach is clearly demonstrated. Further investigations will reveal if cryptotephra layers are commonly preserved in such environmental settings. If this is so then future applications of this approach may prove to be more widely applicable.     

Quench rates in air, water, and liquid nitrogen, and inference of temperature in volcanic eruption columns

We apply a geospeedometer previously developed in this lab to investigate cooling rate profiles of rhyolitic samples initially held at 720–750°C and quenched in water, liquid nitrogen, and air. For quench of mm-size samples in liquid nitrogen and in air, the cooling rate is uniform and is controlled by heat transfer in the quench medium instead of heat conduction in the sample. The heat transfer coefficient in ‘static’ air decreases with increasing sample size. For quench of mm-size samples in water, heat transfer in water is rapid and the cooling rate is largely controlled by heat conduction in the sample. Our experimental results are roughly consistent with previous calculations for cooling in air and in water (although constant heat transfer coefficients were used in these calculations), but cooling rate in liquid nitrogen is only 1.8–2.3 times that in ‘static’ air, and slower by a factor of 2 than calculated by previous authors. Cooling rate in compressed airflow is about the same as that in liquid nitrogen. The experimental results are applied to interpret cooling rates of pyroclasts in ash beds of the most recent eruptions of the Mono Craters. Cooling rates of pyroclasts are inversely correlated with sample size and slower than those in air. The results indicate that the hydrous species concentrations of the pyroclasts were frozen in the eruption column, rather than inside ash beds or in flight in ambient air. From the cooling rates, we infer eruption column temperature in a region where and at a time when hydrous species concentrations in a pyroclast were locked in. The temperature ranges from 260 to 570°C for the most recent eruptions of Mono Craters. These are the first estimates of temperatures in volcanic eruption columns. The ability to estimate cooling rates and eruption column temperatures from eruptive products will provide constraints to dynamic models for the eruption columns.     

A multi-disciplinary study of the 2002–03 Etna eruption: insights into a complex plumbing system

The 2002–03 Mt Etna flank eruption began on 26 October 2002 and finished on 28 January 2003, after three months of continuous explosive activity and discontinuous lava flow output. The eruption involved the opening of eruptive fissures on the NE and S flanks of the volcano, with lava flow output and fire fountaining until 5 November. After this date, the eruption continued exclusively on the S flank, with continuous explosive activity and lava flows active between 13 November and 28 January 2003. Multi-disciplinary data collected during the eruption (petrology, analyses of ash components, gas geochemistry, field surveys, thermal mapping and structural surveys) allowed us to analyse the dynamics of the eruption. The eruption was triggered either by (i) accumulation and eventual ascent of magma from depth or (ii) depressurisation of the edifice due to spreading of the eastern flank of the volcano. The extraordinary explosivity makes the 2002–03 eruption a unique event in the last 300 years, comparable only with La Montagnola 1763 and the 2001 Lower Vents eruptions. A notable feature of the eruption was also the simultaneous effusion of lavas with different composition and emplacement features. Magma erupted from the NE fissure represented the partially degassed magma fraction normally residing within the central conduits and the shallow plumbing system. The magma that erupted from the S fissure was the relatively undegassed, volatile-rich, buoyant fraction which drained the deep feeding system, bypassing the central conduits. This is typical of most Etnean eccentric eruptions. We believe that there is a high probability that Mount Etna has entered a new eruptive phase, with magma being supplied to a deep reservoir independent from the central conduit, that could periodically produce sufficient overpressure to propagate a dyke to the surface and generate further flank eruptions.Editorial responsibility: J. Donnelly-Nolan