Methodological studies on luminescence dating of volcanic ashes

We report here the results of a feasibility study of luminescence dating of polymineralic volcanic ash. Of the several possible protocols that used different emission bands and different IR stimulation regimes, the post infrared-infrared stimulated luminescence (pIR-IRSL) signal (detected using a violet-blue emission window with stimulation temperature in the region 260–320 °C) provided the most stable signal. This involved, i) identification of the most suitable temperature for pIR-IRSL read out, ii) determination of alpha efficiency and, iii) estimation of anomalous (athermal) fading rate. Anomalous fading rate (g-value in %/decade) of pIR-IRSL signal at 300 °C was 0.0–1.6%/decade and it ranged from 2.4 to 5.2%/decade for IRSL at 50 °C, both preheated to 320 °C. Thus, though more stable, pIR-IRSL signals may fade in nature, and even during laboratory extended irradiation. Of the models for fading correction by Huntley and Lamothe (2001) and Kars et al. (2008), the Kars et al. (2008) model performed better as the natural luminescence intensity was closer to the onset of saturation in the luminescence dose response curve. Our measurements suggest that alpha efficiencies of the pIR-IRSL signals are higher than that of IRSL. Fading corrected pIR-IRSL single aliquot regeneration (SAR) protocol based ages on three of the five volcanic ash beds are in agreement with the expected ages of ∼74 ka, based on geochemical association of the present samples to be the Youngest Toba Tuff (YTT). Other ash samples that gave ages of <24 ka and <37 ka, were inferred to have been in their secondary context, reworked from their original depositional sites. The onset of saturation dose of the pIR-IRSL signal (D₀) was ∼330 Gy and this implied a maximum measurable equivalent dose of 660 Gy. The minimum detectable dose was ∼5 Gy. These dose limits correspond to a typical age range of 1–150 ka using the pIR-IRSL signal for volcanic ashes.     

A three-dimensional Eulerian model for transport and deposition of volcanic ashes

Volcanic ash fallout represents a serious threat to people living near active volcanoes because it can produce several undesirable effects such as collapse of roofs by ash loading, respiratory sickness, air traffic disruption, or damage to agriculture. The assessment of such volcanic risk is therefore an issue of vital importance for public safety and its mitigation often requires to evaluate the temporal evolution of the phenomenon through reliable computational models.We develop an Eulerian model, named FALL3D, for the transport and deposition of volcanic ashes. The model is based on the advection–diffusion–sedimentation equation with a turbulent diffusion given by the gradient transport theory, a wind field obtained from a meteorological limited area model (LAM) and the source term derived from by buoyant plume theory. It can be used to forecast either ash concentration in the atmosphere or ash loading on the ground. Model inputs are topography, meteorological data given by a LAM, mass eruption rate, and a particle settling velocity distribution. A test application to the July 2001 Etna eruption is presented.     

The stratigraphic significance of fission-track ages on volcanic ashes in the marine Late Cenozoic of Southern California

Fission-track dates and planktonic microfossil datum levels provide a revised chronology for the marine Late Cenozoic of southern California. In southern California, the Pliocene/Pleistocene boundary has been placed at the first appearance of Globorotalia truncatulinoides within the Pico Formation, Balcom Canyon, Ventura County. A fission-track age on glass shards from the Bailey Ash close to this level yields a result of 1.12 ± 0.36 m.y. B.P. (millions of years before present). In tropical deep-sea cores, however, G. truncatulinoides has been shown to evolve within the Gilsa paleomagnetic event with an estimated age of 1.8 m.y. B.P. Thus, the first appearance of G. truncatulinoides in southern California is cryptogenic and probably related to the delayed migration into this region of water-mass conditions suitable for this species.Two volcanic ashes from the upper part of the Malaga Formation, Malaga Cove, Los Angeles County, yielded fission-track dates on glass shards of 4.42 ± 0.57 m.y. B.P. (lower ash) and 3.364 ± 0.69 m.y. B.P. (upper ash). These dates, in addition to inferred paleomagnetic ages of planktonic microfossil datum levels suggest that the Delmontian Stage of California ranges in age from ～6 to ～3 m.y. B.P. Therefore, the Miocene/Pliocene boundary considered by Berggren and Van Couvering to be ～5 m.y. B.P. must lie in the lower Delmontian Stage but paleontologic criteria for its recognition in California are not yet available.     

An excursion to the Bayuda volcanic field of Northern Sudan

A cluster of well-preserved recent volcanoes in the northern Bayuda Desert make up a more or less continuous field some 520 km² in area surrounded by a number of isolated centres of eruption. The volcanoes are numerous but small; up to 400 m in height and 0.35 km² in volume. Most of them are simple composite volcanoes with a pyroclastic cone skirted by a small lava field erupted from the same vent after explosive eruptions had ceased. In a few instances, however, the cone was eviscerated by more violent eruptions, leaving a deep explosion crater. The lavas are all nepheline-normative alkali basalts and contain a variety of xenocrysts and xenoliths from at least three different sources. The distribution of the recent volcanoes was partly controlled by large granitic ring-intrusions of the Basement Complex country rocks. These intrusions belong to the Younger Granite association of late Precambrian or Lower Palaeozoic age and represent a volcanic-intrusive episode widespread in northern Africa. The complexes are composed of cale-alkaline and peralkaline granites and syenites and a related plexus of dyke swarms.     

Time-scale calibration by high-precision UPb zircon dating of interstratified volcanic ashes in the Ordovician and Lower Silurian stratotypes of Britain

High initial parent/daughter element ratios and a unique dual decay scheme make UPb zicron ages more precise and reliable than most isotopic ages, and thus inherently superior for time-scale calibration. Employing improved techniques to the conventional method of UPb dating, we have analyzed microgram-size (2–12 × 10⁻⁸ g) zircon fractions from biostratigraphically controlled volcanic ashes and dated key Paleozoic time-markers with a precision better than 1% (±2Ma). Four of the stratotype samples from Britain for which fission-track ages [1] were previously reported have yielded improved ages of:438.7 ± 2.0Ma for the lower Silurian zone of Coronograptus cyphus from Llandovery strata at Dob's Linn, southern Scotland;457.5 ± 2.2 Ma for a Middle Ordovician Caradoc (Longvillian) ash near Bala, North Wales, and;465.7 ± 2.1and464.6 ± 1.8 Ma for the Didymograptus artus Zone and the type Didymograptus Murchisoni Zone, respectively, of the Llanvirn Series at Arenig Fawr and Abereiddi Bay, Wales. Another sample from the zone of Dicellograptus anceps (P. pacificus Subzone) of the Ashgill Series at Dob's Linn has been dated at445.7 ± 2.4Ma, suggesting placement of the Ordovician-Silurian time boundary at approximately 441 Ma. A sixth bentonite from Caradocian age strata of North America (Spechts Ferry Shale, Decorah Formation, Missouri) is453.7 ± 1.8Ma old, indicating that the Rocklandian Stage of the Mohawkian Series is only slightly younger than the Longvillian Stage of the Caradoc Series in Britain.     

A preliminary stable isotope study of volcanic ashes discharged by the 1979 eruption of Ontake Volcano,Nagano, Japan

Sulphur isotopic compositions of pyrite, anhydrite and native sulphur in volcanic ashes discharged by the 1979 eruption of Ontake volcano, Nagano, Japan were determined. The isotopic data indicate that sulphate in anhydrite and a part of native sulphur were produced by the disproportionation reaction of sulphite formed by dissolution of SO₂ in volcanic gases into water which filled a mud reservoir probably located just below the crater zone. Some part of H₂S in volcanic gases was fixed as pyrite and some was oxidised to form native sulphur. Hydrothermal alteration of country rocks to form pyrite, anhydrite and clay minerals had proceeded in the mud reservoir before eruption at temperatures ranging from 110° to 185°C which were estimated by oxygen isotopic fractionation between anhydrite and water.

     

Compositional variations in submarine volcanic ashes from the vicinity of the Vanuatu Island Arc: a response to ridge—arc collision?

The late Tertiary-Recent collision of the d'Entrecasteaux Zone with the Vanuatu (New Hebrides) Island Arc appears to have been associated with a shift to generally more potassic compositions. Glass shards analysed from Quaternary volcanic ashes recovered from ODP Leg 134 drill sites follow distinct shoshonitic and calc-alkaline trends, consistent with those previously recognised onshore. In the Aoba Basin (Site 832) a marked compositional change is recognised at 37 m below sea floor (mbsf). From estimates of sedimentation rates this is equivalent to an age of about 100 ka and may be associated with the development of the Central Basin volcanoes of Santa Maria, Aoba and Ambrym on fracture zones transverse to the island arc. Deeper than 37 mbsf differentiated compositions fall on a medium-K calc-alkaline trend but the associated basaltic glasses are strongly potassic. Conversely, above this level the differentiated glasses follow a shoshonitic trend but the basaltic glasses are medium-K calc-alkaline. Separate sources are envisaged, associated with subduction and rifting. Episodic subduction of the d'Entrecasteaux Zone exercises tectonic control over the release of magma from different depths. In contrast, shoshonitic glasses are sparsely represented in the Vanuatu forearc where relatively low-K calc-alkaline compositions probably correspond to the ashes in the lower part of the Aoba Basin sequence. This is consistent with their age (> 100 ka) estimated from lower sedimentation rates at these sites. The scarcity of younger shoshonitic ashes at the forearc sites is attributed to the influence of high-level westerly winds transporting ash from the Central Chain volcanoes eastwards across the North Fiji Basin.     

琼北火山群形成的动力学机制及地震现象的新认识

分析世界火山分布图发现琼北火山群分布在一南北向的火山带上,应用有限元方法模拟计算了双俯冲作用下海南岛所在雷琼\|越东火山带的形成机制,结合海南岛精确定位的地震数据和形变观测结果,认为琼北地区可能存在岩墙侵入或张性断裂膨胀,并根据地震数据模拟分析了岩墙侵入对区域应力场及形变的影响.琼北地区精确地震（2000~2006）定位结果表明地震主要集中在一个垂直面上,并且地震带两端有分叉现象.通过地震时空分布特征推测存在岩墙侵入,并通过数值模拟很好地解释了琼北地区地震的分布特征（狗骨头状）以及地表垂向形变东升西降的特征.     

Carbonatitic volcanism in the Kaiserstuhl alkaline complex: Evidence for highly fluid carbonatitic melts at the earth's surface

Extrusive carbonatites are described from the Miocene alkaline complex of the Kaiserstuhl, Rhinegraben, Western Germany. Agglutinated carbonatitic lapilli form pyroclastic rocks in which all components show forms acquired when a highly fluid melt was sprayed into the air by an explosive eruption: droplets, spherical and elliptical lapilli, rods, dumbbell and pear-shaped forms.Complete morphological analogies suggest a mechanism similar to the formation of “Pele's tears”, basaltic droplets formed by the eruption of the most fluid Hawaiian basaltic magmas. Evidence is provided by this example that CaCO₃-carbonatitic magmas can exist in nature under surface conditions displaying extremely low viscosity.     

Discovery of a late precambrian volcanic ring-complex at Jebel Abu Nahl,Bayouda Desert,Northern Sudan

A photogeologic study of Jebel Abu Nahl in the Bayouda Desert reveals that it is a Late Precambrian ring-complex, larger and more intricate than the three ring-complexes already known from the Northern Sudan. Like the latter, the Abu Nahl complex must have formed as a result of near-surface volcanic activities whereby extrusion of mainly acid lavas and tuffusites and the emplacement of high-level granites were accompanied by repeated subsidence of surface or near-surface blocks along arcuate ring-faults.     

Ashes to ashes, dust to dust

An account of the RAS Presidential Address of 11 February 2000, given by David A Williams , Perren Professor of Astronomy, University College London.     

Particle recycling in volcanic plumes

We have developed a new theoretical model of an eruption column that accounts for the re-entrainment of particles as they fall out of the laterally spreading umbrella cloud. The model illustrates how the mass flux of particles in the plume may increase with height in the plume, by a factor as large as 2.5 because of this recycling. Three important consequences are that (1) the critical velocity required to generate a buoyant eruption column for a given mass flux increases, (2) the total height of rise of the column may decrease, and (3) we infer that in relatively wind-free environments, for eruption columns near the conditions for collapse, the recycling of particles may lead to an unsteady oscillating motion of the plume, which, in time, may lead to the formation of interleaved fall and flow deposits.     

神奇的火山洞穴

洞穴是一种重要的地质地貌现象和地貌类型,也是人类文明的摇篮.火山洞穴主要是指火山熔岩型洞穴,除此之外,还有构造-差异风化洞穴和火山侵入岩崩塌堆叠型洞穴.它们在我国分布范围较广,各具不同的成因机理,旅游及科研价值巨大,极具开发潜力.     

Equilibrium calculations in volcanic gaseous systems

Equilibria calculations of high-temperature volcanic gases from lava lakes are carried out on the basis of best volcanic gas samples. The equilibrium gas composition at temperatures from 800° to 1400°K and pressures up to 25 kilobars (in ideal gas system) was calculated using the free energy minimization model as well as the Newton-Raphson methods. It is shown that the juvenile «magmatic gas » of basaltic magma consists of three components: H₂O, SO₂, CO₂; the water vapor being about 60%. The increase of temperature under constant pressure results in the increase of the SO₂ concentration and in the simultaneous decrease of H₂S. Under the same conditions the ratios CO/CO₂ and H₂/H₂O are found to increase. Methane cannot be a component of «magmatic gas» corresponding to the elemental composition of basaltic lava gases. The calculated values of \(P_{O_2 } \) are in good agreement with the experimental data obtained from direct measurements of \(P_{O_2 } \) in lava lakes and experiments with basaltic melts.     

Base surge in recent volcanic eruptions

A base surge, first identified at the Bikini thermonuclear undersea explosion, is a ring-shaped basal cloud that sweeps outward as a density flow from the base of a vertical explosion column. Base surges are also common in shallow underground test explosions and are formed by expanding gases which first vent vertically and then with continued expansion rush over the crater lip (represented by a large solitary wave in an underwater explosion), tear ejecta from it, and feed a gas-charged density flow, which is the surge cloud. This horizontally moving cloud commonly has an initial velocity of more than 50 meters per second and can carry clastic material many kilometers. Base surges are a common feature of many recent shallow, submarine and phreatic volcanic eruptions. They transport ash, mud, lapilli, and blocks with great velocity and commonly sandblast and knock down trees and houses, coat the blast side with mud, and deposit ejecta at distances beyond the limits of throw-out trajectories. Close to the eruption center, the base surge can erode radial channels and deposit material with dune-type bedding.