Magma chambers beneath the Klyuchevskoy volcanic group (Kamchatka)

A 3D velocity model of the Earth’s crust beneath the Klyuchevskoy volcanic group has been constructed using the seismic tomography method. Anomalies of the velocity parameters related to the zones of magma supply to active volcanoes have been distinguished. Petrological data on the composition, temperature, and pressure of generation and crystallization of parental melts of Klyuchevskoy volcano magnesian basalts have been obtained. The parental melt corresponds to picrite (MgO = 13–14 wt %) with an ultimate saturation of SiO₂ (49–50 wt %), a high H₂O content (2.2–2.9%), and incompatible elements (Sr, Rb, Ba). This melt is formed at pressures of 15–20 kbar and temperatures of 1280–1320°C. Its further crystallization proceeds in intermediate magma chambers at two discrete pressure levels (i.e., greater than 6, and 1–2 kbar). The results of the petrological studies are in good agreement with the seismotomographic model.     

Seismic investigation of the roots of the Kliuchevskaya group volcanoes,Kamchatka

Earthquake data from Δ<2° and of indermediate depth of focus in the upper mantle near the Kliuchevskaya volcanic region show a screening effect of longitudinal and transverse waves. The indicated anomalous zones are identified with the region of magma localization. The anomalous zones are at 35–110 km depths.     

Precursors to dyke-fed eruptions at basaltic volcanoes: insights from patterns of volcano-tectonic seismicity at Kilauea volcano,Hawaii

To investigate the physical controls on volcano-tectonic (VT) precursors to eruptions and intrusions at basaltic volcanoes, we have analyzed the spatial and temporal patterns of VT earthquakes associated with 34 eruptions and 23 dyke intrusions that occurred between 1960 and 1983 at Kilauea, in Hawaii. Eighteen of the 57 magmatic events were preceded by an acceleration of the mean rate of VT earthquakes located close to the main shallow magma reservoir. Using a maximum-likelihood technique and the Bayesian Information Criterion for model preference, we demonstrate that an exponential acceleration is preferred over a power-law acceleration for all sequences. These sequences evolve over time-scales of weeks to months and are consistent with theoretical models for the approach to volcanic eruptions based on the growth of a population of fractures in response to an excess magma pressure. Among the remaining 40 magmatic events, we found a significant correlation between swarms of VT earthquakes located in the mobile south-flank of Kilauea and eruptions and intrusions. The behaviour of these swarms suggests that at least some of the magmatic events are triggered by transient episodes of elevated rates of aseismic flank movement, which could explain why many eruptions and intrusions are not preceded by longer-term precursory signals. In none of the 57 cases could a precursory sequence be used to distinguish between the approach to an eruption or an intrusion, so that, even when a precursory sequence is recognized, there remains an empirical chance of about 40% (24 intrusions from 57 magmatic events) of issuing a false alarm for an imminent eruption.     

镜质体反射率与最高温度及其附近温度变化率的关系——几种镜质体反射率计算模型的比较

对常用的Waples、Middleton及Easy%Ro模型,通过蒙特卡罗方法,得到了相应的简化模型.所有简化模型均表明镜质体反射率(Ro)主要受控于最高温度(Tmax),并且与最高温度附近的温度变化率(Hr)有关.通过对简化模型的比较发现：(1)相对于Easy%Ro模型而言,Waples、Middleton模型对Hr的变化更为敏感;(2)总的来讲,达到相同的Ro值Middleton模型需要的温度最高,Easy%Ro模型其次,Waples模型需要的温度最低.在一定条件下,根据简化模型,利用实测的lnRo-H数据可以求取地层经历的最高温度及相应的地温梯度、热流和时间.实例分析表明岩石热导率等数据的准确性直接影响Ro解释的结果,但这种影响容易分析,而Hr的不确定性则导致多解性.     

Some regularity in the process of re-awakening of andesite and dacite volcanoes: specific features of the 1982 El Chichón volcano,México reactivation

El Chichón volcano is an andesite stratovolcano in southern México. It erupted in March 1982, after about 550 years of quiescence. The 1982 eruption of El Chichón has not been followed by the growth of a lava dome within the newly formed crater. This is rather anomalous since the construction of a new dome after the destruction of an old one is a common process during the eruptions at andesite and dacite volcanoes. To discuss this anomalous aspect of the El Chichón eruption, some regularity in the process of re-awakening of dormant (here defined as a period of quiescence of more than 100 years) andesite and dacite volcanoes are studied based on the seismic activity recorded at the volcanoes Bezymianny, Mount St. Helens, El Chichón, Unzen, Pinatubo and Soufrière Hills. Three stages were identified in the re-awakening activity of these volcanoes: (1) preliminary seismic activity, leading up to the first phreatic explosion; (2) activity between the first and the largest explosions; (3) post-explosion dome-building process. The eruptions were divided into two groups: low-VEI (Volcanic Explosivity Index) and the long duration stage-1 events (Unzen, 1991 and Soufrière Hills volcano, 1995) and high-VEI and the short duration stage-1 events (Bezymianny, 1956; Mount St. Helens, 1980; El Chichón, 1982 and Pinatubo, 1992). The comparative analysis of the seismo-eruptive activity of two eruptions of the second group, the 1980 of Mt. St. Helens and the 1982 of El Chichón, produced an explanation the absence of new dome building during the 1982 eruption of El Chichón volcano. It may be explained in terms of the unusually rapid emission of gas and water from the magmatic and hydrothermal system beneath the volcano during a relatively short sequence of large explosions that could have sharply increased the viscosity of the magma making impossible its exit to the surface.     

Three-dimensional P-wave velocity structure derived from local earthquakes at the Katmai group of volcanoes,Alaska

The three-dimensional P-wave velocity structure beneath the Katmai group of volcanoes is determined by inversion of more than 10,000 rays from over 1000 earthquakes recorded on a local 18 station short-period network between September 1996 and May 2001. The inversion is well constrained from sea level to about 6 km below sea level and encompasses all of the Katmai volcanoes; Martin, Mageik, Trident, Griggs, Novarupta, Snowy, and Katmai caldera. The inversion reduced the average RMS travel-time error from 0.22 s for locations from the standard one-dimensional model to 0.13 s for the best three-dimensional model. The final model, from the 6th inversion step, reveals a prominent low velocity zone (3.6–5.0 km/s) centered at Katmai Pass and extending from Mageik to Trident volcanoes. The anomaly has values about 20–25% slower than velocities outboard of the region (5.0–6.5 km/s). Moderately low velocities (4.5–6.0 km/s) are observed along the volcanic axis between Martin and Katmai Caldera. Griggs volcano, located about 10 km behind (northwest of) the volcanic axis, has unremarkable velocities (5.0–5.7 km/s) compared to non-volcanic regions. The highest velocities are observed between Snowy and Griggs volcanoes (5.5–6.5 km/s). Relocated hypocenters for the best 3-D model are shifted significantly relative to the standard model with clusters of seismicity at Martin volcano shifting systematically deeper by about 1 km to depths of 0 to 4 km below sea level. Hypocenters for the Katmai Caldera are more tightly clustered, relocating beneath the 1912 scarp walls. The relocated hypocenters allow us to compare spatial frequency-size distributions (b-values) using one-dimensional and three-dimensional models. We find that the distribution of b is significantly changed for Martin volcano, which was characterized by variable values (0.8 < b < 2.0) with standard locations and more uniform values (0.8 < b < 1.2) after relocation. Other seismic clusters at Mageik (1.2 < b < 2.2), Trident (0.5 < b < 1.5) and Katmai Caldera (0.8 < b < 1.8) had stable b-values indicating the robustness of the observations. The strong high b-value region at Mageik volcano is mainly associated with an earthquake swarm in October, 1996 that possibly indicates a shallow intrusion or influx of gas. The new velocity and spatial b-value results, in conjunction with prior gravity (Bouguer anomalies up to − 40 mgal) and interferometry (several cm uplift) data, provide strong evidence in favor of partially molten rock at shallow depths beneath the Mageik–Katmai–Novarupta region. Moderately low velocities beneath Martin and Katmai suggest that old, mostly solidified intrusions exist beneath these volcanoes. Higher relative velocities beneath the Griggs and Snowy vents suggest that no magma is resident in the shallow crust beneath these volcanoes.     

Volcan ceboruco: A major composite volcano of the mexican volcanic belt

Ceboruco is a major composite volcano at the western end of the Mexican Volcanic Belt, near the junction between the North American and Pacific plates. The volcano is built from successive eruptions of andesite lavas and pyroclastic rocks, and major eruptions during its history have resulted in the formation of two concentric calderas. The youngest volcanic activity has included the extrusion of dacites within the inner caldera and a voluminous flank eruption of andesite during 1870–72. Fumarolic activity persists to the present day. Chemical analyses show that the lavas are of cale-alkaline type and rangs from andesite (SiO₂=58–61%) to acid dacite (SiO₂=68%) in composition. The rate of increase of K₂O relative to SiO is greater than that in volcanic rocks from the Mexican Volcanic Belt as a whole. This indicates that simple models based on the application of such relationships may not be adequate to explain the petrogenesis of calc-alkaline lavas.     

On the geometrical form of volcanoes

Many volcanic edifices have a remarkably symmetric geometrical form. An example is Mount Fuji in Japan. We model this form assuming that the surface of the volcano is a surface of uniform hydraulic potential; that an erupting magma will follow the path of minimum resistance to the surface. In order to model the resistance to fluid flow we assume the volcanic edifice is a uniform porous medium. The vertical flow of magma is also resisted by the gravitational body force. If the volcano becomes too tall flank eruptions will widen it; if the volcano becomes too wide summit eruptions will increase its elevation. Using the Dupuit approximation for an unconfined aquifer it is shown that the percolation equation is applicable. As magma reaches the surface it is assumed to extend the solid, porous matrix. A similarity solution is obtained to this moving boundary problem. The solution predicts a uniform shape for all volcanoes. This shape is shown to be in excellent agreement with the geometrical form of Mount Fuji.     

Activity of the northern volcano group according to drilling data in the Ushkovsky crater glacier, Kamchatka

Long-term investigations in the Ushkovsky Volcano caldera were completed in June 1998 using deep drilling of the crater glacier filling the Gorshkov summit cone. A 212-m ice core containing 354 bands of volcanic ash was obtained. The possibility of using an analysis of these data in paleovolcanic reconstructions of the northern volcano group is discussed.     

Tectonic and structural evolution of Gough volcano: A volcanological model

Morphostructural, stratigraphic and tectonic data indicate that the evolution of Gough volcano is similar to other oceanic intraplate volcanoes, is older than 1 Ma, and is related to a transform fault. At least six evolutionary stages can be distinguished within two major magmatostructural periods dominated by basaltic and trachytic magmas, respectively.The basaltic shield volcano is characterized by a curved, elongated shape in plan and a rift zone with a high density of dykes, combined with a radial intrusive system. The latter is interpreted as being fed by a magma chamber some 4 km below the surface. The activity of the volcano became more centralized at the end of the basaltic period and its slopes became steeper. This corresponds to the development of a shallower and narrower central conduit in the edifice. The basaltic period was terminated by formation of a shield caldera related to the 4 km deep magma chamber. The term “shield caldera” is used for a collapse structure that is postmagmatic, large in comparison with the diameter of the volcano, and delimited by normal faults that do not show a closed circular pattern but rather a series of arcs. In contrast, summit calderas are defined as smaller, circular-shaped, centrally situated, synmagmatic features, related to a central shallow column. During the basaltic period, landslides were generated on the flanks of the edifice as a result of slope stability factors which are not easy to determine at present, and dynamic factors among which the intrusion of magma along a curved zone certainly played a major role.The trachytic period is characterized by comparatively rare pyroclastic deposits and a large volume of thick flows extruded from domes. These extrusions, as well as plugs, formed from vertical cylindrical columns of magma rising from shallow individual magma pockets fed by the main reservoir.     

Volume estimation of the 1998 flank collapse at Casita volcano,Nicaragua: a comparison of photogrammetric and conventional techniques

In October 1998 a precipitation‐triggered flank collapse occurred at Casita volcano, Nicaragua, leading to a devastating lahar. In this paper the failure volume was calculated using a range of methods. Several pre‐ and post‐failure digital elevation models (DEMs) were created, based on photogrammetric, cartometric and surveying data. The wide range in resulting volumes prompted an assessment of the accuracies and potential problems associated with each of the datasets and techniques used. The best estimate for the failure volume is 1·6 × 10⁶ m³. It is based on a vegetation‐corrected pre‐failure DEM, generated using automated digital photogrammetry, and a post‐failure surface based on a field survey carried out with a Total Station. The volume figure is approximately an order of magnitude higher than values reported in previous publications, all of which are based solely on field estimates. This demonstrates that values reported in the literature, if they are not based on rigorous quantitative analysis, must be regarded with caution. Copyright © 2002 John Wiley & Sons, Ltd.     

A model of diffuse degassing at three subduction-related volcanoes

2 and δ¹³C in soil gas were measured at three active subduction-related stratovolcanoes (Arenal and Poás, Costa Rica; Galeras, Colombia). In general, Rn, CO₂ and δ¹³C values are higher on the lower flanks of the volcanoes, except near fumaroles in the active craters. The upper flanks of these volcanoes have low Rn concentrations and light δ¹³C values. These observations suggest that diffuse degassing of magmatic gas on the upper flanks of these volcanoes is negligible and that more magmatic degassing occurs on the lower flanks where major faults and greater fracturing in the older lavas can channel magmatic gases to the surface. These results are in contrast to findings for Mount Etna where a broad halo of magmatic CO₂ has been postulated to exist over much of the edifice. Differences in radon levels among the three volcanoes studied here may result from differences in age, the degree of fracturing and faulting, regional structures or the level of hydrothermal activity. Volcanoes, such as those studied here, act as plugs in the continental crust, focusing magmatic degassing towards crater fumaroles, faults and the fractured lower flanks. Received: 16 December 1997 / Accepted: 27 January 2000     

Underwater mud volcanoes in the Taman Peninsula: Sonar surveys

Studies of the sea bottom around the Taman Peninsula were used as an example to demonstrate the abilities of sonar systems using signals subjected to linear frequency modulation (LFM) for the detection and study of underwater mud volcanoes.     

Genetic aspects of the Jan Mayen fissure volcano group on the mid-oceanic submarine Mohns Ridge,Norwegian Sea

During six recent expeditions, of which four were led by the author, to the mainly basaltic island of Jan Mayen (length 53.6 km; mean width 7 km; area 380 km²), evidence has been gathered for at least six distinct volcanic phases, coupled with rythmic magmatic variations in the oceanite-trachybasalt-trachyandesite-trachyte lava suite. There are also certain intermediate types and associate pyroclasts, and effusive or explosive uprise of these lavas through two fissure-swarms, intersecting at about 12°, produced a subaerial volcano-group of several hundred cones, elongate north-east — south-west on the north-west margin of a large submarine pedestal possibly capped by a drowned plane of marine erosion at 100–200 m below present mean sea level. These rocks appear to range in age between Tertiary and Recent. Jan Mayen grows from the north-west flank of the submarine Mohns Ridge close to its axial rift within a markedly seismic zone, at a likely junction of crustal fractures immediately north of a sharp east-west flexure in the rift which may indicate a major strike-slip fault. The lavas have affinities with corresponding lavas in Scottish Hebrides and with the basalt-trachyte associations on the islands of Ascension, St Helena, Tristan da Cunha and Gough on the mid-Atlantic Ridge. Both form and structure suggest the island mass has the configuration of a volcanic dome (or possibly two coalescent domes diverging slightly south-west) at least 70×30 km in area and about 1.5 km in height. In the mass are two distinct major volcanic foci: an earlier South Jan or Rudolftoppen « dispersed » or « plexiform » vent, ascribable to numerous «drilled out» fissure-intersections within an area of more than 25 km², and a later North Jan or Beerenberg central vent. A third focus of indeterminate relative age may lie beneath Straumflaket, in the shallow sea off South Cape. Magmas rose through individual fissures and their intersections, to form linear cones of tuff and lava, and extensive basalt floods. Most are vertical dikes but, in places, highly inclined sheets and sills tend to follow bedding and other planes of weakness in tuff and sometimes fed lava flows. Basaltic magma invaded a complex system of intersecting master fissures and subsidiary fractures in tuff near the surface, inflated the mass, distorted and generated local joint systems in the tuff and finally gave rise to meshworks of basaltic sheets in it. Following a long period of repeated fissure eruption, ten of the main basaltic throats at the South Jan dispersed focus, and one near the junction between North Jan and South Jan, were plugged by trachyte, after which there was volcanic quiescence with contemporaneous deep glacial, fluvial and marine erosion. During the subsequent resumption of volcanic activity the North Jan focus of central eruption rose to importance at the expense of the South Jan focus, which remained sealed by trachyte, but numerous small basaltic fissure volcanoes erupted on the seaward edges of the South Jan plateau and through the coastal platform beneath its cliffs, at or near sea level.     

Stochastic modelling at Stromboli: a volcano with remarkable memory

The assessment of potential volcanic eruptions is a critical aspect when evaluating the safety of populated areas. A stochastic approach has been developed for the analysis and simulation of data sampled at active volcanoes. This approach allows for the detection and quantification of time correlation, volcanic event forecasts using Cox model based simulations and volcanic tremor decomposition in order to identify potential precursors of major eruptions. The stochastic approach has been applied to data monitored at Stromboli volcano. Significant time correlation has been detected which makes Stromboli a volcano with a remarkable memory of its recent activity in comparison to other volcanoes. Forecasting of the number of strombolian events for the next few days has been performed by Monte Carlo simulations. Finally, kriging analysis of the tremor intensity has enabled time component estimation which could furnish additional monitoring variables for the forecast of paroxysmal phases at Stromboli.