The identification and diagnostics of active and potentially active volcanic features in the Kuril-Kamchatka island arc and the Commander Islands link of the Aleutian arc

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.     

Physical volcanology and structural development of Cerro Azul Volcano, Isabela Island, Galápagos: implications for the development of Galápagos-type shield volcanoes

3 ) erupted from circumferential vents near the summit. These flows are nearly an order of magnitude smaller in volume than the predominantly aa flows erupted from radial eruptive fissures near the break in slope (0.06–0.1 km³). The differences in volume and flow morphology with altitude are due to slower eruption rates from summit vents than from flank vents, which, in turn, are attributable to the different heights the magmas must ascend from shallow reservoirs. These observations support the contention that the steep upper flanks were constructed by the buildup of short lava flows rather than by the structural deformation of originally gently dipping flanks. In addition to the higher eruption rates, a subdued lower flank geometry is promoted by the deposition of lava deltas onto the shallow Galápagos platform on the western, northern, and eastern flanks of the volcano. ⁴⁰Ar/³⁹Ar geochronology and volume estimates show that, despite their morphologic differences, the growth of the western Galápagos shields has been nearly synchronous, precluding an evolutionary model for their development. The wide variations in elevation, volume, area, and the distribution of slope angles among the western volcanoes can be linked instead to different long-term eruption rates and, to a lesser degree, the position of each volcano relative to the edge of the Galápagos platform. Received: 24 September 1998 / Accepted: 7 August 1999     

Erosion calderas: origins, processes, structural and climatic control

 The origin and development of erosion-modified, erosion-transformed, and erosion-induced depressions in volcanic terrains are reviewed and systematized. A proposed classification, addressing terminology issues, considers structural, geomorphic, and climatic factors that contribute to the topographic modification of summit or flank depressions on volcanoes. Breaching of a closed crater or caldera generated by volcanic or non-volcanic processes results in an outlet valley. Under climates with up to ∼2000–2500 mm annual rainfall, craters, and calderas are commonly drained by a single outlet. The outlet valley can maintain its dominant downcutting position because it quickly enlarges its drainage basin by capturing the area of the primary depression. Multi-drained volcanic depressions can form if special factors, e.g., high-rate geological processes, such as faulting or glaciation, suppress fluvial erosion. Normal (fluvial) erosion-modified volcanic depressions the circular rim of which is derived from the original rim are termed erosion craters or erosion calderas, depending on the pre-existing depression. The resulting landform should be classed as an erosion-induced volcanic depression if the degradation of a cluster of craters produces a single-drained, irregular-shaped basin, or if flank erosion results in a quasi-closed depression. Under humid climates, craters and calderas degrade at a faster rate. Mostly at subtropical and tropical ocean-island and island-arc volcanoes, their erosion results in so-called amphitheater valleys that develop under heavy rainfall (>∼2500 mm/year), rainstorms, and high-elevation differences. Structural and lithological control, and groundwater in ocean islands, may in turn preform and guide development of high-energy valleys through rockfalls, landsliding, mudflows, and mass wasting. Given the intense erosion, amphitheater valleys are able to breach a primary depression from several directions and degrade the summit region at a high rate. Occasionally, amphitheater valleys may create summit depressions without a pre-existing crater or caldera. The resulting, negative landforms, which may drain in several directions and the primary origin of which is commonly unrecognizable, should be included in erosion-transformed volcanic depressions. Received: 4 January 1998 / Accepted: 18 January 1999     

High-temperature emplacement of the Cerro Galán and Toconquis Group ignimbrites (Puna plateau,NW Argentina) determined by TRM analyses

Estimates of pyroclastic flow emplacement temperatures in the Cerro Galán ignimbrite and Toconquis Group ignimbrites were determined using thermal remanent magnetization of lithic clasts embedded within the deposits. These ignimbrites belong to the Cerro Galán volcanic system, one of the largest calderas in the world, in the Puna plateau, NW Argentina. Temperature estimates for the 2.08-Ma Cerro Galán ignimbrite are retrieved from 40 sites in 14 localities (176 measured clasts), distributed at different distances from the caldera and different stratigraphic heights. Additionally, temperature estimates were obtained from 27 sample sites (125 measured clasts) from seven ignimbrite units forming the older Toconquis Group (5.60–4.51 Ma), mainly outcropping along a type section at Rio Las Pitas, Vega Real Grande. The paleomagnetic data obtained by progressive thermal demagnetization show that the clasts of the Cerro Galán ignimbrite have one single magnetic component, oriented close to the expected geomagnetic field at the time of emplacement. Results show therefore that most of the clasts acquired a new magnetization oriented parallel to the magnetic field at the moment of the ignimbrite deposition, suggesting that the clasts were heated up to or above the highest blocking temperature (T _b) of the magnetic minerals (T _b = 580°C for magnetite; T _b = 600–630°C for hematite). We obtained similar emplacement temperature estimations for six out of the seven volcanic units belonging to the Toconquis Group, with the exception of one unit (Lower Merihuaca), where we found two distinct magnetic components. The estimation of emplacement temperatures in this latter case is constrained at 580–610°C, which are lower than the other ignimbrites. These estimations are also in agreement with the lowest pre-eruptive magma temperatures calculated for the same unit (i.e., 790°C; hornblende–plagioclase thermometer; Folkes et al. 2011b). We conclude that the Cerro Galán ignimbrite and Toconquis Group ignimbrites were emplaced at temperatures equal to or higher than 620°C, except for Lower Merihuaca unit emplaced at lower temperatures. The homogeneity of high temperatures from proximal to distal facies in the Cerro Galán ignimbrite provides constraints for the emplacement model, marked by a relatively low eruption column, low levels of turbulence, air entrainment, surface–water interaction, and a high level of topographic confinement, all ensuring minimal heat loss.     

An improved age framework for late Quaternary silicic eruptions in northern Central America

 Five new stepwise-heating ⁴⁰Ar/³⁹Ar ages and one new high-sensitivity ¹⁴C date of ash-fall and ash-flow deposits from late Quaternary silicic volcanoes in northern Central America document the eruption rates and frequencies of five major rhyodacite and rhyolite calderas (Atitlán, Amatitlán, Ayarza, Coatepeque, and Ilopango) located north of the basalt, andesite, and dacite stratovolcanoes of the Central American volcanic front. These deposits form extensive time-stratigraphic horizons that intercalate regionally, and knowledge of dates and stratigraphy provides a valuable framework for age determinations of more localized volcanic and nonvolcanic events. The new data, especially when integrated with previous stratigraphic and dating work, show that all five calderas erupted several times in the past 200 ka and, despite a lack of historic activity, should be considered as active centers that could produce highly explosive eruptions again. Because of their locations near the highly vulnerable economic hearts of Guatemala and El Salvador, the risks of eruptions from these calderas should be carefully considered along with risks of major earthquakes and volcanic front volcanoes, which are much more frequent but inflict less severe and extensive damage. This investigation also includes some examples of dating efforts that failed to produce reasonable results. Received: 15 May 1998 / Accepted: 18 January 1999     

Deep structure,volcanism and Wadati-Benioff zone of the northwestern pacific convergent margin

The morphology of the Wadati-Benioff zone in the region of Kamchatka, the Kurile Islands and Hokkaido, based on the distribution of 6319 earthquake foci, has verified the existence of an intermediate-depth aseismic gap and its relation to active andesitic volcanism. It appeared that deep-focus earthquakes in this region belong to a paleosubduction zone activated by an intermediate-depth collision with the active subduction zone in the area of Hokkaido. A system of deep seismically active fracture zones was delineated in the continental plate and confirmed by the results of deep seismic sounding. Two of these fractures, dipping toward the subduction zone, may be considered as the principal feeding channels for active and Holocene volcanoes of the continental volcanic bels of Kamchatka.     

A reappraisal of shear wave splitting parameters from Italian active volcanic areas through a semiautomatic algorithm

Shear wave splitting parameters represent a useful tool to detail the stress changes occurring in volcanic environments before impending eruptions. In the present paper, we display the parameter estimates obtained through implementation of a semiautomatic algorithm applied to all useful datasets of the following Italian active volcanic areas: Mt. Vesuvius, Campi Flegrei, and Mt. Etna. Most of these datasets have been the object of several studies (Bianco et al., Annali di Geofisica, XXXXIX 2:429–443, 1996, J Volcanol Geotherm Res 82:199–218, 1998a, Geophys Res Lett 25(10):1545–1548, 1998b, Phys Chem Earth 24:977–983, 1999, J Volcanol Geotherm Res 133:229–246, 2004, Geophys J Int 167(2):959–967, 2006; Del Pezzo et al., Bull Seismol Soc Am 94(2):439–452, 2004). Applying the semiautomatic algorithm, we confirmed the results obtained in previous studies, so we do not discuss in much detail each of our findings but give a general overview of the anisotropic features of the investigated Italian volcanoes. In order to make a comparison among the different volcanic areas, we present our results in terms of the main direction of the fast polarization (φ) and percentage of shear wave anisotropy (ξ).     

K–Ar ages of the basaltic rocks from Far East Russia: Constraints on the tectono-magmatism associated with the Japan Sea opening

K–Ar ages of the Cenozoic basaltic rocks from the Far East region of Russia (comprising Sikhote-Alin and Sakhalin) are determined to obtain constraints on the tectono-magmatic evolution of the Eurasian margin by comparison with the Japanese Islands, Northeast China, and the formation of the back-arc basin. In the early Tertiary stage (54–26 Ma), the northwestward subduction of the Pacific Plate produced the active continental margin volcanism of Sikhote-Alin and Sakhalin, whereas the rift-type volcanism of Northeast China, inland part of the continent began to develop under a northeast–southwest-trending deep fault system. In the early Neogene (24–17 Ma), a large number of subduction-related volcanic rocks were erupted in connection with the Japan Sea opening. After an inactive interval of the volcanism ∼ 20–13 Ma ago, the late Neogene (12–5 Ma) volcanism of Sikhote-Alin and Sakhalin became distinct from those of the preceding stages and indicated within-plate geochemical features similar to those of Northeast China, in contrast to the Japan Arc which produces island arc volcanism. During the Japan Sea opening, the northeastern Eurasian margin detached and became a continental island arc system, and an integral part of continental eastern Asia comprising Sikhote-Alin, Sakhalin and Northeast China, and the Japan Arc with a back-arc basin. The convergence between the Eurasian Plate, the Pacific Plate and the Indian Plate may have contributed to the Cenozoic tectono-magmatism of the northeastern Eurasian continent.     

Quaternary volcanism of the Japanese Islands

Abstract Quaternary volcanism of the Japanese Islands is examined from the perspective of experimental petrology, geographic distribution of volcanoes and spatial geochemical variations. The dehydration of amphibole and chlorite at a 110 km depth and of phlogopite at ∼180 km in the downdragged hydrous mantle layer would result in the occurrence of two volcanic chains parallel to the trench axis. Long-term subduction of the old Pacific plate and recent subduction of the young Philippine Sea plate beneath East Japan and West Japan volcanic belts respectively, would be critical for the significant difference in intensity, style and geochemistry of Quaternary volcanism between the two volcanic belts. The geochemistry of volcanic rocks in Northeast Japan and those in the Ryukyu arc is typical of 'island-arcs' having low LIL/HFS element ratios, while alkalic basalts along the Japan Sea coast side in Southwest Japan have high LIL/HFS ratios similar to intra-continental or oceanic island basalts. Across-arc variations in eruptive volume and distributional density of volcanoes and in geochemistry are documented in Northeast Japan and are well explained by the decreasing degrees of partial melting toward back-arc side, and the difference in geochemistry of fluids supplied by the downdragged hydrous layer.     

Offset caldera and crater collapse on Juan de Fuca ridge-flank volcanoes

 Forty-three volcanoes located along the flanks of the Juan de Fuca Ridge were selected to study relationships between their morphologies and off-axis magmatic processes. The volcanoes occur both in chains consisting of up to seven distinct cones and isolated edifices. Nearly all of the volcanoes are circular, truncated cones with steep flanks and large, relatively flat summit plateaus. In addition, most of these volcanoes also have prominent and distinctly offset calderas or craters. The most striking characteristic of the volcanoes' morphology is that nearly all of their collapse structures are located on the sides of the volcanoes which face the Juan de Fuca Ridge and many are breached with openings toward the ridge. A simple model based on these observations accounts for these ridge-facing features. As plate motion transports a volcano away from its magma source beneath the lithosphere, the volcano's magma supply conduits tend to lag behind. Eventually these conduits are abandoned and ridgeward collapse structures are formed. It can be inferred from the model that, on average, individual volcanoes were active for approximately 50 000 years and that most eruptions took place early in this interval. If most of the cone-building eruptions occurred during the first thousand years or so, associated hydrothermal activity may have temporarily rivaled the present-day yearly time-averaged hydrothermal output along the entire Juan de Fuca ridge axis. Received: 1 September 1996 / Accepted: 13 January 1997     

The pattern of circumferential and radial eruptive fissures on the volcanoes of Fernandina and Isabela islands,Galapagos

Maps of the eruptive vents on the active shield volcanoes of Fernandina and Isabela islands, Galapagos, made from aerial photographs, display a distinctive pattern that consists of circumferential eruptive fissures around the summit calderas and radial fissures lower on the flanks. On some volcano flanks either circumferential or radial eruptions have been dominant in recent time. The location of circumferential vents outside the calderas is independent of caldera-related normal faults. The eruptive fissures are the surface expression of dike emplacement, and the dike orientations are interpreted to be controlled by the state of stress in the volcano. Very few subaerial volcanoes display a pattern of fissures similar to that of the Galapagos volcanoes. Some seamounts and shield volcanoes on Mars morphologically resemble the Galapagos volcanoes, but more specific evidence is needed to determine if they also share common structure and eruptive style.     

Geochronology of Gran Canaria,Canary Islands: Age of shield building volcanism and other magmatic phases

Forty-six new K-Ar age determinations are presented on whole rock samples and mineral separates from volcanic and subvolcanic rocks of Gran Canaria. The main subaerial shield building basaltic volcanism with estimated volume of about 1000 km³ was confined to the interval about 13.7 m.y. to 13.5 m.y. ago in the middle Miocene. Substantial volume (～100 km³) of silicic volcanics (trachyte and peralkaline rhyolite) were erupted with no detectable time break following the basaltic volcanism, essentially contemporaneous with formation of a large collapse caldera at 13.4±0.3 m.y. ago. Trachytic to phonolitic volcanism continued intermittently in the waning states of activity until about 9 m.y. ago. Following a long hiatus there was resurgence of volcanism with eruption of about 100 km³ of basanitic to hauyne phonolitic rocks of the Roque Nublo Group between about 4.4 m.y. and 3.4 m.y. ago in the Pliocene. After a hiatus of less than 1.0 m.y., olivine nephelinite magmas were erupted and this activity continued intermittently until relatively recent times, the younger eruptives being mainly basanitic in composition. The volume of volcanic products in this phase probably does not exceed 10 km³. Thus the volume of all the resurgent volcanism comprises less than 10 percent of the subaerially exposed part of Gran Canaria. The results show that the subaerial main shield building phase of volcanism in Gran Canaria, consisting of mildly alkali to transitional basalts, occurred over a time interval that was less than 0.5 m.y. Magmatic evolution on Gran Canaria appears to be similar to that found on other basaltic volcanoes in oceanic regions. Thus volcanoes in the Hawaiian, Marquesas and Society Islands all were built by basaltic lavas in similar short-lived episodes of volcanism. In some Hawaiian volcanoes, a resurgent phase of volcanism of strongly undersaturated basalts of small volume is recognized following a long hiatus, again similar to that found on Gran Canaria. The relatively large volume of silicic lavas erupted in Gran Canaria immediately following the main basaltic shield building phase is, however, not matched in the Pacific volcanoes mentioned.     

大兴安岭焰山、高山火山——一种新的火山喷发型式

在野外地质资料基础上,利用火山形态学方法,探讨了大兴安岭焰山、高山火山的喷发型式。结果表明,大兴安岭哈拉哈河-绰尔河火山群中的焰山和高山火山不同于斯通博利式喷发形成的火山,其早期爆破喷发的火山碎屑形成火山渣锥、空降火山碎屑席和小型火山碎屑流,晚期溢出大量熔岩。两火山具有较高大的锥体(标高200～300m以上),在结构上,松散火山砾、火山弹等构成下部的降落锥,熔结集块岩构成上部的溅落锥。由火山砾和火山灰组成的空降火山碎屑席分布在火山锥体周围。两火山溢出的熔岩经历了从结壳熔岩→翻花石→渣状熔岩的演变。根据喷发产物可推断焰山和高山火山具有以下喷发特征:爆破喷发形成持续的喷发柱→斯通博利式喷发→熔岩喷泉喷溢,其中以持续时间较长的喷发柱区别于典型的斯通博利式喷发。类似焰山、高山火山的喷发特征,在龙岗第四纪火山群、镜泊湖全新世火山群中也都有个例,这是中国大陆火山作用中一种新的喷发型式。     

Volcán Ecuador,Galapágos Islands: erosion as a possible mechanism for the generation of steep-sided basaltic volcanoes

Volcán Ecuador (0°02′S, 91°35′W) consists of two strongly contrasting components: the eroded and vegetated remnant of a once-circular main volcano with a probable caldera, and a prominent rift zone extending to the northeast that is neither strongly eroded nor weathered. There are about 20 young-looking flows and vents on this caldera floor but only one on the higher remnant of the main volcano. The southwest half of the main volcano is faulted into the ocean. The main part of Volcán Ecuador possesses steep erosional slopes (average 30–40°) that cut into a sequence of flows that dip radially outward at <10°. In contrast, the northeast rift zone consists entirely of young flows and vents. The upper 10 km of the rift zone forms a peninsula about 7.5 km wide that connects Volcán Ecuador to Volcán Wolf. The rift zone bends to the southeast and the lower 8 km is tangential to the coast of Volcán Wolf. The rift zone axis dips away from the northeast edge of the main volcano, and its flanks slope roughly northwest and southeast at <4°. The rift zone is the Galápagos structure that most closely resembles a Hawaiian rift zone because it is constructed of lavas from subparallel linear vents, shows evidence of a deep feeder conduit, and has changed its direction to avoid a direct intersection with neighboring Volcán Wolf. The steep erosional slopes extending around the perimeter of the main volcano (except to the southwest where slumping occurred) were probably generated by marine erosion during a prolonged period of eruptive inactivity (perhaps 20 000–30 000 years). Only a few post-erosional eruptions have taken place at the main volcano in and near what was once the caldera. The entire rift zone postdates the period of prolonged erosion. Using the evidence for prolonged inactivity at Volcán Ecuador, we propose that erosion may have helped to produce steep slopes on the other western Galápagos volcanoes. On these more active volcanoes, however, numerous subsequent eruptions have completely mantled the erosional slopes with lava. The mechanism by which the volcanoes may shut off for long periods of time is unknown, but the fact that the Galápagos hotspot is presently supplying nine active volcanoes suggests that the magma supply at an individual volcano could vary greatly over periods of (tens of?) thousands of years.     

Permian to recent volcanism in northern sumatra,indonesia: a preliminary study of its distribution,chemistry, and peculiarities

Sumatra has been a ‘volcanic arc’, above an NE-dipping subduction zone, since at least the Late Permian. The principal volcanic episodes in Sumatra N of the Equator have been in the Late Permian, Late Mesozoic, Palaeogene, Miocene and Quaternary.Late Permian volcanic rocks, of limited extent, are altered porphyritic basic lavas interstratified with limestones and phyllites.Late Mesozoic volcanic rocks, widely distributed along and W of the major transcurrent.Sumatra Fault System (SFS), which axially bisects Sumatra, include ophiolite-related spilites, andesites and basalts. PossiblePalaeogene volcanic rocks include an altered basalt pile with associated dyke-swarm in the extreme NW, intruded by an Early Miocene (19 my) dioritic stock; and variable pyroxene rich basic lavas and agglomerates ranging from alkali basaltic to absarokitic in the extreme SW.Miocene volcanic rocks, widely distributed (especially W of the SFS), and cropping out extensively along the W coast, include calc-alkaline to high-K calc-alkaline basalts, andesites and dacites.Quaternary volcanoes (3 active, 14 dormant or extinct) are irregularly distributed both along and across the arc; thus they lie fore-arc of the SFS near the Equator but well back-arc farther north. The largest concentration of centres, around Lake Toba, includes the >2000 km³ Pleistocene rhyolitic Toba Tuffs. Quaternary volcanics are mainly calc-alkaline andesites, dacites and rhyolites with few basalts; they seem less variable, but on the whole more acid, than the Tertiary. The Quaternary volcanism is anomalous in relation to both southern Sumatra and adjacent Java/Bali: in southern Sumatra, volcanoes are regularly spaced along and successively less active away from the SFS, but neither rule holds in northern Sumatra. Depths to the subduction zone below major calc-alkaline volcanoes in Java/Bali are 160–210 km, but little over 100 km in northern Sumatra, which also lacks the regular K₂O-depth correlations seen in Java. These anomalies may arise because Sumatra — being underlain by continental crust — is more akin to destructive continental margins than typical island-arcs such as E Java or Bali, and because the Sumatran subduction zone has a peculiar structure due to the oblique approach of the subducting plate. A further anomaly — an E-W belt of small centres along the back-arc coast — may relate to an incipient S-dipping subduction zone N of Sumatra and not the main NE-dipping zone to its W. Correlation of the Tertiary volcanism with the present tectonic regime is hazardous, but the extensive W coastal volcanism (which includes rather alkaline lavas) is particularly anomalous in relation to the shallow depth (<100 km) of the present subduction zone. The various outcrops may owe their present locations to extensive fault movements (especially along the SFS), to the peculiar structure of the fore-arc (suggested by equally anomalous Sn- and W-bearing granitic batholiths also along the W coast), or they may not be subduction-related at all.     

Evaluating fracture patterns within a resurgent caldera: Campi Flegrei, Italy

Understanding deformation of active calderas allows their dynamics to be defined and their hazard mitigated. The Campi Flegrei resurgent caldera (Italy) is one of the most active and hazardous volcanoes in the world, characterized by post-collapse resurgence, eruptions, ground deformation, and seismicity. An original structural analysis provides an overview of the main fracture zones. NW-SE and NE-SW fractures (normal or transtensive faults and extensional fractures) predominate along the rim and within the caldera, suggesting a regional control, both during and after the collapses. While the NE-SW fractures are ubiquitous in the deposits of the last ∼37 ka, NW-SE fractures predominate in the last 4.5 ka, during resurgence. The most recently (<4.5 ka) strained area lies in the caldera center (Solfatara area), where the faults, with an overall ∼ENE-WSW extension direction, appear to be associated with the bending due to resurgence. Solfatara lies immediately to the east of the most uplifted part of the caldera (Pozzuoli area), where domes form and culminate both on the long-term (resurgence, accompanied by volcanic activity) and short-term deformation (1982–1984 bradyseism, accompanied by seismic and hydrothermal activity). Similar volcano-tectonic behavior characterizes the short- and long-term uplifts, and only the intensity of the tectonic and volcanic activity varies, being related to varying amounts of uplift. Seismicity and hydrothermal manifestations occur during the bradyseisms, with moderate uplift, while surface faulting and eruptions occur during resurgence, with higher uplift. The features observed at Campi Flegrei are found at other major calderas, suggesting consistent behavior of large magmatic systems.     

Volcanism offshore of Vesuvius Volcano in Naples Bay

 High-resolution seismic reflection data are used to identify structural features in Naples Bay near Vesuvius Volcano. Several buried seismic units with reflection-free interiors are probably volcanic deposits erupted during and since the formation of the breached crater of Monte Somma Volcano, which preceded the growth of Vesuvius. The presumed undersea volcanic deposits are limited in extent; thus, stratigraphic relationships cannot be established among them. Other features revealed by our data include (a) the warping of lowstand marine deposits by undersea cryptodomes located approximately 10 km from the summit of Vesuvius, (b) a succession of normal step faults that record seaward collapse of the volcano, and (c) a small undersea slump in the uppermost marine deposits of Naples Bay, which may be the result of nueé ardentes that entered the sea during a major eruption of Vesuvius in 1631. Detection of these undersea features illustrates some capabilities of making detailed seismic reflection profiles across undersea volcanoes. Received: 16 September 1997 / Accepted: 23 November 1997     

Subsidence of ash-flow calderas: relation to caldera size and magma-chamber geometry

 Diverse subsidence geometries and collapse processes for ash-flow calderas are inferred to reflect varying sizes, roof geometries, and depths of the source magma chambers, in combination with prior volcanic and regional tectonic influences. Based largely on a review of features at eroded pre-Quaternary calderas, a continuum of geometries and subsidence styles is inferred to exist, in both island-arc and continental settings, between small funnel calderas and larger plate (piston) subsidences bounded by arcuate faults. Within most ring-fault calderas, the subsided block is variably disrupted, due to differential movement during ash-flow eruptions and postcollapse magmatism, but highly chaotic piecemeal subsidence appears to be uncommon for large-diameter calderas. Small-scale downsag structures and accompanying extensional fractures develop along margins of most calderas during early stages of subsidence, but downsag is dominant only at calderas that have not subsided deeply. Calderas that are loci for multicyclic ash-flow eruption and subsidence cycles have the most complex internal structures. Large calderas have flared inner topographic walls due to landsliding of unstable slopes, and the resulting slide debris can constitute large proportions of caldera fill. Because the slide debris is concentrated near caldera walls, models from geophysical data can suggest a funnel geometry, even for large plate-subsidence calderas bounded by ring faults. Simple geometric models indicate that many large calderas have subsided 3–5 km, greater than the depth of most naturally exposed sections of intracaldera deposits. Many ring-fault plate-subsidence calderas and intrusive ring complexes have been recognized in the western U.S., Japan, and elsewhere, but no well-documented examples of exposed eroded calderas have large-scale funnel geometry or chaotically disrupted caldera floors. Reported ignimbrite "shields" in the central Andes, where large-volume ash-flows are inferred to have erupted without caldera collapse, seem alternatively interpretable as more conventional calderas that were filled to overflow by younger lavas and tuffs. Some exposed subcaldera intrusions provide insights concerning subsidence processes, but such intrusions may continue to evolve in volume, roof geometry, depth, and composition after formation of associated calderas. Received: 13 February 1997 / Accepted: 9 August 1997     

Ages of calderas,large explosive craters and active volcanoes in the Kuril-Kamchatka region,Russia

The ages of most of calderas, large explosive craters and active volcanoes in the Kuril-Kamchatka region have been determined by extensive geological, geomorphological, tephrochronological and isotopic geochronological studies, including more than 600 ¹⁴C dates. Eight Krakatoa-type and three Hawaiian-type calderas and no less than three large explosive craters formed here during the Holocene. Most of the Late Pleistocene Krakatoa-type calderas were established around 30 000–40 000 years ago. The active volcanoes are geologically very young, with maximum ages of about 40 000–50 000 years. The overwhelming majority of recently active volcanic cones originated at the very end of the Late Pleistocene or in the Holocene. These studies show that all Holocene stratovolcanoes in Kamchatka were emplaced in the Holocene only in the Eastern volcanic belt. Periods of synchronous, intensified Holocene volcanic activity occurred within the time intervals of 7500–7800 and 1300–1800 ¹⁴C years BP.     

Earthquake distribution and volcanism in Kamchatka,Kurile Islands,and Hokkaido Part 1: Kamchatka and northern Kuriles

Summary The morphology of the Wadati-Benioff zone in the region of Kamchatka and Northern Kuriles, based on the distribution of 1102 earthquake foci, verified the existence of an intermediate depth aseismic gap and its relation to active andesitic volcanism. A system of deep seismically active fracture zones, genetically connected with the process of subduction, was delineated in the continental plate and confirmed by the results of deep seismic sounding. Two of these fractures, dipping toward the subduction zone, may be considered as the principal feeding channels for active and Holocene volcanoes of the continental volcanic belts of Kamchatka.