The subsurface structure of the AIRA caldera and its vicinity in Southern Kyushu, Japan

Southern Kyushu, Japan, includes a chain of large and small calderas and active volcanoes, and the greatest part of it is covered with thick pyroclastic ejecta. The regional and local structures of this area are discussed from the standpoint of physical volcanology, with consideration of all available data.The regional structure of this area is examined in the light of gravity and geomagnetic anomalies. Two layers of the earth's uppermost crust are defined by spectrum analysis of the gravity anomalies. These two layers are identical with the two identified by seismicwave velocities. The Bouguer gravity anomalies are relatively high and rather monotonous over outcrops of the Mesozoic basement and the granite, but are relatively low and perturbed over calderas and caldera-like structures. Two low-gravity anomalies in Kagoshima Bay are remarkable. One is circular, with its center on the Aira caldera. The other is elongated between the Satsuma and Oosumi peninsulas. The southern end of the latter anomaly is occupied by the Ata caldera. Discussion of the gravity anomalies of the Aira caldera suggests that the subsurface basement has a funnel shape and is overlain by ‘fallback’. The sub bottom geology of the caldera suggests that it is formed by a few smaller depressions, though the distribution of the overall gravity anomalies is parallel with its shape.The southern part of Kagoshima Bay is characterized by a graben-like topography and low-gravity anomalies and, moreover, by several calderas. The middle part, between the Aira and Ata calderas, may have a graben-like structure. A profile crossing the bay through Sakurajima volcano is modeled on the basis of results from drilling and gravity surveys. The basement has a graben-like structure and is filled with coarse and low-density deposits, and the structure continues northwards to the Aira caldera with a funnel shape.A comparison of this area with the Taupo-Rotorua depression in New Zealand and Lake Toba in Indonesia, leads the authors to the conclusion that such major volcanic depressions may have been formed by amalgamation of a series of caldera-like structures which were formed by multiple violent explosions accompanied by ejection of a tremendous amount of pyroclastic material.     

Lithofacies and eruption ages of Late Cretaceous caldera volcanoes in the Himeji–Yamasaki district, southwest Japan: Implications for ancient large-scale felsic arc volcanism

Abstract The Himeji–Yamasaki region in the Inner Zone of southwest Japan is underlain mainly by Late Cretaceous volcanic rocks called the Ikuno Group or the Hiromine and Aioi Groups. A new stratigraphic and geochronological study shows that the volcanic rocks in this area consist of 15 eroded caldera volcanoes between 82 and 65 Ma; they are, in order of decreasing age, the Hiromine, Hoden, Ibo, Okawachi, Seppikosan, Hayashida, Shinokubi, Fukusaki, Kurooyama, Ise, Fukadanigawa, Nagusayama, Matobayama, Yumesaki and Mineyama Formations. These calderas vary in diameter from 1 to 20 km and are bounded by steep unconformities; they coalesce and overlap each other. The individual caldera fills are composed mainly of single voluminous pyroclastic flow deposits, which are often interleaved with debris avalanche deposits and occasionally underlie lacustrine deposits. The intracaldera pyroclastic flow deposits are made up of massive, welded or non‐welded tuff breccia to lapilli tuff, and are characterized by their great thickness. The debris avalanche deposits are ill‐sorted breccia, generated by the collapse of the caldera wall toward the caldera floor during the pyroclastic‐flow eruption. The large calderas that are more than 10 km in diameter contain original values of approximately 100 km³ of intracaldera pyroclastic flow deposits. These large calderas are similar to the well‐known Valles‐type calderas in their dimensions, although it is uncertain whether their caldera floors are coherent plates or incoherent pieces. Conversely, the small calderas have diatreme‐like subsurface structures. The variety of the caldera volcanoes in this area is caused by the difference in the volume of caldera‐forming pyroclastic eruptions, as the large and small calderas coexisted. The caldera‐forming eruption rates in Late Cretaceous southwest Japan, including the studied area, were similar to those in late Cenozoic central Andes and northeast Honshu arc, Japan, but obviously smaller than those of late Cenozoic intracratonic caldera clusters in western North America and the Quaternary extensional volcanic arcs in Taupo, New Zealand. The widespread Late Cretaceous felsic igneous rocks in southwest Japan were generated by a long‐term accumulation of low‐rate granitic magmatism at the eastern margin of the Eurasian Plate.     

Shishimuta caldera,the buried source of the Yabakei pyroclastic flow in the Hohi volcanic zone,Japan

Drill-hole, geochronologic, and gravity data identify the buried Shishimuta caldera beneath post-caldera lava domes and lacustrine deposits in the center of the Hohi volcanic zone. The caldera is the source of the Yabakei pyroclastic flow, which erupted 1.0 Ma ago with a bulk volume of 110 km³. The caldera is a breccia-filled funnel-shaped depression 8 km wide and > 3 km deep with a V-shaped negative Bouguer gravity anomaly up to 36 mgal. Neither ring vents nor resurgence was recognized; instead, post-caldera monogenetic volcanism in an extensional setting dominated the area. The andesitic breccia has a relatively low density and fills the caldera; it possibly formed by fragmentation of disrupted roof rock during the violent Yabakei eruption and related collapse. Fewer normal faults and shallow microearthquakes occur inside the caldera than around it, possibly because rocks beneath the caldera are structurally incoherent. A profile of Shishimuta caldera may be more elongated vertically, and have a more intensely fractured zone, than that of a Valles-type caldera.     

Crustal structure beneath Aso Caldera,Southwest Japan,as derived from receiver function analysis

Aso Volcano experienced a huge pyroclastic eruption 90 thousand years ago, and formed a large caldera (18 km × 25 km). In order to test the hypothesis of a magma body in the mid and lower crust that has been suggested geophysically and geochemically, we investigated seismic velocity discontinuities and velocity structure beneath Aso Caldera using receiver functions and a genetic algorithm inversion. We confirm the existence of the Moho at depths between 30 km and 35 km and a large velocity anomaly should exist in the deep portion of the crust beneath Aso Caldera, from imaging of receiver functions observed only at stations outside the caldera. As a result of a more detailed examination with GA inversion, a low velocity layer is detected at depths between 10 km and 24 km beneath the western part of the caldera. S-wave velocity of the layer is estimated to be 2.0–2.4 km/s. We estimate that the low velocity layer contains at most 15% melt or 30% aqueous fluid. The layer exists near the Conrad and at the same depths as the swarm of the low frequency earthquakes and a compressional and dilatational deformation source which are expected to be caused by fluid movement beneath the middle-eastern part of the caldera. Fluid contained in the layer might be related with huge pyroclastic eruptions of Aso Volcano.     

K–Ar ages of the Akan-Shiretoko volcanic chain lying oblique to the Kurile trench: Implications for tectonic control of volcanism

The Akan‐Shiretoko volcanic chain, situated in the Southwestern Kurile arc, consists mainly of nine subaerial andesitic stratovolcanoes and three calderas. The chain extends in a SW–NE direction for 200 km, situated oblique to the Kurile trench at an angle of 25 degrees. Thirty‐seven new K–Ar ages, plus previous data, suggest that volcanic activity along the Akan‐Shiretoko volcanic chain began at ca 4 Ma at Akan, at the southwestern end of the chain, and systematically progressed northeastward, resulting in the southwest‐northeast‐trending volcanic chain. This spatial and temporal distribution of volcanoes can be explained by anticline development advancing northeastward from the Akan area, accompanied by magma rising through northeast‐trending fractures that developed along the anticlinal axis. The northeastward development of the anticline caused uplifting of the Akan‐Shiretoko area and changed the area from submarine to subaerial conditions. Anticline formation was likely due to deformation of the southwestern Kurile arc, with southwestward migration of the Kurile forearc sliver caused by oblique subduction of the Pacific plate. The echelon topographic arrangement of the Shiretoko, Kunashiri, Etorofu and Urup was formed at ca 1 Ma.     

New multibeam mapping and geochemistry of the 30°–35° S sector,and overview,of southern Kermadec arc volcanism

New multibeam mapping and whole-rock geochemistry establish the first order definition of the modern submarine Kermadec arc between 30° and 35° S. Twenty-two volcanoes with basal diameters > 5 km are newly discovered or fully-mapped for the first time; Giggenbach, Macauley, Havre, Haungaroa, Kuiwai, Ngatoroirangi, Sonne, Kibblewhite and Yokosuka. For each large volcano, edifice morphology and structure, surficial deposits, lava fields, distribution of sector collapses, and lava compositions are determined. Macauley and Havre are large silicic intra-oceanic caldera complexes. For both, concentric ridges on the outer flanks are interpreted as recording mega-bedforms associated with pyroclastic density flows and edifice foundering. Other stratovolcanoes reveal complex histories, with repeated cycles of tectonically controlled construction and sector collapse, extensive basaltic flow fields, and the development of summit craters and/or small nested calderas.Combined with existing data for the southernmost arc segment, we provide an overview of the spatial distribution and magmatic heterogeneity along ∼780 km of the Kermadec arc at 30°–36°30′ S. Coincident changes in arc elevation and lava composition define three volcano–tectonic segments. A central deeper segment at 32°20′–34°10′ S has basement elevations of > 3200 m water-depth, and relatively simple stratovolcanoes dominated by low-K series, basalt–basaltic andesite. In contrast, the adjoining arc segments have higher basement elevations (typically < 2500 m water-depth), multi-vent volcanic centres including caldera complexes, and erupt sub-equal proportions of dacite and basalt–basaltic andesite. The association of silicic magmas with higher basement elevations (and hence thicker crust), coupled with significant inter- and intra-volcano heterogeneity of the silicic lavas, but not the mafic lavas, is interpreted as evidence for dehydration melting of the sub-arc crust. Conversely, the crust beneath the deeper arc segments is thinner, initially cooler, and has not yet reached the thermal requirements for anatexis. Silicic calderas with diameters > 3 km coincide with the shallower arc segments. The dominant mode of large caldera formation is interpreted as mass-discharge pyroclastic eruption with syn-eruptive collapse. Hence, the shallower arc segments are characterized by both the generation of volatile-enriched magmas from crustal melting and a reduced hydrostatic load, allowing magma vesiculation and fragmentation to initiate and sustain pyroclastic eruptions. Proposed initiation parameters for submarine pyroclastic eruptions are water-depths < 1000 m, magmas with 5–6 wt.% water and > 70 wt.% SiO₂, and a high discharge rate.     

Geologic evolution of the Donguinyó-Huichapan caldera complex,central Mexican Volcanic Belt,Mexico

The Donguinyó-Huichapan caldera complex is located 110 km to the NNW of Mexico City, in the central sector of the Mexican Volcanic Belt. It is a 10 km in diameter complex apparently with two overlapping calderas, each one related to an ignimbrite sequence that contrasts in composition, mineralogy, welding, distribution, and physical aspect. The geologic evolution of this complex includes the following phases, 1) A first caldera formed at 5.0 ± 0.3 Ma, with the eruption of several discrete pulses of andesitic to trachydacitic pyroclastic flows that produced a series of densely welded ignimbrites; 2) At 4.6 ± 0.3 Ma, several small shield volcanoes and cinder cones built the rim of this caldera and erupted basaltic-andesite and andesitic lava flows; 3) At 4.2 ± 0.2 Ma, a second caldera was formed associated to the eruption of the Huichapan Tuff, which is a rhyolitic pyroclastic sequence consisting of minor unwelded ignimbrites, pumice fall and surge deposits, and a voluminous welded ignimbrite; 4) Also yielding an age of 4.2 ± 0.2 Ma, several trachydacitic lava domes were extruded along the new ring fracture and formed the rim of the Huichapan caldera, as well as five intra-caldera domes of dacitic and trachydacitic composition. Peripheral volcanism includes a large 2.5 ± 0.1 Ma shield volcano that was emplaced on the Huichapan caldera rim.The two calderas that form the Donguinyó-Huichapan complex have contrasting differences in volcanic styles that were apparently due to their differences in composition. Products erupted by the Donguinyó caldera are basaltic-andesite to trachydacitic in composition, whereas Huichapan caldera products are all high-silica rhyolites.     

Non-explosive,constructional evolution of the ice-filled caldera at Volcán Sollipulli,Chile

 A radar and gravity survey of the ice-filled caldera at Volcán Sollipulli, Chile, indicates that the intra-caldera ice has a thickness of up to 650 m in its central part and that the caldera harbours a minimum of 6 km³ of ice. Reconnaissance geological observations show that the volcano has erupted compositions ranging from olivine basalt to dacite and have identified five distinct volcanic units in the caldera walls. Pre- or syn-caldera collapse deposits (the Sharkfin pyroclastic unit) comprise a sequence which evolved from subglacial to subaerial facies. Post-caldera collapse products, which crop out along 17 of the 20 km length of the caldera wall, were erupted almost exclusively along the caldera margins in the presence of a large body of intra-caldera ice. The Alpehué crater, formed by an explosive eruption between 2960 and 2780 a. BP, in the southwest part of the caldera is shown to post date formation of the caldera. Sollipulli lacks voluminous silicic pyroclastic rocks associated with caldera formation and the collapse structure does not appear to be a consequence of a large-magnitude explosive eruption. Instead, lateral magma movement at depth resulting in emptying of the magma chamber may have generated the caldera. The radar and gravity data show that the central part of the caldera floor is flat but, within a few hundred metres of the caldera walls, the floor has a stepped topography with relatively low-density rock bodies beneath the ice in this region. This, coupled with the fact that most of the post-caldera eruptions have taken place along the caldera walls, implies that the caldera has been substantially modified by subglacial marginal eruptions. Sollipulli caldera has evolved from a collapse to a constructional feature with intra-caldera ice playing a major role. The post-caldera eruptions have resulted in an increase in height of the walls and concomitant deepening of the caldera with time. Received: 12 June 1995 / Accepted: 7 December 1995     

Acid volcanism on the south Arabian coast

Along the south coast of Arabia, between Aden and the southern entrance to the Red Sea, there are six central vent volcanoes of probable Pliocene age. All are characterised by the interstratification of basic and acidic extrusives, the formation of large central calderas at a late stage in the volcanic cycle and the subsequent infilling of these calderas with horizontal acidic ignimbrites and basic lavas. Lying 60 miles to the west of Aden and of particular interest is Jebel Khariz, the largest and best preserved of the six volcanic centres, covering a roughly circular area of about 100 square miles and rising to a height of 2,766 feet. The volcanic sequence of Jebel Khariz is broadly divisible into two suites: a) alkali-rich rhyolites and trachytes which occur as flows and pyroclastic horizons and form about 80 per cent of the volume of the cone, and b) effusives of basaltic composition that occur in the caldera, locally on the south-east and south-west flanks and in a small parasitic cone on the northern flank. The alkali-rich acidic suite includes lavas, ash-flow and ash-fall rocks as well as vent and flow breccias, Generally, all rocks of this suite have phenocrysts of anorthoclase, and may contain phenocrysts of fayalitic olivine, aegirine-augite, magnetite and/or quartz. The fine grained matrix is composed of the same minerals with skeletal riebeckite and, in some cases, cossyrite. The basaltic suite is characteristically porphyritic, the phenocrysts being of calcic plagioclase, clinopyroxene, olivine and magnetite in a fine-grained mesostasis of plagioclase, olivine, clinopyroxene and ore. The plagioclase, on initial investigation, appears to lie in the labradorite-bytownite range, the olivine is commonly replaced by iddingsite and the clinopyroxene is most commonly a pale mauve titanaugite. Near the centre of the volcanic pile, as exposed in the caldera wall, masses of rhyolitic composition can be seen to form over half of the volcanic sequence. These masses are markedly lenzoid in cross-section normal to the flow direction and display intricate flow folding; they are considered to have been extruded as viscous lava. Further from the volcanic centre, these acidic extrusives become less markedly lenzoid until in the distal areas of individual units, some 5 miles from the caldera, they have spread out to form sheet-like masses covering as much as 10 square miles to a uniform thickness rarely exceeding 25 feet. The presence of agglomeratic bases, hard compact central sections and less compact upper divisions, together with the ubiquitous presence of columnar jointing and occasional shard textures suggest that these distal parts of each extrusive unit have been formed by an ash-flow/ash-fall mechanism. It is postulated that the majority of the Jebel Khariz volcanic pile was formed by emission of acidic material, effusive in the central area, but deposited mainly by an ash-flow mechanism around the flanks of the cone. This could be due to either the synchronous eruption of viscous lava from the central vent with ash flow eruptions on the flanks; or, more probably, to the progression of an individual volcanic episode through an initial ash-flow phase followed by the effusion of viscous lava, all emanating from the central vent.     

Volcanoes of the Cape Hoskins area,New Britain,territory of Papua and New Guinea

One active and ten extinct Quaternary volcanoes are described from the Cape Hoskins area, on the north coast of New Britain. They are mostly strato volcanoes built up of lava flows, lava domes, pyroclastic flows, lahars, tephra, and derived alluvial sediments. The volcanic products range in composition from basalt to rhyolite, but basaltic andesite and andesite predominate. Much of the area is covered by tephra, several metres thick, consisting mainly of rhyolitic pumice. The active volcano, Pago, is built up of several glacier-like lava flows, the last of which was formed during an eruption in 1914–18. Pago lies within a well-preserved caldera forming the central part of a broad low-angle cone, named Witori, which consists largely of welded and unwelded pyroclastic flow deposits. C-14 dates obtained on charcoal indicate that the caldera eruption occurred about 2500 years B. P. Another caldera of similar age lies south of Witori. Of the other eight volcanoes described four are relatively well-preserved steep-sided cones formed mainly of lava flows, one is a remnant of a low-angle cone with a caldera, and three are deeply eroded cones which have none of their constructional surfaces preserved.     

Identification of the source caldera for the Middle Miocene ash-flow tuffs in the Kii Peninsula based on apatite trace-element composition

A large caldera cluster consisting of at least four calderas (Omine, Odai, Kumano-North and Kumano calderas) existed in the central–southern part of the Kii Peninsula approximately 14–15 Ma. On the other hand, thick Middle Miocene ash-flow tuffs, referred to as the Muro Ash-flow Tuff and the Sekibutsu Tuff Member, are distributed in the northern part of the Kii Peninsula. Although these tuffs are considered to have erupted from the caldera cluster in the central-southern Kii Peninsula, identifying the source caldera in the cluster has been controversial because of similarities in the petrological characteristics and identical radiometric ages of the volcaniclastic rocks of these calderas. We successfully discriminated the characteristics of the eruptive products of each caldera in the caldera cluster based on the apatite trace-element compositions of the pyroclastic dikes and ash-flow tuffs of the calderas. We also demonstrated that the source caldera of at least the lower main part of the Muro Ash-flow Tuff and the Sekibutsu Tuff Member was the Odai Caldera, which is located in the central Kii Peninsula. Our findings show possible correlations among the pyroclastic conduits and ash-flow tuffs of the caldera-fill and/or outflow deposits, even in cases where they have been densely welded and diagenetically altered. This method is useful for the study of deeply eroded ancient calderas.     

Large-scale failures on domes and stratocones situated on caldera ring faults: sand-box modeling of natural examples from Kamchatka, Russia

Edifices of stratocones and domes are often situated eccentrically above shallow silicic magma reservoirs. Evacuation of such reservoirs forms collapse calderas commonly surrounded by remnants of one or several volcanic cones that appear variously affected and destabilized. We studied morphologies of six calderas in Kamchatka, Russia, with diameters of 4 to 12 km. Edifices affected by caldera subsidence have residual heights of 250–800 m, and typical amphitheater-like depressions opening toward the calderas. The amphitheaters closely resemble horseshoe-shaped craters formed by large-scale flank failures of volcanoes with development of debris avalanches. Where caldera boundaries intersect such cones, the caldera margins have notable outward embayments. We therefore hypothesize that in the process of caldera formation, these eccentrically situated edifices were partly displaced and destabilized, causing large-scale landslides. The landslide masses are then transformed into debris avalanches and emplaced inside the developing caldera basins. To test this hypothesis, we carried out sand-box analogue experiments, in which caldera formation (modeled by evacuation of a rubber balloon) was simulated. The deformation of volcanic cones was studied by placing sand-cones in the vicinity of the expected caldera rim. At the initial stage of the modeled subsidence, the propagating ring fault of the caldera bifurcates within the affected cone into two faults, the outermost of which is notably curved outward off the caldera center. The two faults dissect the cone into three parts: (1) a stable outer part, (2) a highly unstable and subsiding intracaldera part, and (3) a subsiding graben structure between parts (1) and (2). Further progression of the caldera subsidence is likely to cause failure of parts (2) and (3) with failed material sliding into the caldera basin and with formation of an amphitheater-like depression oriented toward the developing caldera. The mass of material which is liable to slide into the caldera basin, and the shape of the resulted amphitheater are a function of the relative position of the caldera ring fault and the base of the cone. A cone situated mostly outside the ring fault is affected to a minor degree by caldera subsidence and collapses with formation of a narrow amphitheater deeply incised into the cone, having a small opening angle. Accordingly, the caldera exhibits a prominent outward embayment. By contrast, collapse of a cone initially situated mostly inside the caldera results in a broad amphitheater with a large opening angle, i.e. the embayment of the caldera rim is negligible. The relationships between the relative position of an edifice above the caldera fault and the opening angle of the formed amphitheater are similar for the modeled and the natural cases of caldera/cone interactions. Thus, our experiments support the hypothesis that volcanic edifices affected by caldera subsidence can experience large-scale failures with formation of indicative amphitheaters oriented toward the caldera basins. More generally, the scalloped appearance of boundaries of calderas in contact with pre-caldera topographic highs can be explained by the gravitational influence of topography on the process of caldera formation.Editorial responsibility: J. Stix     

Eruptive history of the Colima volcanic complex (Mexico)

The evolution of the Colima volcanic complex can be divided into successive periods characterized by different dynamic and magmatic processes: emission of andesitic to dacitic lava flows, acid-ash and pumice-flow deposits, fallback nuées ardentes leading to pyroclastic flows with heterogeneous magma, plinian air-fall deposits, scoriae cones of alkaline and calc-alkaline nature. Four caldera-forming events, resulting either from major ignimbrite outbursts or Mount St. Helens-type eruptions, separate the main stages of development of the complex from the building of an ancient shield volcano (25 × 30 km wide) up to two summit cones, Nevado and Fuego.The oldest caldera, C1 (7–8 km wide), related to the pouring out of dacitic ash flows, marks the transition between two periods of activity in the primitive edifice called Nevado I: the first one, which is at least 0.6 m.y. old, was mainly andesitic and effusive, whereas the second one was characterized by extrusion of domes and related pyroclastic products. A small summit caldera, C2 (3–3.5 km wide), ended the evolution of Nevado I.Two modern volcanoes then began to grow. The building of the Nevado II started about 200,000 y. ago. It settled into the C2 caldera and partially overflowed it. The other volcano, here called Paleofuego, was progressively built on the southern side of the former Nevado I. Some of its flows are 50,000 y. old, but the age of its first outbursts is not known. However, it is younger than Nevado II. These two modern volcanoes had similar evolutions. Each of them was affected by a huge Mount St. Helens-type (or Bezymianny-type) event, 10,000 y. ago for the Paleofuego, and hardly older for the Nevado II. The landslides were responsible for two horseshoe-shaped avalanche calderas, C3 (Nevado) and C4 (Paleofuego), each 4–5 km wide, opening towards the east and the south. In both cases, the activity following these events was highly explosive and produced thick air-fall deposits around the summit craters.The Nevado III, formed by thick andesitic flows, is located close to the southwestern rim of the C3 caldera. It was a small and short-lived cone. Volcan de Fuego, located at the center of the C4 caldera, is nearly 1500 m high. Its activity is characterized by an alternation of long stages of growth by flows and short destructive episodes related to violent outbursts producing pyroclastic flows with heterogeneous magma and plinian air falls.The evolution of the primitive volcano followed a similar pattern leading to formation of C1 and then C2. The analogy between the evolutions of the two modern volcanoes (Nevado II–III; Paleofuego-Fuego) is described. Their vicinity and their contemporaneous growth pose the problem of the existence of a single reservoir, or two independent magmatic chambers, after the evolution of a common structure represented by the primitive volcano.     

Application of synthetic aperture radar (SAR) imagery to volcano mapping in the humid tropics: a case study in East Java, Indonesia

 Volcanoes in humid tropical environments are frequently cloud covered, typically densely vegetated and rapidly eroded. These factors complicate field and laboratory studies and even the basic identification of potentially active volcanoes. Numerous previous studies have highlighted the potential value of radar remote sensing for volcanology in equatorial regions. Here, cloud- and vegetation-penetrating L_HH-band (λ≈24 cm) synthetic aperture radar (SAR) data from the Japanese Earth Resources Satellite (JERS-1) are used to investigate persistently active volcanoes and prehistoric calderas in East Java, Indonesia. The L_HH-band JERS-1 SAR produces high-spatial-resolution (18 m) imagery with relatively high incidence angle that highlights structures and topographic variations at or greater than the wavelength scale while minimising geometrical distortions such as layover and foreshortening. These images, along with Internet browse data derived from the Canadian RADARSAT mission, provide new evidence relating regional tectonics to volcanism throughout East Java. Volcanic events, such as caldera collapse at the Tengger caldera, appear to have been partly controlled by northwest-aligned faults related to intra-arc sedimentary basins. Similar regional controls appear important at historically active Lamongan volcano, which is encircled by numerous flank maars and cinder cones. A previously undocumented pyroclastic sheet and debris avalanche deposit from the Jambangan caldera complex is also manifested in the synoptic radar images. At the currently active Semeru volcano these data permit identification of recent pyroclastic flow and lahar deposits. Radar data therefore offer a valuable tool for mapping and hazard assessment at late Quaternary volcanoes. The criteria developed in the analysis here could be applied to other regions in the humid tropics. Received: 25 June 1998 / Accepted: 20 January 1999     

Gravity and aeromagnetic constraints on the structure of the Woods Mountains volcanic center, southeastern California

 The Woods Mountain volcanic center is a well-exposed, mildly alkaline volcanic center that formed during the Miocene in southeastern California. Detailed geologic mapping and geochemical studies have distinguished three major volcanic phases: precaldera, caldera forming, and postcaldera. Geologic mapping indicates that caldera formation occurred incrementally during eruptions of three large ignimbrites and continued into a period of voluminous intracaldera lava-flow eruptions. Rhyolitic ignimbrites and lava flows within the caldera are associated with large amplitude, circular gravity, and magnetic minima that are among the most prominent gravity and magnetic anomalies in southeastern California. Analysis of a Bouguer gravity anomaly map, reduced-to-the-pole magnetic intensity map, and three-dimensional gravity and magnetic models indicates that there is a single, funnel- to bowl-shaped caldera approximately 4 km thick and approximately 10 km wide at the surface. This model is consistent with other siliceous, pyroclastic-filled calderas on continental crust, except that most siliceous volcanic centers associated with more than one eruption are characterized by more than one caldera. Received: 20 December 1997 / Accepted: 15 October 1998     

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     

Isotope geochemistry (Sr–Nd–Pb) and petrogenesis of leucite-bearing volcanic rocks from “Colli Albani” volcano, Roman Magmatic Province, Central Italy: inferences on volcano evolution and magma genesis

The “Colli Albani” composite volcano is made up of strongly silica-undersaturated leucite-bearing rocks. Magmas were erupted during three main periods, but a complex plumbing system dominated by regional tectonics channelled magmas into different reservoirs. The most alkali-rich magmas, restricted to the caldera-forming period (pre-caldera), are extremely enriched in incompatible trace elements and display more radiogenic Sr (⁸⁷Sr/⁸⁶Sr?=?0.71057–0.71067), with slightly less radiogenic Pb with respect to those of the post-caldera period. Post-caldera volcanic activity was concentrated in three different volcanic environments: external to the caldera, along the caldera edge and within the caldera. The post-caldera magmas produced melilite- to leucitite-bearing, plagioclase-free leucitites. In contrast to the pre-caldera lavas, they are characterised by lower incompatible trace element abundances and less radiogenic Sr (⁸⁷Sr/⁸⁶Sr?=?0.71006–0.71039). Magmas evolved through crystal fractionation plus minor crustal assimilation in a large magma chamber during the pre-caldera period. The multiple caldera collapses dissected and partially obliterated the early magma chamber. During the post-caldera stage, magmas were channelled through several pathways and multiple shallow-level magma reservoirs were established. A lithospheric mantle wedge previously depleted in the basaltic component and subsequently enriched by metasomatic slab-derived component is suggested as the mantle source of Colli Albani parental magmas. Two different parental magmas are recognised for the pre- and post-caldera stages. The differences may be related to the interplay between smaller degrees of melting for the pre-caldera magmas and more carbonate-rich recycled subducted lithologies in the post-caldera magmas.     

江西相山火山盆地地质结构研究:来自大地电磁测深及三维地质建模的证据

江西省相山火山盆地是我国第一大、世界第三大火山岩型铀矿田,其西部牛头山一带铀矿勘探中发现深部有大垂幅的Pb-Zn-Ag矿化.60多年来,以该矿田为对象开展的研究取得了一系列丰硕成果,但对火山机构的认识仍不确定.我们采集了涵盖该火山盆地主要地质体的1386块钻孔岩芯标本和243块地表岩石标本,开展了电阻率、磁化率、密度等物性参数测量,并在火山盆地中施测了19条MT剖面(2条骨干剖面和17条精细剖面),对3000 m以浅主要地层、岩体和断裂带等目标地质体的三维展布特征进行了解译和三维建模.研究结果表明:(1)相山火山盆地具有变质岩-花岗岩双基底.基底变质岩系顶界面表现为南北分带(三隆间两凹)和东西分块(两垒夹一堑)的三维地质格局;南西部有加里东期花岗岩侵人,具有似层状的空间展布特征;盆地基底变质岩系与上覆火山-沉积岩盖层之间呈连续的水平低阻异常带,不整合界面清晰.(2)打鼓顶组火山岩呈似层状产出,主要分布于盆地西部;在河元背一船坑一杏树下一带识别出近东西走向厚层的流纹英安岩凹槽,相山铀矿田西部探明的主要铀矿床分布在该凹槽内或其边缘.鹅湖岭组火山岩总体形态呈蘑菇状,在盆地中部厚度较大.在相山主峰半径约2 km的范围内,发现自下而上贯通式的低阻异常,推测是鹅湖岭组碎斑熔岩喷发的通道相(火山颈相),其火山颈呈陡立管状,深部向南东倾伏,浅部向南东撒开.后期花岗斑岩呈岩墙-岩床组合状,总体构成一个向西开口的环形岩体.打鼓顶期主要岩浆通道位于相山顶一巴山之间,次岩浆通道位于河元背;鹅湖岭期火山活动主岩浆通道也位于相山顶一巴山之间,次岩浆通道位于河元背、阳家山(芙蓉山)、严坑和柏昌.(3)火山盆地中断裂构造发育,MT测量结果显示存在7条北东向、4条北西向和1条南北向格架性断裂构造(其中一条新发现的北东向断裂隐伏于白垩纪红盆之下),盆地北部发育1条弧形火山塌陷构造,表现为大规模延续的低阻异常带.     

The Reporoa Caldera,Taupo Volcanic Zone: source of the Kaingaroa Ignimbrites

The Reporoa Caldera occupies the northern end of the Reporoa Depression, previously described as a tectonic fault-angle depression. Earlier confirmation of the topographic basin as a caldera had been hindered by the lack of an associated young pyroclastic flow deposit of large enough volume to have caused caldera collapse. New exposures on the eastern margin of the Reporoa basin reveal thick lithic lag breccias (>30 m) interbedded within the 0.24 Ma Kaingaroa Ignimbrites. These ignimbrites were previously attributed to the adjacent Okataina Volcanic Centre. Lag breccia thicknesses and maximum clast sizes decrease rapidly outward from the caldera rim, and discrete breccias are absent from ignimbrite sections more than 3 km from the rim. The lithic lag breccias, together with structural and geophysical evidence, confirm Reporoa Caldera as the source of the c. 100 km³ Kaingaroa Ignimbrites, adding another major rhyolitic volcanic centre to the seven previously recognized in the Taupo Volcanic Zone. Other, older, calderas may also be present in the Reporoa Depression.     

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.