The geology and geophysics of the ambrym caldera,New Hebrides

Ambrym Island has an unusually large, well-preserved basaltic caldera 13 km across. The caldera occurs in the central region of an early broad composite cone, which formed a north-south line with three smailer volcanoes. Alter the caldera was formed volcanism occurred within it and along fissure lines running nearly east-west. Two volcanic cones are active almost continuously and historic fissure cruptions have been recorded. The caldera formed by quiet subsidence, or by subsidence accompanied by eruption of scoria lappili similar to that erupted prior and subsequent to caldera formation. The collapse was at least 600 metres and radiocarbon dating suggests it took place less than 2000 years ago. The caldera is detined by gravity anomalies 10 to 14 milligals lower than those at its rim suggesting predominantly ash infilling. Aeromagnetic anomalies show a prominent. nearly east-west lineation, with normally magnetised bipole anomalies over the centre of the caldera and over fissure lines east of it. The source of the present volcanic activity is believed to be located along dyke fissures, with a perched magma chamber beneath the caldera. The geophysical evidence on Ambrym, together with that of regional east trending magnetic anomalies and recent bathymetric results, suggests that the volcanic activity is localised by the intersection of an east-west fracture zone with the axis of the New Hebrides island are.     

Structure of the cantal strato-volcano, French Central Massif, from electric, magnetotelluric soundings and aeromagnetic data

The Neogene composite volcano of Cantal is the largest volcano in Europe. Its area (2500 km²) is larger than that of Etna (1200 km²). It was formed between 23 and 3 Ma ago.An aeromagnetic survey, in addition to electrical (ES) and magnetotelluric soundings (MTS) carried out in recent years have made it possible to describe the structure of this volcano.The terrain effects of the various pyroclastics and lavas that make up the volcanic edifice have been computed and removed from the aeromagnetic data; the analysis of the resulting terrain-corrected residual anomalies gives clear evidence for the presence of three internal or deep-seated magnetized bodies which are also revealed by the ES and MTS investigations:

1. (1) The main hypostructure corresponds to a large volcano-tectonic trough filled with low-resistivity (10-5 ωm) and moderately magnetized (1 A/m) volcanic material. However, a part of the trough is filled with non-magnetic formations and it may be an Oligocene sedimentary graben cut across by the present volcanic trough.

2. (2) The hypovolcanic intrusion (monzonites and gabbros) of the Jordanne upper valley was previously known from an outcrop and a borehole (260 m). Its large extent (15 km²) is shown by magnetic study as well as by ES and MTS. According to the magnetic interpretation, its thickness would be 300 m.

3. (3) The nature of the structure inside the Plomb du Cantal is still uncertain. In view of its high resistivity and high magnetization, it may be an unexposed pile of lavas inside the cone or a large intrusion.

In the area where a caldera affects the volcanic cone, ES and MTS underline the importance of a hydrothermal process (propylitization) that lowers and homogenizes the resistivity (p = 30 ω m) of all volcanic rocks and probably that of the uppermost part of the crystallophyllian basement.     

Internal structure of Tenerife (Canary Islands) based on gravity, aeromagnetic and volcanological data

Gravity and magnetic methods have been applied to the Tenerife Island, to provide new information about its internal structure. For this study, 365 gravity stations covering the central part of the island have been selected. The anomalous density maps at different depths were obtained by means of an inversion global adjustment, on fixed density contrast, to describe the three-dimensional (3D) geometry of the anomalous bodies. On the other hand, several analysis techniques, such as reduction to the pole, spectral analysis, low-pass filtering, terrain correction and forward modelling, were applied to process the high-resolution data obtained in an aeromagnetic survey, completed with marine and terrestrial data.The joint analysis of gravity and magnetic anomalies has shown tectonic and volcanic features that define some fundamental aspects of the structural framework and volcanic evolution of the island. A strong gravity anomaly produced by a large and deep source has been associated with an uplifted block of the basement beneath the southern part of Tenerife. The sources of the observed gravity highs from 8 km b.s.l. may be associated with the growth of the submarine shield stage that was clearly controlled by regional tectonic.The long-wavelength magnetic anomalies reveal highly magnetic sources, interpreted as gabbro-ultramafic cumulates associated with the root zone of a large dyke swarm. This intrusive body could be topped by the emplacement zone of magma chambers that correlate with a magnetic horizon at 5.7±0.8 km depth. Rooted in this highly magnetic zone, two dike–like structures can be associated with the magmatic feeding system of large recent basaltic volcanoes. A shallow magnetic horizon (1.4 km a.s.l.) can be correlated with the bottom phonolites of the Las Cañadas Edifice.In the central part of the island the coincidence of some gravity and magnetic lows is consistent with the presence of low-density and low-magnetic materials, that infill a collapsed caldera system. The structures close to the surface are characterised by low-density areas connected with the recent volcanism, in particular the minimum over the Teide volcano. Hydrothermal alteration is assumed to be the cause of a short-wavelength magnetic low over the Teide volcano.     

The monzogabbroic intrusion in the island of Vulcano,Aeolian Archipelago,Italy

Two drill-holes were carried out during 1983–84 by the Joint Venture AGIP-EMS-ENEL on the island of Vulcano southwest of the Cratere della Fossa. After passing through pyroclastics and lavas of the young volcanic centres of Vulcano the drill-holes penetrated an intrusion of monzogabbro to leuco-monzogabbro composition. In one of the holes the top of the intrusion occurs at 1360 m and the intrusive rocks are found to the bottom of the well at 2050 m. At this depth the temperature exceeds 419 °C and the temperature gradients are sufficiently steep that magma could well be reached only a few hundred metres deeper. Lava of the South Vulcano centre is metamorphosed by the intrusion.A massive pyroclastic bed, underlying the welded scoriae deposits associated with collapse of the Caldera del Piano system, contains blocks of the intrusion. Radiometric data suggest an intrusion age of 30 000 years. Geophysical data indicate that the main intrusion is a shallow level and is located in the stretch of sea west of Mt. Lentìa.     

New geological insights and structural control on fluid circulation in La Fossa cone (Vulcano,Aeolian Islands,Italy)

Electric resistivity tomography (ERT), self-potential (SP), soil CO₂ flux, and temperature are used to study the inner structure of La Fossa cone (Vulcano, Aeolian Islands). Nine profiles were performed across the cone with a measurement spacing of 20 m. The crater rims of La Fossa cone are underlined by sharp horizontal resistivity contrasts. SP, CO₂ flux, and temperature anomalies underline these boundaries which we interpret as structural limits associated to preferential circulation of fluids. The Pietre Cotte crater and Gran Cratere crater enclose the main hydrothermal system, identified at the centre of the edifice on the base of low electrical resistivity values (<20 Ω m) and strong CO₂ degassing, SP, and temperature anomalies. In the periphery, the hydrothermal activity is also visible along structural boundaries such as the Punte Nere, Forgia Vecchia, and Palizzi crater rims and at the base of the cone, on the southern side of the edifice, along a fault attributed to the NW main tectonic trend of the island. Inside the Punte Nere crater, the ERT sections show an electrical resistive body that we interpret as an intrusion or a dome. This magmatic body is reconstructed in 3D using the available ERT profiles. Its shape and position, with respect to the Pietre Cotte crater fault, allows replacing this structure in the chronology of the development of the volcano. It corresponds to a late phase of activity of the Punte Nere edifice. Considering the position of the SP, soil CO₂ flux, and temperature maxima and the repartition of conductive zones related to hydrothermal circulation with respect to the main structural features, La Fossa cone could be considered as a relevant example of the strong influence of pre-existing structures on hydrothermal fluid circulation at the scale of a volcanic edifice.     

A gravity study of the Island of vulcano,tyrrhenian sea,Italy

A detailed gravity survey was carried out on the island of Vulcano, Aeolian Islands, Italy. Gravity was measured on 107 stations and the Bouguer anomalies were computed by assuming geological densities. Aim of this survey was to complete the island structural pattern relatively to the shallower structures. Separation of the gravity anomaly field was carried out by means of data filtering, and two main components were discerned. The λ>2.2 km wavelength component, filtered out of the longer wavelength components, was interpreted quantitatively along a NW profile. The best fitting model consists of an upper layer of recent pyroclastic products (p=2.1 g/cm³) lying upon a highly compacted pyroclastic series or lavas (p=2.4 g/cm³). The shorter wavelength residual gravity field (λ<2.2 km) is characterized by two anomalies, located on Vulcanello and the «Fossa di Vulcano» crater. Vulcanello anomaly could be interpreted, given the geothermal state of the area, as due to an increase of the rock density consequent to propylization processes by high temperature fluids (T>200°C). «Fossa di Vulcano» anomaly is instead attributable to the local volcanic chimney. A schematic comprehensive model of Vulcano is also presented, which accounts for the available main geological and geophysical data.     

Stratigraphy and significance of Brown Tuffs on the Aeolian Islands (southern Italy)

Brown Tuffs (BT) are volcaniclastic ash deposits prominently represented in the stratigraphic profiles of all the Aeolian Islands (and Capo Milazzo on the northern coast of Sicily). Detailed stratigraphy and tephrochronology together with available radiometric ages suggest that they were emplaced over a long time interval spanning from the end of the last interglacial period (ca. 80 ka BP) up to 4–5 ka BP (age of the overlying Punte Nere pyroclastic products on Vulcano). The most complete BT succession is documented on Lipari where 14 distinct and successive units are subdivided by the interbedding of widespread tephra layers, local volcanic products, paleosols and epiclastic deposits and the occurrence of local erosive surfaces. Inter-island occurrence of Ischia-Tephra (a widely known tephra layer in the Aeolian archipelago dated at 56 ka BP) and Monte Guardia pyroclastics from Lipari (dated at 22–20 ka BP) subdivides the BT succession in Upper (UBT), Intermediate (IBT) and Lower BT units (LBT), which can be correlated at regional level: the LBT was emplaced between 80 and 56 ka BP, the IBT between 56 and 22 ka BP and the UBT between 20 and 4–5 ka BP. On the basis of stratigraphy, similarity in lithology and textural features, morphology of glass fragments, composition and consistency of thickness and grain-size variations, UBT units correlate with Piano Grotte dei Rossi tuffs on Vulcano island. They were generated by pulsating hydromagmatic explosive activity giving rise to pyroclastic density currents spreading laterally from a source located inside the La Fossa caldera on Vulcano island. Composition is in agreement with this hypothesis since UBT compositional features match those of Vulcano magmas erupted in that period. The effect of co-ignimbrite ash clouds (or associated fallout processes from sustained eruptive columns) is seen to explain the presence of UBT in areas further away from the suggested source (e.g. Salina and Lipari islands and Capo Milazzo). The origin of UBT exposed on Panarea island is still a matter of debate, due to contrasting compositional data. Due to large uniformity of lithological, textural and componentry characters with respect to the UBT, the lower portions of the BT succession (LBT-IBT) are considered to be the result of recurrent, large scale hydromagmatic eruptions of similar type. Moreover, for the IBT units, the correlation with Monte Molineddo 3 pyroclastics of Vulcano island (on the basis of lithological, compositional and stratigraphic matching) again suggests source(s) related to the Vulcano plumbing system and located within the La Fossa Caldera.     

Aeromagnetic surveys of Kuttyaro and Aso caldera regions,Japan

Detailed total-intensity aeromagnetic surveys of the Kuttyaro and Aso caldera regions, eastern Hokkaido and central Kyushu, were made during early 1964 under the auspices of the U.S.-Japan Co-operative Science Program in conjunction with a project for geophysical studies of calderas in Japan. Each caldera has a maximum diameter of about 22 km; the flights cover a 60 × 60 km rectangular area in each region. The Kuttyaro survey also encompasses the older caldera Akan, south-west of Kuttyaro, and the younger caldera Mashu to the east. All three lie within the Chīshīma (Kurile) volcanic zone. The isomagnetic contour map shows this zone as a belt of short wave-length anomaies which trends east-northeast across the region. Broad wavelength anomalies with trends intersecting the Chīshīma belt at an acute angle probably reflect structural relief on the Neogene volcanic basement concealed beneath Kuttyaro pyroclastic flows. The centre of Kuttyaro caldera coincides with the sharp southern termination of a strong basement high, whereas caldera faults and post-caldera domes have little magnetic expression. Mashu caldera is marked by a minimum in the position of the caldera lake; a symmetrical positive anomaly centering southeast of the caldera suggests either a buried older volcanic edifice or an intrusion. Akan caldera is represented by a magnetic depression encompassing a positive anomaly produced by its central post-caldera cone. The depression extends north of the geologically-deduced boundary of the caldera and may include an earlier collapse structure. Several volcanoes and lava sequences in the region produce negative anomalies due to inverse polarization. The most significant feature of the Aso isomagnetic map is a large, elongate positive anomaly that occupies the southern half of the caldera and extends about one caldera diameter to the south-west along the trend of the Median Tectonic Line of south-west Japan. Whether the anomaly represents the pre-Tertiary basement complex or a younger intrusion perhaps associated with Aso eruptive activity is uncertain. However, the causative body is abruptly truncated within the caldera by a major east-south-east structure passing through the eastern rim and coincident with the approximate locus of resurgent central vent eruptions. The structure may be a fault system that provided egress for the Aso pyroclastic flows. Superimposed on the basement anomaly are the effects of the topography of the caldera, the superficial caldera structure, and the post-caldera cones. An area of intense solfataric activity in the Kuju group of young volcanoes north of Aso has a pronounced negative anomaly. These two surveys illustrate the utility of the magnetic method for investigations of basement structure in caldera regions. They have served as a guide in interpreting reconnaissance aeromagnetic profiles flown concurrently for this project across some 14 other calderas or caldera-like structures in the Japanese islands.     

Role of magma mixing during the recent activity of La Fossa di vulcano (Aeolian Islands, Italy)

Relatively homogeneous trachytes have been erupted for approximately 3800 years at la Fossa di Vulcano. From the Punte Nere eruptive cycle up to the Palizzi cycle the products varied little, while after the Palizzi cycle (1600 + 1000 a B.P.) to the latest eruption, 1888–1890 AD, a spectrum of compositions, with rhyolite dominating, characterized the erupted products.A stratigraphic sequence, starting with the Palizzi lava flow, has been studied, focussing the attention on lavas and volcanic bombs, to define the role that magma mixing processes have played in the recent history of La Fossa di Vulcano. Textural and chemical analyses of whole rocks, glass, groundmass, and mineral phases indicate that only the breadcrust bombs, erupted during the 1888–1890, show evidence of mixing between trachytic and rhyolitic end-members. Interestingly, in the deposits of the same eruption, trachytic bombs also occur.The lava flows erupted before 1888–1890 display general features suggesting that they entrained crystals and lava fragments during magma ascent. During the 1888–1890 eruption the trachytic bombs were erupted before the breadcrust bombs, which have a more evolved and hybrid composition. These characteristics, together with the change of the nature of the products after the Palizzi cycle, require a complex volcanological model for the recent history of la Fossa di Vulcano.     

A 3D magnetic structure of Izu-Oshima Volcano and their changes after the eruption in 1986 as estimated from repeated airborne magnetic surveys

A 3D magnetic inversion method using a conjugate gradient method (CG method) was developed for constructing 3D magnetization models of a volcanic edifice and applied to aeromagnetic anomalies of Izu-Oshima Volcano surveyed in 1986 and in 1997. The calculated results of the 1986 data show that the volcanic edifice of Izu-Oshima Volcano has a mean magnetization intensity ranging from 10.4 to 12.1 A/m. The derived 3D magnetic structure shows low magnetization zones beneath the west-northwest of the western caldera rim, beneath the west-southwest of Mt. Mihara and beneath Mt. Shiroishi. These features may be related with demagnetizations, reflecting a high thermal state due to magma activities in the 1986 eruption. The comparison between 3D magnetization models in 1986 and in 1997, indicates meaningful changes beneath the C-craters erupted in 1986, suggesting a recovery process of demagnetizations and a considerable decrease of magnetization intensities in the foot of Mt. Futago, indicative of demagnetizations. A derived magnetization model including Izu-Oshima Volcano and its surrounding sea areas clarifies the submerged volcanic edifices around Izu-Oshima Island, and suggests that the old volcanic edifices of Fudeshima, Gyojyanoiwaya, and Okata Volcanoes have been affected by eastward migrations due to massive intrusions of a dike-like structure inferred at the base of Izu-Oshima Volcano.     

How natural remanent magnetization of basaltic units can dominate the reduced to pole magnetic value: A case study from the Faroe Islands

Early attempts to utilize magnetic data to understand the volcanic and subvolcanic succession on the Faroese Continental Shelf have shown that conventional interpretation and modelling of magnetic data from this area leads to ambiguous results. Interpretation of the aeromagnetic data on the Faroese Continental Shelf shows that some previously identified basement highs coincide with reduced-to-pole magnetic highs, whereas others coincide with negative or mixed magnetic features. Similarly, igneous centres are characterized by different polarity magnetic anomalies. Palaeomagnetic analysis of the onshore volcanic succession has demonstrated that the thermoremanent magnetization of the basaltic lavas is stronger than the induced magnetism, and both reversely and normally magnetized units are present. We have tested this with 2½D profile modelling using the palaeomagnetic information to correlate high-amplitude magnetic anomalies with basalt successions containing changes in magnetic polarity. This approach has enabled us to map the termination of the differently magnetized units offshore and thereby extend the mapping of the Faroe Island Basalt Group on the Faroese Platform and into adjacent areas.     

Eruptive and emplacement mechanisms of widespread fine-grained pyroclastic deposits on Vulcano Island (Italy)

 Tufi di Grotte dei Rossi Inferiori are thick ash deposits, representing the most voluminous stratigraphic unit on Vulcano Island. The deposits are related to hydromagmatic eruption which occurred under shallow water inside a caldera depression. Grain-size data and results of SEM investigation allow the character of the transporting medium, solid material concentration in the cloud during the lateral expansion, and the nature and role of the fluids present at the time of deposition to be constrained. We suggest that the eruption was characterized by closely timed hydromagmatic pulses giving rise to eruption clouds rich in water vapor and steam. The coarser material was not significantly transported in the eruptive cloud and it probably deposited in the caldera depression area. The finer material was extensively transported in the cloud, creating turbulent flows which surmounted the caldera rim barrier and dispersed in a southward direction, forming widespread deposits in the Piano area. Lower concentrated flows produced laminated deposits of more limited dispersion, whereas higher concentrated flows formed more dispersed thicker massive layers. Received: 26 February 1996 / Accepted: 17 June 1997     

History and mechanism of eruption of soda-rhyolite and alkali basalt,Socorro Island,Mexico

Socorro Island is the summit of a large volcanic mountain located on the Clarion Fracture Zone in the east Pacific. Two major periods of volcanic activity can be recognized on the island. The first (pre-caldera) period was characterized by eruptions of olivine-poor alkali basalt, followed by quiet effusion of soda rhyolite including varieties transitional to pantellerite. This period of activity terminated with the formation of a caldera by collapse. A relatively prolonged period of quiescence ended with rifting and down-faulting of the western side of the island along a north-south fracture system, accompanied by violently explosive eruptions of soda rhyolite which built a large tephra cone over the position of the old caldera. The locus of eruptive activity moved outward and downward along tension fractures and old tectonic rifts as the central vents became blocked by domes of dense obsidian. Low level eruptions of viscous soda rhyolite including pantellerite commenced without preliminary explosive eruptions and built numerous endogenous and exogenous domes. Basaltic eruptions were rare and confined to low-level vents. During the growth of the volcano the direction of active rifting appears to have changed from east-west to northwest-southeast to north-south. Little is known of the submarine portion of the volcano, but the topography seems to reflect the three directions of rifting. The oldest submarine lavas are assumed to be basaltic and are probably of late Tertiary age. The eruptive history of Socorro suggests that the underlying magma column became stratified toward the end of the active period.     

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.     

Lead isotope composition of the sublimates from the fumaroles of Vulcano (Aeolian Islands,Italy): inferences on the deep fluid circulation

The fumarolic fluids of Vulcano (Aeolian Islands, Italy) consist of a mixture of both deep and shallow components. The final products, the fumarolic gases and the sublimates associated with them, provide information on the complex interactions that occur at depth. As radiogenic isotopes do not undergo fractionation after they are incorporated in a fumarolic gas, they can be used directly to characterize the components that mixed. Lead isotopes are particularly suitable, because seawater, which plays an important part in the formation of the fumarolic fluids of Vulcano, contains only negligible amounts of it (10^-12 g/g). Therefore, the lead present in the fumarolic gases (and sublimates) is derived from the magmatic component and a water-rock interaction process. The lead isotope compositions of the lead sulfosalt sublimates collected from the Fossa Crater of Vulcano in 1924, and between 1989 and 1993, are given. The lead isotope ratios of most of the samples are the same within the range of analytical error, regardless of their collection date. The only samples that display slight variations are those collected in 1993. On the whole, the compositional trend of the lead isotopes of the sublimates coincides with that of the latitic-rhyolitic activity of Fossa and differs substantially from that of the pre-Fossa trachy-basaltic activity. The lead composition of the sublimates is very different from that of the Calabrian basement rocks. The data presented here show that the magma presently degassing at Vulcano has the same lead isotopic composition as the products of the recent activity of Fossa, whereas the fumarolic fluid circulation of Vulcano has not involved basement rocks similar to the Calabrian metasediments.     

Origin of a magnetic lineation on Kyushu Island, Japan

Abstract Several linear magnetic anomalies over continental crust have been identified in and around the Japanese Islands. The anomalies are probably related to island arc tectonic structures, but identifying specific sources has been difficult. Several deep holes were drilled in and around Aso caldera, where a linear anomaly occurs along an active fault. One drillhole located on the linear anomaly encountered a zone of highly magnetized and altered basement rocks at least 100 m thick at a depth of ∼1000 m. The other hole was located away from the anomaly and did not encounter any high-magnetic zones. Rocks from the zone have exceptionally strong remanent magnetization (several tens of A/m) sub-parallel to the present field. AF demagnetization experiments indicated that the magnetization is hard and stable. Magnetic modeling indicates that the linear anomaly is caused mainly by this layer. Microscopic examination of core samples shows that the highly magnetized zone includes secondary magnetic minerals and abundant hydrothermal alterations. Temperatures determined by fluid inclusions and down-hole temperatures show that the temperature of the highly magnetized zone was elevated in the past relative to surrounding rocks. The high temperature could destroy primary magnetic minerals and replace them with secondary magnetic minerals. Thus, the past hydrothermal system may have enhanced thermo-chemical remanent magnetization. The results can produce a model indicating that there was a past hydrothermal system related to the tectonic structure.     

磁异常揭示的峨眉山大火成岩省的深部结构

峨眉山大火成岩省位于中国西南部,在晚二叠纪约260 Ma喷发出巨量的大陆溢流型玄武岩.对于大火成岩省的岩浆喷发,在地下必定有一个相应的大规模岩浆聚集和运移系统.地球物理方法是探测岩石圈内部的有效方式.峨眉山大火成岩省为镁铁质岩浆喷发,由于镁铁质-超镁铁质岩石一般具有强磁性,因此,在喷发结束之后,地下岩浆系统如果被镁铁质岩浆填充,冷却固化成为岩石圈的一部分,很有可能会引起磁异常.本文使用区域磁异常数据来对峨眉山大火成岩省的深部构造进行研究.该区域的磁异常由一系列离散的异常组成,通过3D磁化率反演可以得到磁性体的空间分布.由于磁异常中具有明显的剩磁,直接使用经典的反演方法会有较大误差,我们首先将磁异常转换为弱敏感于磁化方向的磁异常模量,再使用模量数据进行3D反演,得到地下空间内磁异常源的分布.经过分析认为这些离散分布的磁异常源反映了岩石圈内部的镁铁质-超镁铁质侵入体.侵入体的位置可能反映了底侵和内侵的镁铁质岩浆固化形成的侵入体,代表镁铁质岩浆房位置或者岩浆运移的主要通道.     

Volcanic hazards at Fossa of Vulcano: Data from the last 6,000 years

Stratigraphic reconstruction of the complete sequence of deposits that formed the Fossa cone of Vulcano has distinguished four principal eruptive cycles: Punte Nere, Palizzi, Commenda, and Pietre Cotte. At least three additional eruptive cycles, one of which ends with the Campo Sportivo lava, occur between deposits of the Punte Nere and Palizzi cycles. However, exposure is inadequate for their characterization. The assignment of the modern deposits that follow the Pietre Cotte lava is uncertain.Deposits of each cycle follow a similar stochastic pattern that is controlled by a decrease in the effect of water/melt interaction. The normal sequence of pyroclastic products for each cycle starts with wet-surge beds, followed by dry-surge horizons, fall deposits, and finally lava flows. Absolute age determinations have been made on each cycle-ending lava flow.Wet-surge deposits normally occur near the crater rim, whereas dry-surge deposits are more widespread, reaching the surrounding caldera wall in many places. Thick fall deposits are confined to a zone extending about 800 m from the crater rim. Lava flows normally reach the base of the cone. The greatest hazard at Fossa is related to surge eruptions. The thickness of dry-surge deposits on the flanks of the cone increases away from the crater, but they pinch out toward the source near the crater rim. SEM analysis of the surface textures of juvenile glass clasts from dry-surge deposits confirms that the dominant control on the eruptive mechanism is water/melt interaction. Only slight modifications are induced on grain surfaces during transport. Particles from the Palizzi dry-surge beds lack surface textures characteristic of fall pyroclasts which suggests that ballistic fragments were not incorporated into the dense portion of the turbulent surge cloud. A quantitative analysis of the dispersal of products from the Palizzi cycle allowed creation of a computer-generated map for this eruption.Paper presented at the IUGG Inter-disciplinary Symposium on Volcanic Hazard, Hamburg, August 1983.     

Structural features of the shallow plumbing system of Vulcano Island Italy

In this study, we integrate information gathered from surface geology and tectonics with the results of a shallow (0–2 km b.s.l.) seismic tomography of Vulcano Island (Italy), obtained from the analysis of local earthquakes. The observed low Vp regions correspond to caldera filling products, mainly consisting of pyroclastics, tuffs, lava flows and hyaloclastites. High-velocity anomalies represent intrusive bodies. The striking correspondence between the stratigraphy from deep wells and the calculated velocity structure allows us to reconstruct the geometry and distribution of a main intrusion and to recognize some intra-caldera depressions. The shape and location of the high and low Vp anomalies are consistent with NW–SE and N–S strikes. Eruptive centres younger than 42 kyr, as well as the structural depressions of Vulcano and of the neighbour Lipari Island, align along a N–S direction. The combined interpretation of the available structural data and of the results from the tomography suggests that magmatic reservoirs of Vulcano at shallow depth (>0.5 km) align along a NW–SE strike but their shape is controlled by N–S striking normal faults and/or cracks that accommodate the right-lateral movements of the NW–SE strike-slip fault system.Editorial responsibility: T. Druitt     

The effect of stress on faulting and minor intrusions in the vicinity of a magma body

The stress field in the vicinity of a body of fluid of simple geometry contained within a non-homogeneously stressed solid has been calculated and the result applied to the case of a magma body within a region of the crust subject to triaxial stresses. The types of faulting and minor intrusion which result are described. The theory indicates that the regional stresses in the crust together with the magma pressure control the type of faulting, the form of the minor intrusions and the occurrence of eruptions from the magma body. The stress conditions favouring caldera formation, the intrusion of radial dykes, dyke swarms and cone sheets are described.