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     

A model for radial dike emplacement in composite cones based on observations from Summer Coon volcano,Colorado, USA

We mapped the geometry of 13 silicic dikes at Summer Coon, an eroded Oligocene stratovolcano in southern Colorado, to investigate various characteristics of radial dike emplacement in composite volcanoes. Exposed dikes are up to about 7 km in length and have numerous offset segments along their upper peripheries. Surprisingly, most dikes at Summer Coon increase in thickness with distance from the center of the volcano. Magma pressure in a dike is expected to lessen away from the pressurized source region, which would encourage a blade-like dike to decrease in thickness with distance from the center of the volcano. We attribute the observed thickness pattern as evidence of a driving pressure gradient, which is caused by decreasing host rock shear modulus and horizontal stress, both due to decreasing emplacement depths beneath the sloping flanks of the volcano. Based on data from Summer Coon, we propose that radial dikes originate at depth below the summit of a host volcano and follow steeply inclined paths towards the surface. Near the interface between volcanic cone and basement, which may represent a neutral buoyancy surface or stress barrier, magma is transported subhorizontally and radially away from the center of the volcano in blade-like dikes. The dikes thicken with increasing radial distance, and offset segments and fingers form along the upper peripheries of the intrusions. Eruptions may occur anywhere along the length of the dikes, but the erupted volume will generally be greater for dike-fed eruptions far from the center of the host volcano owing to the increase in driving pressure with distance from the source. Observed eruptive volumes, vent locations, and vent-area intrusions from inferred post-glacial dike-fed eruptions at Mount Adams, Washington, USA, support the proposed model. Hazards associated with radial dike emplacement are therefore greater for longer dikes that propagate to the outer flanks of a volcano.     

The May 2005 eruption of Fernandina volcano,Galápagos: The first circumferential dike intrusion observed by GPS and InSAR

The May 2005 eruption of Fernandina volcano, Galápagos, occurred along circumferential fissures parallel to the caldera rim and fed lava flows down the steep southwestern slope of the volcano for several weeks. This was the first circumferential dike intrusion ever observed by both InSAR and GPS measurements and thus provides an opportunity to determine the subsurface geometry of these enigmatic structures that are common on Galápagos volcanoes but are rare elsewhere. Pre- and post- eruption ground deformation between 2002 and 2006 can be modeled by the inflation of two separate magma reservoirs beneath the caldera: a shallow sill at ~1 km depth and a deeper point-source at ~5 km depth, and we infer that this system also existed at the time of the 2005 eruption. The co-eruption deformation is dominated by uplift near the 2005 eruptive fissures, superimposed on a broad subsidence centered on the caldera. Modeling of the co-eruption deformation was performed by including various combinations of planar dislocations to simulate the 2005 circumferential dike intrusion. We found that a single planar dike could not match both the InSAR and GPS data. Our best-fit model includes three planar dikes connected along hinge lines to simulate a curved concave shell that is steeply dipping (~45–60°) toward the caldera at the surface and more gently dipping (~12–14°) at depth where it connects to the horizontal sub-caldera sill. The shallow sill is underlain by the deep point source. The geometry of this modeled magmatic system is consistent with the petrology of Fernandina lavas, which suggest that circumferential eruptions tap the shallowest parts of the system, whereas radial eruptions are fed from deeper levels. The recent history of eruptions at Fernandina is also consistent with the idea that circumferential and radial intrusions are sometimes in a stress-feedback relationship and alternate in time with one another.     

Caldera morphology in the western Galápagos and implications for volcano eruptive behavior and mechanisms of caldera formation

Caldera morphology on the six historically active shield volcanoes that comprise Isabela and Fernandina islands, the two westernmost islands in the Galapagos archipelago, is linked to the dynamics of magma supply to, and withdrawal from, the magma chamber beneath each volcano. Caldera size (e.g., volumes 2–9 times that of the caldera of Kilauea, Hawai'i), the absence of well-developed rift zones and the inability to sustain prolonged low-volumetric-flow-rate flank eruptions suggest that magma storage occurs predominantly within centrally located chambers (at the expense of storage within the flanks). The calderas play an important role in the formation of distinctive arcuate fissures in the central part of the volcano: repeated inward collapse of the caldera walls along with floor subsidence provide mechanisms for sustaining radially oriented least-compressive stresses that favor the formation of arcuate fissures within 1–2 km outboard of the caldera rim. Variations in caldera shape, depth-to-diameter ratio, intra-caldera bench location and the extent of talus slope development provide insight into the most recent events of caldera modification, which may be modulated by the episodic supply of magma to each volcano. A lack of correlation between the volume of the single historical collapse event and its associated volume of erupted lava precludes a model of caldera formation linked directly to magma withdrawal. Rather, caldera collapse is probably the result of accumulated loss from the central storage system without sufficient recharge and (as has been suggested for Kilauea) may be aided by the downward drag of dense cumulates and intrusives.     

The fissure swarm of the Askja volcanic system along the divergent plate boundary of N Iceland

Divergent plate boundaries, such as the one crossing Iceland, are characterized by a high density of subparallel volcanic fissures and tectonic fractures, collectively termed rift zones, or fissure swarms when extending from a specific volcano. Volcanic fissures and tectonic fractures in the fissure swarms are formed during rifting events, when magma intrudes fractures to form dikes and even feeds fissure eruptions. We mapped volcanic fissures and tectonic fractures in a part of the divergent plate boundary in northern Iceland. The study area is ~1,800 km², located within and north of the Askja central volcano. The style of fractures changes with distance from Askja. Close to Askja the swarm is dominated by eruptive fissures. The proportion of tectonic fractures gets larger with distance from Askja. This may indicate that magma pressure is generally higher in dikes close to Askja than farther away from it. Volcanic fissures and tectonic fractures are either oriented away from or concentric with the 3–4 identified calderas in Askja. The average azimuth of fissures and fractures in the area deviates significantly from the azimuth perpendicular to the direction of plate velocity. As this deviation decreases gradually northward, we suggest that the effect of the triple junction of the North American, Eurasian and the Hreppar microplate is a likely cause for this deviation. Shallow, tectonic earthquakes in the vicinity of Askja are often located in a relatively unfractured area between the fissure swarms of Askja and Kverkfjöll. These earthquakes are associated with strike-slip faulting according to fault plane solutions. We suggest that the latest magma intrusions into either the Askja or the Kverkfjöll fissure swarms rotated the maximum stress axis from being vertical to horizontal, causing the formation of strike-slip faults instead of the dilatational fractures related to the fissure swarms. The activity in different parts of the Askja fissure swarm is uneven in time and switches between subswarms, as shown by a fissure swarm that is exposed in an early Holocene lava NW of Herðubreið but disappears under a younger (3500–4500 BP) lava flow. We suggest that the location of inflation centres in Askja central volcano controls into which part of the Askja fissure swarm a dike propagates. The size and amount of fractures in the Kollóttadyngja lava shield decrease with increasing elevation. We suggest that this occurred as the depth to the propagating dike(s) was greater under central Kollóttadyngja than under its flanks, due to topography.     

Tectonic and structural evolution of Gough volcano: A volcanological model

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

Shallow magma transport for the 2002–3 Mt. Etna eruption inferred from thermal infrared surveys

The 26 October 2002–28 January 2003 eruption of Mt. Etna volcano was characterised by lava effusion and by an uncommon explosivity along a 1 km-long-eruptive fissure on the southern, upper flank of the volcano. The intense activity promoted rapid growth of cinder cones and several effusive vents. Analysis of thermal images, recorded throughout the eruption, allowed investigation of the distribution of vents along the eruptive fissure, and of the nature of explosive activity. The spatial and temporal distribution of active vents revealed phases of dike intrusion, expansion, geometric stabilization and drainage. These phases were characterised by different styles of explosive activity, with a gradual transition from fire fountaining through transitional phases to mild strombolian activity, and ending with non-explosive lava effusion. Here we interpret the mechanisms of the dike emplacement and the eruptive dynamics, according to changes in the eruptive style, vent morphology and apparent temperature variations at vents, detected through thermal imaging. This is the first time that dike emplacement and eruptive activity have been tracked using a handheld thermal camera and we believe that its use was crucial to gain a detailed understanding of the eruptive event.     

Dike propagation within active central volcanic edifices: constraints from Somma-Vesuvius,Etna and analogue models

Dikes within stratovolcanoes are commonly expected to have radial patterns. However, other patterns may also be found, due to regional stresses, magmatic reservoirs and topographic variations. Here, we investigate dike patterns within volcanic edifices by studying dike and fissure complexes at Somma-Vesuvius and Etna (Italy) using analogue models. At the surface, the dikes and fissures show a radial configuration. At depths of tens to several hundreds of metres, in areas exposed by erosion, tangential and oblique dikes are also present. Analogue models indicate that dikes approaching the flanks of cones, regardless of their initial orientation, reorient to become radial (parallel to the maximum gravitational stress). This re-orientation is a significant process in shallow magma migration and may also control the emplacement of dike-fed fissures reaching the lower slopes of the volcano.     

Trachyte shield volcanoes: a new volcanic form from South Turkana,Kenya

Seven Pliocene volcanoes, one of which is described in detail, occur in the northern part of the Kenya Rift. They have low-angle, shield like forms, and comprise lavas, pumice tuffs and ash-flow tuffs almost wholly of trachytic composition. Each volcano possesses a structurally complex source zone in which plugs, dykes and pumice tuffs are concentrated and in which clearly defined craters and calderas are uncommon. By contrast, the flank zones are stratiform with slopes of about 5° and are composed of lavas and ash-flow sheets erupted in a highly fluid condition. The volcanoes range up to 50 km in diameter and are elongated parallel to the general trend of the rift reflecting a tectonic control on the distribution of the vents and their products. This combination of morphological, structural and compositional features suggests that the volcanoes are of a type not described before. Notes on the petrography of the lavas are included and it is suggested that the trachytes are petrogenetically related to alkali basalts, compositionally similar to those which form the substrate to the trachyte volcanoes.     

The caldera of Volcan Fernandina: a remote sensing study of its structure and recent activity

Air photographs taken in 1946, 1960, and 1982, together with SPOT HVR-1 images obtained in April and October of 1988, are used to characterize recent activity in and around the caldera of Fernandina Volcano, West Galapagos Islands. The eruptive and collapse events during this time span appear to be distributed in a NW-SE band across the summit and caldera. On the flanks of the volcano, subtle topographic ridges indicate that this is a long-term preferred orientation of extra-caldera activity as well (although radial and arcuate fissures are found on all sectors). The caldera is formed from the coalescence of multiple collapse features that are also distributed along a NW-SE direction, and these give the caldera its elongate and scalloped outline. The NW and SE benches consist of lavas that ponded in once-separated depressions that have been incorporated into the caldera by more recent collapse. The volume of individual eruptions within the caldera over the observed 42 years appears to be small (4x10⁶ m³) in comparison to the volumes of individual flows exposed in the caldera walls (120–150x10⁶ m³). Field observations (in 1989) of lavas exposed in the caldera walls and their cross-cutting relationships show that there have been at least three generations of calderas, and that at times each was completely filled. An interplay between a varying supply rate to the volcano and a regional stress regime is suggested to be the cause of long-term spatial and volumetric variations in activity. When supply is high, the caldera is filled in relative to collapse and dikes tend to propagate in all directions through the edifice. At other times (such as the present) supply is relatively low; eruptions are small, the caldera is far from being filled in, and dike propagation is influenced by an extra-volcano stress regime.     

Pyroclastic density currents resulting from the interaction of basaltic magma with hydrothermally altered rock: an example from the 2006 summit eruptions of Mount Etna,Italy

After 16 months of quiescence, Mount Etna began to erupt again in mid-July 2006. The activity was concentrated at and around the Southeast Crater (SEC), one of the four craters on the summit of Etna, and eruptive activity continued intermittently for 5 months. During this period, numerous vents displayed a wide range of eruptive styles at different times. Virtually all explosive activities took place at vents at the summit of the SEC and on its flanks. Eruptive episodes, which lasted from 1 day to 2 weeks, became shorter and more violent with time. Volcanic activity at these vents was often accompanied by dramatic mass-wasting processes such as collapse of parts of the cone, highly unusual flowage processes involving both old rocks and fresh magmatic material, and magma–water interaction. The most dramatic events took place on 16 November, when numerous rockfalls and pyroclastic density currents (PDCs) were generated during the opening of a large fracture on the SE flank of the SEC cone. The largest PDCs were clearly triggered explosively, and there is evidence that much of the energy was generated during the interaction of intruding magma with wet rocks on the cone’s flanks. The most mobile PDCs traveled up to 1 km from their source. This previously unknown process on Etna may not be unique on this volcano and is likely to have taken place on other volcanoes. It represents a newly recognized hazard to those who visit and work in the vicinity of the summit of Etna.     

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.     

Morphology and mechanism of eruption of postglacial shield volcanoes in Iceland

Postglacial Icelandic shield volcanoes were formed in monogenetic eruptions mainly in the early Holocene epoch. Shield volcanoes vary in their cone morphology and in the areal extent of the associated lava flows. This paper presents the results of a study of 24 olivine tholeiite and 7 picrite basaltic shield volcanoes. For the olivine tholeiitic shields the median slope is 2.7°, the median height 60 m, the median diameter 3.6 km, the median aspect ratio (height against diameter) 0.019, and the median cone volume 0.2 km³. The picritic shield volcanoes are considerably steeper and smaller. A shield-volcano cone forms from successive lava lake overflows which are of shelly-type pahoehoe. A widespread apron surrounding the cone forms from tube-fed P-type pahoehoe. The slopes of the cones have (a) a planar or slightly convex form, (b) a concave form, or (c) a convex-concave form. A successive stage of a shield volcano is determined on the basis of cone morphology and lava assemblages. A shield-producing eruption has alternating episodes of lava lake overflows and tube-fed delivery to the distal parts of the flow field. In the late stages of eruption, the cone volume increases in response to the increased amount of rootless outpouring on the cone flanks. Normally, only a small percentage of the total erupted volume of a shield volcano, sometimes as little as 1–3%, is in the shield volcano cone itself, the main volume being in the apron of the shield.     

Eruptive styles and inferences about plumbing systems at Hidden Cone and Little Black Peak scoria cone volcanoes (Nevada,U.S.A.)

We describe two small scoria cone volcanoes, Hidden Cone and Little Black Peak (ages between ~320–390 ka), in the Southwestern Nevada Volcanic Field and discuss their eruption mechanisms and inferences about their plumbing systems. Cone-forming pyroclastic deposits are consistent with eruptive styles ranging from Strombolian to violent Strombolian, and lavas emanated from near the bases of the cones. The volcanoes are monogenetic (rather than polycyclic, as allowed by previous geomorphic interpretations). Vents at each volcano appear to coincide with pre-existing normal faults, consistent with observations at older, deeply eroded volcanoes in the region. The existence of these two volcanoes on a topographically high area (particularly Hidden Cone) provides evidence for short feeder dike lengths (~500 m at the surface). We infer that this short length reflects the small length scale of the mantle source region that was tapped to feed each volcano. Editorial responsibility: J Stix     

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     

Evolution of Aoba caldera volcano,New Hebrides

Aoba is a basalt volcano situated in the northern part of a chain containing all the active volcanoes in the New Hebrides. The chain extends the length of the New Hebrides. Growing from a depth of 2,400 meters on the sea floor, the volcano probably emerged above sea level in the late Pliocene or early Pleistocene. The age of the oldest exposed rocks is unknown. Relatively fluid lavas with autobrecciated surfaces probably issued from tissures, initiating a shield-building stage as the volcano emerged. Airfall pyroclastics increase towards the top of these lavas and are overlain by agglomerates marking a more explosive episode. Activity continued with the effusion of picrite basalt, accompanied by spasms of ash emission that formed crystal tuff. Subsequently a more explosive episode produced agglomerate and tuff with occasional tongues of lava. The two oval summit calderas are apparently related to deep-seated subsidence. Lack of pumice deposits, and the basic nature of the magma suggest that the foundering of the calderas was a quiet event, possibly due to massive outpourings of lava at a lower level, although a substantial volume also erupted from the summit volcanoes at this time. A broad pyroclastic cone, which was still growing 360 years ago, occupies the centre of the inner caldera. It is surmounted by a wide crater, or possibly small caldera, containing a lake in which palagonite tuff cones have formed. The western end of the inner caldera is occupied by an explosion crater, and the eastern end by a semicircular lake. A thermal area containing a solfatara on the southeast shore of the eastern lake, and staining in the crater lake suggestive of fumarole activity, are the only evidence of vulcanicity at the present time. It is difficult to correlate events at the centre of the volcano with those at the lateral fissures. Later episodes at the centre are probably broadly contemporaneous with activity along the fissures, the inner ends of which are mantled by younger deposits of the central volcano. Accumulation of material about this axial fiissure system, marked by no less than 64 cruptive foci, mainly spatter cones, and phreatic explosion craters where they intersect the coast, has extended the island to the northeast and southwest, producing the present oval shape. Numerous flows spilled from these fissures, the last reaching the sea at N’dui N’dui only 300 years ago according to local legend. Abundant ash was emitted from both the summit calderas and flank fissures at a late stage, forming a tuff mantle with layers of accretionary lapilli. The last volcanic event was the formation of a lahar which destoyed a village on the northeast slope of the volcano about 100 years ago. No consistent variation with time is evident in the composition of the magma, although plagiophyric and aphyric lava erupted during the later stages. All the rocks are basaltic, and differ only in the presence or absence of phenocryst-forming minerals, and the proportions in which they occur. Picrite basalt and ankaramite erupted from the central volcano and flank fissures, respectively.     

Compositional variation in three Cascade stratovolcanoes: Jefferson,Rainier, and Shasta

Detailed major and trace element studies of volcanic rocks from Jefferson, Rainier, and Shasta stratovolcanoes in the Cascade Range indicate that each volcano has distinct geochemical distribution patterns. Silica variation diagrams are not smooth nor, in general, continuous for any volcano. Portions of stratigraphic sections within the volcanoes exhibit compositional coherency and are interpreted as eruptive groups which were extruded over time intervals which are short compared to the lifetimes of the volcanoes. The results of this investigation indicate the leasibility of geochemically mapping eruptive groups within stratovolcanoes. Systematic compositional trends are not observed within thick (500–1000 m) eruptive groups but may occur over thicknesses of <200 m. Compositional variations within eruptive groups are commonly non-systematic and show ranges similar to the ranges observed in individual flows. Correlations between the amounts or kinds of phenocryst phases present and intra-group compositional variation is not observed. Inter-group compositional differences are sometimes accompanied by mineralogical differences. Late andesites and dacites at Rainier and Shasta are characterized by decreases in K and Rb while at Jefferson increases in these elements and other compositional changes occur in the late eruptives. Progressive fractional crystallization models do not seem capable of explaining the element distributions observed in the three volcanoes. Existing data are consistent with a model involving varying degrees of melting of some combination of amphibolite, eclogite or peridotite in or above a subduction zone with varying water contents. Segregation and sequential eruption of small batches of magma may produce the eruptive groups characterizing the volcanoes. Late mafic magmas erupted at satellite vents appear to be produced in different (deeper?) mantle source areas.     

Microgravity and height changes caused by volcanic activity: Four

Microgravity measurements and levelling surveys on volcanoes are not always easy to make, but are useful for studying volcanic processes quantitatively. Gravity changes associated with volcanic activity are not always significant. Precision of microgravity measurements depend critically on the procedures adopted, and those applied in the present paper are described. Levelling technique is now orthodox, and some empirical laws relating ground deformation to volcanic activity are deduced from the accumulated data. Gravity changes occur at the same time and places as ground deformations. The relationship between microgravity and height changes are discussed from the standpoint of analyzing the data obtained on volcanoes. The observational results obtained on four volcanoes in Japan are separately analyzed because each volcano exhibits different patterns of gravity changes and deformations. During the 1977–1982 activity of Usu volcano, deformation was accompanied by microgravity changes frequently observed at a particular benchmark at the base of the volcano for about five years. The gravity changes prove to be not a direct effect of magma movements but to be caused by the deformations of ground strata and aquifers around the benchmark. The 1983 eruption of Miyakejima volcano was associated with local gravity changes around the eruptive fissures due to magma intrusion which was approximately modelled. Similarly the 1986 eruption of Ooshima volcano caused gravity changes on the volcano, but these were poorly correlated with elevation changes and their origins were not uniquely interpreted. To detect gravity changes associated with the activity of Sakurajima volcano, an equigravity point was selected at the north of the volcano besides the gravity points on and around the volcano itself. The probable gradual accumulation of magmas beneath the volcano for eight years is substantiated by observed microgravity and elevation changes.     

The July–August 2001 eruption of Mt. Etna (Sicily)

The July–August 2001 eruption of Mt. Etna stimulated widespread public and media interest, caused significant damage to tourist facilities, and for several days threatened the town of Nicolosi on the S flank of the volcano. Seven eruptive fissures were active, five on the S flank between 3,050 and 2,100 m altitude, and two on the NE flank between 3,080 and 2,600 m elevation. All produced lava flows over various periods during the eruption, the most voluminous of which reached a length of 6.9 km. Mineralogically, the 2001 lavas fall into two distinct groups, indicating that magma was supplied through two different and largely independent pathways, one extending laterally from the central conduit system through radial fissures, the other being a vertically ascending eccentric dike. Furthermore, one of the eccentric vents, at 2,570 m elevation, was the site of vigorous phreatomagmatic activity as the dike cut through a shallow aquifer, during both the initial and closing stages of the eruption. For 6 days the magma column feeding this vent was more or less effectively sealed from the aquifer, permitting powerful explosive and effusive magmatic activity. While the eruption was characterized by a highly dynamic evolution, complex interactions between some of the eruptive fissures, and changing eruptive styles, its total volume (~25×10 ⁶ m ³ of lava and 5–10×10 ⁶ m ³ of pyroclastics) was relatively small in comparison with other recent eruptions of Etna. Effusion rates were calculated on a daily basis and reached peaks of 14–16 m ³ s ^-1, while the average effusion rate at all fissures was about 11 m ³ s ^-1, which is not exceptionally high. The eruption showed a number of peculiar features, but none of these (except the contemporaneous lateral and eccentric activity) represented a significant deviation from Etna's eruptive behavior in the long term. However, the 2001 eruption could be but the first in a series of flank eruptions, some of which might be more voluminous and hazardous. Placed in a long-term context, the eruption confirms a distinct trend, initiated during the past 50 years, toward higher production rates and more frequent eruptions, which might bring Etna back to similar levels of activity as during the early to mid seventeenth century.