Surface deformation at the Krafla volcano,North Iceland, 1982–1992

Extensive measurements of ground deformation at the Krafla volcano, Iceland, have been made since the beginning in 1975 of a series of eruptions and intrusions into the fissure system that extends north and south of the volcano. I concentrate on measurements before and after the eruption of September 1984, the last event of this series when the largest volume of lava was erupted. The patterns of ground deformation associated with the 1984 eruption, determined by precision levelling, electronic distance measurements and lake level observations, were similar to earlier intrusions and eruptions, in that the surface of the volcano subsided and the fissure system widened as magma moved laterally from a shallow central reservoir into the fissure system. The shallow magma reservoir of Krafla continued to expand for about five years after the eruption, but a slow subsidence of the central area began in 1989. Besides the presence of an inflating and deflating shallow magma reservoir at a depth of 2.5 km beneath the Krafla caldera, another inflating magma reservoir may exist at much greater depth below Krafla. The accumulation of compressive strain by numerous rift intrusions and eruptions since 1975 along the flanks of the north-south Krafla fissure swarm is being released slowly and will probably be reflected in the results of deformation measurements near Krafla for the next several decades. The total horizontal extension of the Krafla rift system in 1975–1984 was about 9 m, equal to about 500 years of constant plate divergence. The extension is twice the accumulated divergence since previous rifting events and eruptions in 1724–1729     

Widening of the Krafla fissure swarm during the 1975–1981 volcano-tectonic episode

Frequent distance measurements across the Krafla fissure swarm, North Iceland, recorded the extension accompanying the sequence of rifting events which started in December, 1975, and lasted for 6 years.An 80 to 90 km long section of the fissure swarm extended during this sequence of rifting and volcanic events. Maximum widening of about 8 m occurred 10 to 12 km north of Leirhnjúkur. which is located above the center of the Krafla magma reservoir. From that location, the amount of widening decreased north-wards and is estimated to exceed 2 m where the seismicity indicated the northern termination of the present rifting, off the north coast, about 70 km north of Leirhnjúkur. The amount of widening also decreased southwards and approached zero at 15 to 20 km south of Leirhnjúkur.The ground deformation associated with these rifting events can be summarised as:A narrow strip, 1 to 2 km wide, along the fissure swarm is heavily fractured with numerous open cracks parallel to the fissure swarm. This fractured strip has subsided 2 to 3 m relative to its flanks.The flanks of the fractured zone have been uplifted relative to regions farther away. The uplift is not well constrained, but tilt observations at several locations indicate about 1 m uplift. The flanks of the rift zone have contracted, perpendicular to the fissure swarm. The maximum contractional strain exceeds 300 mm per km.The amount of areal expansion (windening of the fissure swarm times the length of the fractured zone) associated with these rifting events is estimated to be about 0.30 km². For individual events, the area of expansion has been roungly proportional to the volume of subsidence above the Krafla magma reservoir.If the width of a new dike is equal widening of surface fissures, the ratio of the subsidence volume to the area of expansion for the best observed events indicated a height of a new dike system as 2.4 to 2.8 km. This ratio is significantly less for events of large lava production, but even during these events, the majority of magma leaving the Krafla reservoir was apparently emplaced in subsurface fissures.     

The Krafla fissure swarm, Iceland, and its formation by rifting events

Fissure swarms at divergent plate boundaries are activated in rifting events, during which intense fracturing occurs in the fissure swarm accompanied by intrusion of magma to form dikes that sometimes lead to eruptions. To study the evolution of fissure swarms and the behaviour of rifting events, detailed mapping was carried out on fractures and eruptive fissures within the Krafla fissure swarm (KFS). Fracture densities of dated lava flows ranging from 10,000?years bp to ~30?years old were studied, and the fracture pattern was compared with data on the historical Myvatn rifting episode (1724–1729) and the instrumentally recorded Krafla rifting episode (1975–1984). Additionally, the interaction of transform faults and fissure swarms was studied by analysing the influence of the Húsavík transform faults on the KFS. During the historical rifting episodes, eruptions on the fissure swarm occurred within ~7?km from the Krafla central volcano, although faults and fractures were formed or activated at up to 60–70?km distance. This is consistent with earlier rifting patterns, as Holocene eruptive fissures within the KFS are most common closer to the central volcano. Most fractures within the central Krafla caldera are parallel to the overall orientation of the fissure swarm. This suggests that the regional stress field is governing in the Krafla central volcano, while the local stress field of the volcano is generally weak. A sudden widening of the graben in the northern KFS and a local maximum of fracture density at the junction of the KFS and the extrapolation of the Húsavík transform fault zone indicates possible buried continuation of the Húsavík transform fault zone which extends to the KFS. Eruptive fissures are found farther away from the Krafla central volcano in the southern KFS than in the northern KFS. This is either due to an additional magma source in the southern KFS (the Heiearsporeur volcanic system) or caused by the Húsavík transform faults, transferring some of the plate extension in the northern part. Fracture density within particular lava flow fields increases with field age, indicating that repeated rifting events have occurred in the fissure swarm during the last 10,000?years bp. The fracture density in the KFS is also generally higher closer to the Krafla central volcano than at the ends of the fissure swarm. This suggests that rifting events are more common in the parts of the fissure swarm closer to the Krafla central volcano.     

Integration of micro-gravity and geodetic data to constrain shallow system mass changes at Krafla Volcano, N Iceland

New and previously published micro-gravity data are combined with InSAR data, precise levelling and GPS measurements to produce a model for the processes operating at Krafla volcano, 20 years after its most recent eruption. The data have been divided into two periods: from 1990 to 1995 and from 1996 to 2003 and show that the rate of deflation at Krafla is decaying exponentially. The net micro-gravity change at the centre of the caldera is shown, using the measured free air gradient, to be −85 μGal for the first and −100 μGal for the second period. After consideration of the effects of water extraction by the geothermal power station within the caldera, the net gravity decreases are −73±17 μGal for the first and −65±17 μGal for the second period. These decreases are interpreted in terms of magma drainage. Following a Mogi point source model, we calculate the mass decrease to be ∼2×10¹⁰ kg/year reflecting a drainage rate of ∼0.23 m³/s, similar to the ∼0.13 m³/s drainage rate previously found at Askja volcano, N. Iceland. Based on the evidence for deeper magma reservoirs and the similarity between the two volcanic systems, we suggest a pressure-link between Askja and Krafla at deeper levels (at the lower crust or the crust-mantle boundary). After the Krafla fires, co-rifting pressure decrease of a deep source at Krafla stimulated the subsequent inflow of magma, eventually affecting conditions along the plate boundary in N. Iceland, as far away as Askja. We anticipate that the pressure of the deeper reservoir at Krafla will reach a critical value and eventually magma will rise from there to the shallow magma chamber, possibly initiating a new rifting episode. We have demonstrated that by examining micro-gravity and geodetic data, our knowledge of active volcanic systems can be significantly improved.Editorial responsibility: A. Harris     

Volume of magma accumulation or withdrawal estimated from surface uplift or subsidence,with application to the 1960 collapse of Kilauea volcano

An elastic point source model proposed by Mogi for magma chamber inflation and deflation has been applied to geodetic data collected at many volcanoes. The volume of ground surface uplift or subsidence estimated from this model is closely related to the volume of magma injection into or withdrawal from the reservoir below. The analytical expressions for these volumes are reviewed for a spherical chamber and it is shown that they differ by the factor 2(1-v), where v is Poisson's ratio of the host rock. For the common estimate v=0.25, as used by Mogi and subsequent workers, the uplift volume is 3/2 the injection volume. For highly fractured rocks, v can be even less and the uplift volume can approach twice the injection volume. Unfortunately, there is no single relation between the inflation of magma reservoirs and the dilation or contraction of host rocks. The inflation of sill-like bodies, for instance, generates no overall change in host rock volume. Inflation of dike-like bodies generates contraction such that, in contrast with Mogi's result, the uplift volume is generally less than the injection volume; for v=0.25, the former is only 3/4 of the latter. Estimates of volumes of magma injection or withdrawal are there-fore greatly dependent on the magma reservoir configuration. Ground surface tilt data collected during the 1960 collapse of Kilauea crater, one of the first events interpreted with Mogi's model and one of the largest collapses measured at Kilauea, is not favored by any one of a variety of deformation models. These models, however, predict substantially different volumes of both magma withdrawal and ground surface subsidence.     

S-wave shadows in the Krafla Caldera in NE-Iceland,evidence for a magma chamber in the crust

During the present tectonic activity in the volcanic rift zone in NE-Iceland it has become apparent that the attenuation of seismic waves is highly variable in the central region of the Krafla volcano. Earthquakes associated with the inflation of the volcano have been used to delineate two regions of high attenuation of S-waves within the caldera. These areas are located near the center of inflation have horizontal dimensions of 1–2 km and are interpreted as the expression of a magma chamber. The top of the chamber is constrained by hypocentral locations and ray paths to be at about 3 km depth. Small pockets of magma may exist at shallower levels. The bottom of the chamber is not well constrained, but appears to be above 7 km depth. Generally S-waves propagate without any anomalous aftenuation through laver 3 (v_p=0.5 km sec^?1) across the volcanic rift zone in NE-Iceland. The rift zone therefore does not appear to be underlain by an estensive magma chamber at crustal levels. The Krafla magma chamber is a localized feature of the Krafla central volcano.     

Multiple volcano deformation sources in a post-rifting period: 1989–2005 behaviour of Krafla,Iceland constrained by levelling,tilt and GPS observations

The Krafla rifting episode, which occurred in North Iceland in 1975–1984, was followed by inflation of a shallow magma chamber until 1989. At that time, gradual subsidence began above the magma chamber and has continued to the present at a declining rate. Pressure decrease in a shallow magma chamber is not the only source of deformation at Krafla, as other deformation processes are driven by exploitation of two geothermal fields, together with plate spreading. In addition, deep-seated magma accumulation appears to take place, with its centre ∼ 10 km north of the Krafla caldera. The relative strength of these sources has varied with time. New results from a levelling survey and GPS measurements in 2005 allow an updated view on the deformation field. Deformation rates spanning 2000–2005 are the lowest recorded in the 30-year history of geodetic studies at the volcano. The inferred rate of 2000–2005 subsidence related to processes in the shallow magma chamber is less than 0.3 cm/yr whereas it was ∼ 5 cm/yr in 1989–1992. Currently, the highest rate of subsidence takes place in the Leirbotnar area, within the Krafla caldera, and appears to be a result of geothermal exploitation.     

Dike intrusion associated with the 2000 eruption of Miyakejima Volcano, Japan

The model for the 2000 dike intrusion event between Kozushima and Miyakejima volcano, Japan, was reinvestigated. After the sudden earthquake swarm in Miyakejima volcano, a dike intrusion of large volume was detected by the nationwide GPS network (Geonet). The displacements detected with GPS stations over an area with a radius of about 200 km shows a distribution that is consistent with the dike source being located near Miyakejima volcano.The dike was intruded northwestwards between Miyakejima and the neighboring Kozushima volcano. We searched for the parameters in the models that reproduce the regional displacements due to dike intrusion between Miyakejima and Kozushiima islands. We tested three models, (1) the model with a single dike, (2) the model with a dike and a point dislocation source which represents a creep dislocation source and (3) the model with a dike and a deflation source which represents a magma reservoir. Though all three models can match the horizontal displacements near the source area, model 1 fails to reproduce the regional displacements in the central part of Japan. Both models 2 and 3 can reproduce the regional displacement for horizontal components. Model 3 produces slightly better results than model 2 for vertical components. The balance in the volume budget for models 2 and 3 is also consistent with the observations. These results show that we cannot distinguish between the two models using only GPS observation. As there is no direct evidence for such a large creep or ductile source (corresponds to M7 or more) as proposed in model 2 and the active seismic region migrated back and forth within the linear swarm region, the model with a dike and a deep magma source is preferable. For the deflation point source, we obtained a deflation volume of 1.5 km³ at the depth of 20 km below the dike. An additional ~0.95 km³ of volume loss through caldera collapse and edifice deflation took place at Miyakejima. We conclude that the magma that intruded the dike came in part from below Miyakejima and in part from below the sea floor between Miyakejima and Kozushima, perhaps from reservoirs at the Moho.Editorial responsibility: S Nakada, T Druitt     

Net gravity decrease at Askja volcano,Iceland: constraints on processes responsible for continuous caldera deflation, 1988–2003

Askja caldera in northeast Iceland has been in a state of unrest for decades. Ground-deformation surveys show that the rate of deformation, i.e., deflation, is much higher then observed at any other dormant volcano in Iceland. This work presents the results from microgravity and deformation studies at Askja from 1988 to 2003. The deflation reaches a maximum of −0.46 m in the centre of the caldera, relative to a station outside the caldera, during the study period. The source of deformation is inferred to be at ∼3 km depth and a recent study infers a second deeper source at ∼16 km depth. The deflation is consistent with a subsurface volume change of −0.018 km³. We find a net microgravity decrease of 115 μGal in the centre of the caldera relative to the same station. This corresponds to a subsurface mass decrease of 1.6×10¹¹ kg between 1988 and 2003 based on the use of a point source model. A combination of magma drainage and cooling and contraction of the shallow magma reservoir at 3 km depth is our favoured model, consistent with the integrated observations. We suggest that extensional tectonic forces generate space in the ductile part of the crust to accommodate ongoing magma drainage from the shallow magma chamber.     

Detecting volcanic eruption precursors: a new method using gravity and deformation measurements

One of the fundamental questions in modern volcanology is the manner in which a volcanic eruption is triggered; the intrusion of fresh magma into a reservoir is thought to be a key component. The amount by which previously ponded reservoir magma interacts with a newly intruded magma will determine the nature and rate of eruption as well as the chemistry of erupted lavas and shallow dykes. The physics of this interaction can be investigated through a conventional monitoring procedure that incorporates the simple and much used Mogi model relating ground deformation (most simply represented by Δh) to changes in volume of a magma reservoir. Gravity changes (Δg) combined with ground deformation provide information on magma reservoir mass changes. Our models predict how, during inflation, the observed Δg/Δh gradient will evolve as a volcano develops from a state of dormancy through unrest into a state of explosive activity. Calderas in a state of unrest and large composite volcanoes are the targets for the methods proposed here and are exemplified by Campi Flegrei, Rabaul, Krafla, and Long Valley. We show here how the simultaneous measurement of deformation and gravity at only a few key stations can identify important precursory processes within a magma reservoir prior to the onset of more conventional eruption precursors.     

Magma reservoir systems inferred from tilt patterns

Inflation patterns based on water-tube tiltmeter and levelling observation show different features for Krafla Volcano in Iceland and Kilauea Volcano in Hawaii. Monotonous sawtooth shape inflation is observed at Krafla, while inflation curves at Kileauea are more or less complicated. The difference was attributed to differences in the system of magma reservoir for the two volcanoes. By using the electrical equivalent of a magma reservoir and volcanic conduit as a capacitor and a resistor, an electrical oseillator was considered to be a possible model for a magma reservoir system. In the case of Krafla, the magma reservoir system is replaced with one electric oscillator called «Single system» or «Icelandic type» system. The complicated inflation pattern of Kilauea was interpreted as the assembly of a main magma reservoir and the group of surrounding small reservoirs. The equivalent electric analogue is the composite parallel and serial connection of a single oscillator which generates irregular output voltage during a charging process. The proposed magma reservoir system of Kilauea is called «Multi-coupled system» or «Hawaiian type system» which also help in interpreting the wondering of the uplift center and tidal phenomena of the Halemaumau lava lake.     

Crustal deformations associated with the 1986 fissure eruption of Izu-Oshima volcano, Japan, and their tectonic significance

We have investigated crustal deformations associated with the 1986 eruption of Izu-Oshima volcano, Japan, which was accompanied by an intensive fissure eruption. Two fissure crater chains, with NW-SE trend were created in the northern part of the caldera and on its northwestern flank. Their trend is consistent with the direction of compressive stress in this region. Depression of > 30 cm in the central zone including the caldera, and in the northwestern and southeastern parts in the island, was detected by precise leveling. On the other hand, uplifts up to 20 cm in the northeastern and southwestern parts were observed. Tide observations revealed that the Okada tide station, the leveling datum in Izu-Oshima, may have subsided by 5 cm after the eruption. An 1 m opening of fissure craters was detected by distance measurements of the baselines which cross fissure craters. Horizontal displacements obtained by reoccupation of control points showed a symmetrical pattern which was consistent with the opening of fissure craters. Anomalous strain changes were also observed in the surrounding regions—contractions were observed in the Boso and the Miura peninsula, northeast of Izu-Oshima, and extensions in the Izu peninsula.

To interpret these crustal deformations, a model which consists of a nearly vertical tensile fault and a deflation source is presented. The tensile fault lies parallel to the fissures and is divided into two parts according to depth. The deeper part of the tensile fault is 12 km long, 10 km wide, and has 2 km burial depth and 2.7 m opening displacement. The shallower part, which may represent the fissure craters, is 4 km long, 2 km wide, and the amount of opening is estimated to be 1 m. However, the deflation source may be located at a depth of 10 km beneath the northwestern flank of the caldera and depression just above the source is estimated to be 30 cm. A deflation source is required to explain the subsidence at the Okada tide station and the extension in the Izu peninsula. This model suggests that the eruption might have released tensile stresses in and around the Izu region which result from bending of the subducting Philippine Sea plate.     

Plate boundary deformation and continuing deflation of the Askja volcano,North Iceland,determined with GPS, 1987–1993

GPS geodetic measurements were conducted around the Askja central volcano located at the divergent plate boundary in north Iceland in 1987, 1990, 1992 and 1993. The accuracy of the 1987 and 1990 measurements is in the range of 10 mm for horizontal components; the accuracy of the 1992 and 1993 measurements is about 4 mm in the horizontal plane. Regional deformation in the Askja region is dominated by extension. Points located outside a 30–45 km wide plate boundary deformation zone indicate a displacement of 2.4±0.5 cm/a in the direction N 99°E±12° of the Eurasian plate relative to the North American plate in the period 1987–1990. Within the plate boundary deformation zone extensional strain accumulates at a rate of 0.8 strain/a. Displacement of control points next to Askja (>7 km from the caldera center) in the periods 1990–1993 and 1992–1993 show deflation and contraction towards the caldera. These results are in accordance with the results obtained by other geodetic methods in the area, which indicate that the deflation at Askja occurs in response to a pressure decrease at about 2.8 km depth, located close to the center of the main Askja caldera. A Mogi point source was fixed at this location and the GPS data used to solve for the source strength. A central subsidence of 11±2.5 cm in the period 1990–1993 is indicated, and 5.5±1.5 cm in the period 1992–1993. The maximum tensional strain rate, according to the point source model, occurs at a horizontal distance of 2.5–6 km from the source, at the same location as the main caldera boundary. Discrepancies between the observed displacements and predicted displacements from the Mogi model near the Askja caldera can be attributed to the regional eastwest extension that occurs at Askja.     

Storage, migration, and eruption of magma at Kilauea volcano, Hawaii, 1971–1972

The magmatic plumbing system of Kilauea Volcano consists of a broad region of magma generation in the upper mantle, a steeply inclined zone through which magma rises to an intravolcano reservoir located about 2 to 6 km beneath the summit of the volcano, and a network of conduits that carry magma from this reservoir to sites of eruption within the caldera and along east and southwest rift zones. The functioning of most parts of this system was illustrated by activity during 1971 and 1972. When a 29-month-long eruption at Mauna Ulu on the east rift zone began to wane in 1971, the summit region of the volcano began to inflate rapidly; apparently, blockage of the feeder conduit to Mauna Ulu diverted a continuing supply of mantle-derived magma to prolonged storage in the summit reservoir. Rapid inflation of the summit area persisted at a nearly constant rate from June 1971 to February 1972, when a conduit to Mauna Ulu was reopened. The cadence of inflation was twice interrupted briefly, first by a 10-hour eruption in Kilauea Caldera on 14 August, and later by an eruption that began in the caldera and migrated 12 km down the southwest rift zone between 24 and 29 September. The 14 August and 24–29 September eruptions added about 10⁷ m³ and 8 × 10⁶ m³, respectively, of new lava to the surface of Kilauea. These volumes, combined with the volume increase represented by inflation of the volcanic edifice itself, account for an approximately 6 × 10⁶ m³/month rate of growth between June 1971 and January 1972, essentially the same rate at which mantle-derived magma was supplied to Kilauea between 1952 and the end of the Mauna Ulu eruption in 1971.The August and September 1971 lavas are tholeiitic basalts of similar major-element chemical composition. The compositions can be reproduced by mixing various proportions of chemically distinct variants of lava that erupted during the preceding activity at Mauna Ulu. Thus, part of the magma rising from the mantle to feed the Mauna Ulu eruption may have been stored within the summit reservoir from 4 to 20 months before it was erupted in the summit caldera and along the southwest rift zone in August and September.The September 1971 activity was only the fourth eruption on the southwest rift zone during Kilauea's 200 years of recorded history, in contrast to more than 20 eruptions on the east rift zone. Order-of-magnitude differences in topographic and geophysical expression indicate greatly disparate eruption rates for far more than historic time and thus suggest a considerably larger dike swarm within the east rift zone than within the southwest rift zone. Characteristics of the historic eruptions on the southwest rift zone suggest that magma may be fed directly from active lava lakes in Kilauea Caldera or from shallow cupolas at the top of the summit magma reservoir, through fissures that propagate down rift from the caldera itself at the onset of eruption. Moreover, emplacement of this magma into the southwest rift zone may be possible only when compressive stress across the rift is reduced by some unknown critical amount owing either to seaward displacement of the terrane south-southeast of the rift zone or to a deflated condition of Mauna Loa Volcano adjacent to the northwest, or both. The former condition arises when the forceful emplacement of dikes into the east rift zone wedges the south flank of Kilauea seaward. Such controls on the potential for eruption along the southwest rift zone may be related to the topographic and geophysical constrasts between the two rift zones.     

Rifting episode in north iceland in 1874–1875 and the eruptions of askja and sveinagja

In 1874 and 1875 the fissure swarm of Askja central volcano was activated during a major rifting episode. This rifting resulted in a fissure eruption of 0.3 km³ basaltic magma in Sveinagja graben, 50 to 70 km north of Askja and subsequent caldera collapse forming the Oskjuvatn caldera within the main Askja caldera. Five weeks after initial collapse, an explosive mixed magma eruption took place in Askja. On the basis of matching chemistry, synchronous activity and parallels with other rifted central volcanoes, the events in Askja and its lissure swarm are attributed to rise of basaltic magma into a high-level reservoir in the central volcano, subsequent rifting of the reservoir and lateral flow magma within the fissure swarm to emerge in the Sveinagja eruption. This lateral draining of the Askja reservoir is the most plausible cause for caldera collpse. The Sveinagja basalt belong to the group of evolved tholejites characteristie of several Icelandic central volcanoes and associated fissure swarms. Such tholeiites, with Mgvalues in the 40 to 50 tange, represent magmas which have suffered extensive fractional crystallization within the crust. The 12% porphyritic Sveinagja basalt contains phenocrysts of olivine (Fo_62–67), plagioclase (An_57–62), clinopyroxene (Wo₃₈En₄₆Wo₁₆) and titanomagnetite. Extrusion temperature of the lava, calculated on the basis of olivine and plagioclase geothermometry, is found to be close to 1150°C.     

Simple stochastic modelling of the eruption history of a basaltic volcano: Nyamuragira,Zaire

Three simple models of the behaviour of a series of basaltic eruptions have been tested against the eruptive history of Nyamuragira. The data set contains the repose periods and the volumes of lava emitted in 22 eruptions since 1901. Model 1 is fully stochastic and eruptions of any volume with random repose intervals are possible. Models 2 and 3 are constrained by deterministic limits on the maximum capacity of the magma reservoir and on the lowest drainage level of the reservoir respectively. The method of testing these models involves (1) seeking change points in the time series to determine regimes of uniform magma supply rate, and (2) applying linear regression to these regimes, which for models 2 and 3 are the determinsstic limits to those models. Two change points in the time series for Nyamuragira, in 1958 and 1980, were determined using a Kolmogorov-Smirnov technique. The latter change involved an increase in the magma supply rate by a factor of 2.5, from 0.55 to 1.37 m³s^-1. Model 2 provides the best fit to the behavior of Nyamuragira with the ratio of variation explained by the model to total variation. R², being greater than 0.9 for all three regimes. This fit can be interpreted to mean that there is a determinstic limit to the elastic strength of the magma reservoir 4–8 km below the summit of the volcano.     

Formation of caldera periphery faults: an experimental study

Changing stresses in multi-stage caldera volcanoes were simulated in scaled analogue experiments aiming to reconstruct the mechanism(s) associated with caldera formation and the corresponding zones of structural weakness. We evaluate characteristic structures resulting from doming (chamber inflation), evacuation collapse (chamber deflation) and cyclic resurgence (inflation and deflation), and we analyse the consequential fault patterns and their statistical relationship to morphology and geometry. Doming results in radial fractures and subordinate concentric reverse faults which propagate divergently from the chamber upwards with increasing dilation. The structural dome so produced is characterised bysteepening in the periphery, whereas the broadening apex subsides. Pure evacuation causes the chamber roof to collapse along adjacent bell-shaped reverse faults. The distribution of concentric faults is influenced by the initial edifice morphology; steep and irregular initial flanks result in a tilted or chaotic caldera floor. The third set of experiments focused on the structural interaction of cyclic inflation and subsequent moderate deflation. Following doming, caldera subsidence produces concentric faults that characteristically crosscut radial cracks of the dome. The flanks of the edifice relax, resulting in discontinuous circumferential faults that outline a structural network of radial and concentric faults; the latter form locally uplifted and tiltedwedges (half-grabens) that grade into horst-and-graben structures. This superimposed fault pattern also extends inside the caldera. We suggest that major pressure deviations in magma chamber(s) are reflected in the fault arrangement dissecting the volcanoflanks and may be used as a first-order indication of the processes and mechanisms involved in caldera formation.     

Modeling the Mogi magma source centre of the Santorini (Thera) volcano, Aegean Sea, Greece, 1994–1999, based on a numerical-topological approach

The aim of this study is the refinement of the dynamics of a recent (1994?C1999) minor, slow-inflation episode of the Santorini (Thera) volcano, famous for the Minoan (??3600 B.C.) eruption and the identification of the parameters of the magmatic source responsible for the inflation. Based on the Mogi source equations, on geodetic observations of base-line changes, on a topological, grid-search approach and on the reasonable assumption that the magma source remained practically stable in map view during the inflation period, we have been able to refine the location and depth (approximately 2.7 km) of the magma center. A tendency for increase of the magma pressure with time, roughly corresponding to a sphere with radius between 30 and 60 m, and a short deflation interval were also documented. The overall modeling was based on a topological method of inversion in two steps and for a selected 4-D grid. At a first step the system of Mogi-source equations was approximated by the intersection of the 4-D subspaces (defined by sets of grid points) each satisfying one observation equation on the basis of a grid-search procedure. At a second step, the best estimate of the Mogi source solution and its full variance-covariance matrix were defined using a common stochastic approach. The overall approach leads to a solution of a system of equations focusing on a 4-D space bounding significant minima in the misfits between model and observed values, and not on solutions focusing on single points, usually trapped in local minima. This study is important to understand a new phase of volcanic unrest since January 2011, while the proposed methodology, inspired from traditional navigation methods may be useful for other inversion problems leading to redundant systems of highly non-linear equations with n unknowns (i.e. topological solutions in the n-D space).     

Explosive volcanism (VEI 6) without caldera formation: insight from Huaynaputina volcano, southern Peru

Through examination of the vent region of Volcán Huaynaputina, Peru, we address why some major explosive eruptions do not produce an equivalent caldera at the eruption site. Here, in 1600, more than 11 km³ DRE (VEI 6) were erupted in three stages without developing a volumetrically equivalent caldera. Fieldwork and analysis of aerial photographs reveal evidence for cryptic collapse in the form of two small subsidence structures. The first is a small non-coherent collapse that is superimposed on a cored-out vent. This structure is delimited by a partial ring of steep faults estimated at 0.85 by 0.95 km. Collapse was non-coherent with an inwardly tilted terrace in the north and a southern sector broken up along a pre-existing local fault. Displacement was variable along this fault, but subsidence of approximately 70 m was found and caused the formation of restricted extensional gashes in the periphery. The second subsidence structure developed at the margin of a dome; the structure has a diameter of 0.56 km and crosscuts the non-coherent collapse structure. Subsidence of the dome occurred along a series of up to seven concentric listric faults that together accommodate approximately 14 m of subsidence. Both subsidence structures total 0.043 km³ in volume, and are much smaller than the 11 km³ of erupted magma. Crosscutting relationships show that subsidence occurred during stages II and III when ∼2 km³ was erupted and not during the main plinian eruption of stage I (8.8 km³). The mismatch in erupted volume vs. subsidence volume is the result of a complex plumbing system. The stage I magma that constitutes the bulk of the erupted volume is thought to originate from a ∼20-km-deep regional reservoir based on petrological constraints supported by seismic data. The underpressure resulting from the extraction of a relatively small fraction of magma from the deep reservoir was not sufficient enough to trigger collapse at the surface, but the eruption left a 0.56-km diameter cored-out vent in which a dome was emplaced at the end of stage II. Petrologic evidence suggests that the stage I magma interacted with and remobilized a shallow crystal mush (∼4–6 km) that erupted during stage II and III. As the crystal mush erupted from the shallow reservoir, depressurization led to incremental subsidence of the non-coherent collapse structure. As the stage III eruption waned, local pressure release caused subsidence of the dome. Our findings highlight the importance of a connected magma reservoir, the complexity of the plumbing system, and the pattern of underpressure in controlling the nature of collapse during explosive eruptions. Huaynaputina shows that some major explosive eruptions are not always associated with caldera collapse. Editorial responsibility: J Stix     

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.