Earthquake swarms preceding the 2000 eruption of Miyakejima volcano, Japan

The first sign of magma accumulating beneath Miyakejima, an island volcano in the northern Izu islands, Japan, came at around 18:00 on 26 June 2000, when a swarm of earthquakes was detected by a volcano seismic network on the island. Earthquakes occurred initially beneath the southwest flank near the summit and gradually migrated west of the island, where a submarine eruption occurred the next morning. Earthquakes then migrated further to the northwest between Miyakejima and Kozushima, another volcanic island and developed to the most intense earthquake swarm ever observed in and around Japanese archipelago. To better image how the initial magma intrusion occurred, we relocated hypocenters by using a station-correction method and a double-difference method. The relocated epicenters are generally concentrated near the upper bound of dyke intrusions inferred from geodetic studies throughout the initial stages of the 2000 eruption at Miyakejima from 26 to 27 June 2000. As for seismic activity westward off Miyakejima in the morning on 27 June, hypocenters from both a nationwide seismic network that were relocated by the double-difference method, and those from the volcano seismic network relocated by the station-correction method, formed a very shallow cluster that ascended slowly with time as it propagated northwestward from Miyakejima. This suggests that the dykes have both a radial and upward component of movement.Editorial responsibility: S. Nakada, T. Druitt     

The November 2002 eruption at Piton de la Fournaise volcano,La Réunion Island: ground deformation,seismicity, and pit crater collapse

An eruption on the eastern flank of Piton de la Fournaise volcano started on 16 November, 2002 after 10 months of quiescence. After a relatively constant level of activity during the first 13 days of the eruption, lava discharge, volcanic tremor and seismicity increased from 29 November to 3 December. Lava effusion suddenly ceased on 3 December while shallow earthquakes beneath the Dolomieu summit crater were still recorded at a rate of about one per minute. This unusual activity continued and increased in intensity over the next three weeks, ending with the formation of a pit crater within Dolomieu. Based on ground deformation, measured by rapid-static and continuous GPS and an extensometer, seismic data, and lava effusion patterns, the eruptive period is divided into five stages: 1) slow summit inflation and sporadic seismicity; 2) rapid summit inflation and a short seismic crisis; 3) rapid flank inflation, onset of summit deflation, sporadic seismicity, accompanied by stable effusion; 4) flank inflation, coupled with summit deflation, intense seismicity, and increased lava effusion; and finally 5) little deflation, intense shallow seismicity, and the end of lava effusion. We propose a model in which the pre-intrusive inflation of Stage 1 in the months preceding the eruption was caused by a magma body located near sea level. The magma reservoir was the source of an intrusion rising under the summit during Stage 2. In Stage 3, the magma ponded at a shallow level in the edifice while the lateral injection of a radial dike reached the surface on the eastern flank of the basaltic volcano, causing lava effusion. Pressure decrease in the magmatic plumbing system followed, resulting in upward migration of a collapse front, forming a subterranean column of debris by faulting and stoping. This caused intense shallow seismicity, increase in discharge of lava and volcanic tremor at the lateral vent in Stage 4 and, eventually the formation of a pit crater in Stage 5.     

Temporal development of the 1999 intrusive episode in the Eyjafjallajökull volcano, Iceland, derived from InSAR images

Eyjafjallajökull volcano, located in southern Iceland, is characterized by its quiet nature. Only about a handful of earthquakes associated with the volcanic system had been detected prior to the 1990s. Earthquake swarms did, however, occur in 1994 and 1999. Here we investigate the spatio-temporal evolution of a magmatic intrusion associated with the 1999 earthquake swarm via analysis of produced surface deformation. A series of interferometric synthetic aperture radar (InSAR) images, spanning various periods of the intrusion, show that in 1999 surface deformation occurred mainly on the southern flanks of the volcano. The deformation amounts to more than 20 cm of range change. Inverse modeling resolves the deformation source to be a sill intrusion at 6.3 km depth. Sill opening was up to 1 m and the total intruded volume amounts to ～0.03 km³. The InSAR data display a migration of the center of deformation through time, enabling us to create time-dependant sill-opening models. Furthermore, we investigate the spatio-temporal distribution of earthquakes and find that the distribution supports the InSAR derived model and additionally provides indications for a possible site of a feeder channel. Magmatic flow-rate estimates indicate an initial intrusion rate of 4–6 m³/s, declining over a few weeks.     

Nature of the magma conduit under the east rift zone of Kilauea Volcano,Hawaii

From a combination of results of gravity, magnetic and seismic refraction surveys, the dike complex under the east rift zone of Kilauea Volcano in Hawaii was found to extend for 110 km from the summit area of the volcano to a point 60 km at sea beyond the eastern tip of the island. Near the summit the complex is 20 km wide, and at about 40 km distance from the summit, the complex narrows to 12 km wide. The main body of the dike complex is 2.3 km deep, but some parts are as shallow as 1 km. From extrapolation of temperature data of a deep well and from analysis of magnetic data, it was inferred that temperature of the dike complex is above the Curic point of 540°C. The internal part of the complex can approach the melting point of 1060°C. The dike complex was formed by numerous excursions of magma from the holding reservoir under the volcano summit. The theory of forceful intrusion of magma into rift zones accounts for the magma excursions and migration of the passageways. Gravity and seismic velocity data indicate that density of the material left in the dike complex is 3.1 g/cm³. In the light of recent density determinations of Hawaiian rocks under high pressure and temperature, it is concluded that during Hawaiian volcanic activity, less dense components of the parent magma crupt through surface vents while the more dense components remain trapped below. Samples of the dense material from the dike complex are required before we can have a complete picture of the parent magma of Hawaiian volcanoes. The dike complex is the source of thermal energy for a commercial quality geothermal reservoir that was found by drilling.     

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     

Elastic deformation models of Krafla Volcano,Iceland, for the decade 1975 through 1985

The dynamics of the shallow magma reservoir at Krafla Volcano in NE Iceland have been analyzed with three types of elastic models based on over 70 surveys of tilt and displacement made from 1975 to 1985, a period of continuous volcano-tectonic activity. Modeling results are integrated with geophysical and geological information to estimate the position, geometry, and volume change of magma reservoir domains subjected to periodic inflation and deflation. Dominating influences on magma reservoir dynamics are examined in the context of activity in Krafla's associated rift system and the deformation of its caldera from 1975–1985. Rather than the fluid-filled cavity concept of Mogi (1958) and most recent workers, our models are idealized as strained regions with spherical, double-spherical, or general ellipsoidal symmetry. The models are mathematically generalized from that of Mogi (1958) and are derived by inversion of displacements. Model results from different displacement components are remarkably consistent, although models based on vertical displacements typically have errors-of-fit much closer to expected measurement errors than those based on tilt or horizontal displacements. About one half of the double-sphere and ellipsoid models have significantly better fits than single-sphere models. Double-sphere model results are consistent with a 2 to 3 km center-to-center separation of magma storage zones from at least 2 km to 4 km depth by portions of the fissure system, as implied by S-wave attenuation patterns for 1976–1977. However, all models suggest pressurization zones of more limited extent than possible domains of storage inferred from S-wave attenuation. Ellipsoid models typically implied unrealistically shallow depths of magma storage. Caldera inflation rates decreased after January 1978 when the caldera periodically reinflated to its level prior to the initial December 1975 deflation event. From 1975–1980 the volume and duration of inflation between deflation events was strongly correlated with the volume of the previous deflation. After 1980 there was a significant increase in the duration of inflation periods and a decrease in rates of caldera inflation and fissure system widening. Consistent with these results, the magma reservoir is conceptualized as a hot rock mass containing numerous magma chambers and pressure-sensitive conduits connecting the chambers and deep magma sources. Magma is injected into the fissure system at critical pressures determined by the confining stress and rock mass strength. The duration and volume of inflation required to reach a critical pressure threshold is largely dependent on the volume of magma released in the previous deflation. Reduction of the extension rate across the Krafla fissure system after 1980 suggests that extensional forces were also reduced. A resultant rise of confining pressure on the magma reservoir and a reduced capacity of the fissure system to accommodate dike injection in combination would have increased critical stress levels for reservoir deflation and reduced the pressure gradient driving magma supply from deep sources.     

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.     

Sensitivity test of the geodetic network in Las Cañadas Caldera, Tenerife, for volcano monitoring

A 17-benchmark geodetic network in the volcanic area of the Teide Caldera, Canary Islands has been utilised several times since 1982 to detect crustal movements associated with volcanic activity within the network, as well as a procedure for solving configuration problems. The network is located on the mid-western side of the Caldera, where there are two different morphological zones that both have benchmarks. The authors performed a sensitivity test of this geodetic network for volcano monitoring purposes. To do so, we use a deformation model to calculate surface displacement caused by a dike intrusion in a homogenous half space. The depth and location of dike are changed to study the variation of the effects produced (displacements). The size and location of the intruded dike are found to play a major role in determining both the displacement pattern and magnitude. When the dike is close to the surface, there is an inversion of the surface displacement pattern and very large surface displacement at certain benchmarks. Such phenomena can serve as precursors of such dike eruptions. Our study show a clear need to extend the existing geodetic network to cover the full island for volcano monitoring purposes.     

火山与地震的共性特征及有关问题讨论

对火山与地震及其伴随现象进行了对比分析,发现二者除外在形式不同外,在分布范围、深部构造及其它方面具有明显的共性特征．讨论了火山与地震具有共性特征的原因,认为它们都与岩浆活动有关．因此,内陆地震的发生有可能是地壳内岩浆上涌的直接结果     

Stabilization of volcanic flanks by dike intrusion: an example from Kilauea

 Dike propagation and dilation increases the compression of adjacent rocks. On volcanoes, especially oceanic shields, dikes are accordingly thought to be structurally destabilizing. As compression is incremented, volcanic flanks are driven outward or downslope and thus increase their susceptibility to destructive earthquakes and giant landslides. We show, however, that the 2-m-thick dike emplaced along the east rift zone of Kilauea in 1983 actually stabilized that volcano's flank. Specifically, production of flank earthquakes dropped more than twofold after 1983 as maximum downslope motion slowed to 6 cm·year^–1 from approximately 40 cm·year^–1 during 1980–1982. As much as 65 cm of deflationary subsidence above Kilauea's summit and upper rift zones accompanied the dike intrusion. According to recent estimates, this deflation corresponds to a reduction in magma-reservoir pressure of approximately 4 MPa, probably about as much as the driving pressure of the 1983 dike. The volume of the dike, approximately 0.10–0.15 km³, is orders of magnitude less than the estimated 200- to 250-km³ volume of Kilauea's reservoir of magma and nearby hot, mushy rock. Thus, deflation of that reservoir reduces the compressional load on the flank over a much larger area than intrusion of the dike adds to it, particularly at the dominant depth of seismicity, 8–9 km. A Coulomb block model for flank motion during intervals between major earthquakes requires the low-angle fault beneath Kilauea's flank to exhibit slip weakening, conducive to earthquake instability. Accordingly, the triggering mechanism of destructive earthquakes, several of which have struck Hawaii during the past 150 years, need not require stresses accumulated by dike intrusions. Received: 27 October 1998 / Accepted: 24 May 1999     

地壳中岩浆沿已有断层运移的数值模拟——在长白山天池火山的应用

本文利用有限元及有限差分的方法,对壳内岩浆房或岩浆囊中的岩浆在构造应力及由于围岩与岩浆的密度差产生的浮力作用下,沿已有断层向上运移的动力学过程进行了数值模拟.在岩浆囊顶部与上覆岩层接触处,沿着已有微小破裂,岩浆在一定超压力条件下使已有断层张开并继续向上延伸,从而形成岩浆向上运移的通道.研究了岩浆黏度、密度差、模型深度对最小超压力（岩浆运移到地表所需的最小岩浆房超压力）的影响.在10 km深度的地壳中,若岩浆黏度为0.1~10³ Pa·s,当超压力达到17~20 MPa时,岩浆压力可以驱动岩浆运移到地表层;同时,岩浆动力黏度越大,使岩脉运移到地表需要的超压力就越大.当密度差为300~700 kg·m^-3,其变化对超压力的影响比较小.本文亦对比了三维应力条件和二维平面应变条件下不同结果,比较了不同条件下岩浆运移造成的地表垂直位移变化.结合长白山天池火山地区的区域地质环境,对长白山天池火山岩浆运移条件进行了参数试验性计算分析,估算了在给定长白山天池火山模型条件下地下可能存在的岩浆囊的大小,其结果对认识长白山天池火山地区岩浆活动及相关的预测和监控有参考意义.     

On the interpretation of vertical gravity gradients produced by magmatic intrusions

A method for the computation and interpretation of gravity and height changes and vertical gravity gradients produced by magmatic intrusions in a layered elastic–gravitational medium is presented. The methodology assumes a planar medium geometry, which consists of welded layers overlying a half-space. The medium is elastic and gravitating. The intrusion is treated as a point source and can be located at any depth inside the medium. The theoretical elastic–gravitational model allows the computation of the so-called geometric and orthometric vertical displacements as well as gravity changes of different types. The corresponding vertical gravity gradients can also be computed. We present several examples of theoretical computations and study different types of these geometric and orthometric gravity gradients and the information we can get from the application of the methodology described. The results presented show that the use of these gradients is a useful tool to obtain information on the dynamics of the injection processes, including the detection of new magma recharge. We show that using the elastic–gravitational deformation model we can explain non-linear gravity–height relationships that appear in volcanic areas. We also present the application of the methodology to Mayon volcano, Philippines, delineating the intrusion of new magma, consistent with the produced eruptions after the observation period.     

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.     

腾冲火山及周边地区双差层析成像

利用云南腾冲火山地区15个固定台站记录到的7923次地震的P波到时资料,采用双差层析成像方法,反演得到腾冲火山及周边地区地壳及上地幔顶部三维P波速度结构和地震重定位结果.研究发现,腾冲火山区域地壳内存在明显的地震波低速区,P波速度低于整个区域地壳速度平均值超过15%,上地幔顶部存在规模较大的低速异常区.推测腾冲火山地区存在较大规模的地幔热物质上涌以及向地壳的侵入,热物质在地壳内以岩浆囊形式存储,并且壳内岩浆囊之间可能存在岩浆通道.通过联合反演获得的地震重定位结果显示,丛集地震位置更加集中,其展布特征与断裂构造具有显著的对应关系,表明研究区域断裂构造比较活跃.获得的高分辨率三维P波层析成像结果,为进一步认识火山地区岩浆存储特征以及地震分布与区域构造之间的关系提供了新的地震学依据.     

Evolution of a magma-driven earthquake swarm and triggering of the nearby Oldoinyo Lengai eruption, as resolved by InSAR, ground observations and elastic modeling, East African Rift, 2007

An earthquake swarm struck the North Tanzania Divergence, East African Rift over a 2 month period between July and September 2007. It produced approximately 70 M > 4 earthquakes (peak magnitude Mw 5.9), and extensive surface deformation, concurrent with eruptions at the nearby Oldoinyo Lengai volcano. The spatial and temporal evolution of the entire deformation event was resolved by Interferometric Synthetic Aperture Radar (InSAR) observations, owing to a particularly favorable acquisition programming of the Envisat and ALOS satellites, and was verified by detailed ground observations. Elastic modeling based on the InSAR measurements clearly distinguishes between normal faulting, which dominated during the first week of the event, and intermittent episodes of dike propagation, oblique dike opening and dike-induced faulting during the following month. A gradual decline in the intensity of deformation occurred over the final weeks. Our observations and modeling suggest that the sequence of events was initiated by pressurization of a deep-seated magma chamber below Oldoinyo Lengai which opened the way to lateral dike injection, and dike-induced faulting and seismicity. As dike intrusion terminated, silicate magma ascended the volcano conduit, reacted with the carbonatitic magma, and set off a major episode of explosive ash eruptions producing mixed silicate-carbonatitic ejecta. The rise of the silicate magma within the volcano conduit is attributed to bubble growth and buoyancy increase in the magma chamber either due to a temporary pressure drop after the termination of the diking event, or due to the dynamic effects of seismic wave passage from the earthquake swarm. Similar temporal associations between earthquake swarms and major explosive ash eruptions were observed at Oldoinyo Lengai over the past half century.     

Magma eruption rates of Merapi volcano,Central Java,Indonesia during one century (1890–1992)

The magma eruption rates of Merapi volcano form 1890 to 1992 are re-examined chronologically. For this volcano, movements of extruded lavas and domes as well as their extrusions are important because they control the modes of the subsequent activities and cause nuées ardentes and lahars. The monthly eruption rates varied widely, but the cumulative volume of lavas has increased linearly and is expressed as 0.1x10⁶ m³/month. The magma production rate of this volcano may have been constant for these 100 years. Recurrent excessive effusion of lavas is tentatively interpreted by assuming a magma reservoir. The averaged eruption rate is small in comparison with other volcanoes such as Nyramuragia, Kilauea and Vesuvio. However, it is remarkable that the activity has been continuous for these 100 years and the total amount of lava discharged during this period reached more than 10⁸ m³. A simple model for the formation of the 1992 lava dome is presented. The viscosity of the lavas is probably between 10⁶ and 10⁷ P and the length of the magma conduit is probably less than 10 km.     

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.     

Effect of tensile stress concentration around magma chambers on intrusion and extrusion frequencies

What controls the intrusion and extrusion frequencies associated with volcanoes is still poorly understood. I propose that for volcanoes at divergent plate boundaries, these frequencies may be largely determined by the tensile stress concentration around the magma chambers that feed them. This stress concentration is mainly a function of the applied tensile stress, associated with spreading, and the aspect (height/width) ratios of the chambers. High spreading rates and/or aspect ratios lead to high rates of tensile stress concentration around the chambers and to an increase in their intrusion frequencies. It is found that for chambers at the same depth in a volcanic zone, the one of the highest aspect ratio will normally intrude magma most frequently. Also, if the chambers are of equal aspect ratios, the one at the greatest depth will intrude magma most frequently. Because the extrusion frequency of a volcano is a fraction of its intrusion frequency, the extrusion frequency may also be largely determined by the rate of tensile stress concentration around the magma chamber that feeds the volcano. These results are applied to the divergent plate boundary in Iceland, where many of the volcanoes appear to be fed by “double chambers”, that is, shallow chambers fed by deep-seated chambers. It is found that, except when the aspect ratio of the deep-seated chamber is much less than that of the shallow chamber, the intrusion frequency of the shallow chamber is normally largely controlled by that of the deep-seated chamber. It is concluded that whereas the short-term (i.e., ≤10³ yrs) extrusion frequencies of volcanoes at the plate boundary in Iceland may be similar to the dike intrusion frequencies of the source chambers, the long-term (i.e., ≥10⁴ yrs) extrusion frequencies may be about ten times lower than the intrusion frequencies.     

A comparison of rift-zone tectonics in Iceland and Hawaii

Observations of eroded volcanic rift zones indicate that dikes in Iceland are typically several times thicker than those in Hawaii. Geodetic and seismic observations of active rifts, however, suggest that dike heights in the two regions are similar. Provided the elastic properties of the rift zones are the same, this implies that dikes are intruded with higher driving pressures (magma pressure minus compressive stress perpendicular to the dike plane) in Iceland than Hawaii. A second major difference between the two regions is the greater prevalence of large normal fault scarps in rift zones in Iceland. From this it can be infered that a lower percentage of dikes breach the surface in Iceland than in Hawaii. Thus, although dikes in Iceland are intruded with higher driving pressures, they possess lower absolute magma pressures than in Hawaii. These differences can be interpreted in terms of the tectonic settings in the two regions. In Iceland, a steady remote extension reduces the horizontal stress perpendicular to the rift zone, allowing dikes to be intruded with low absolute pressures but high driving pressures when magma becomes available. In Hawaii, a more continuous magma supply on the timescale over which the dike-induced stresses are relaxed, and perhaps a greater role for intrusions in driving long-term rift extension, ensure that the rift-compressive stress is not relaxed significantly before the next dike is intruded. Thus the magma pressure must be nearly sufficient for eruption in order for intrusion to occur. If the mechanism for relaxing the rift-compressive stress were less efficient still, then an even higher percentage of dikes would erupt, and at times the rift zone trend could become an unfavorable orientation for dike intrusion. Such might be the case at Mauna Loa, which lacks large rift-zone faults and fissures and possesses numerous radial vents outside its two main rift zones.     

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.