V42A-01 INVITED
Multi-temporal behaviour of a persistently active volcano - Masaya, Nicaragua
An in depth understanding of the processes responsible for persistent volcanism can only be achieved through the integration of multi-temporal geophysical and geochemical techniques. Gravity changes combined with ground deformation and gas flux have provided important information on magma reservoir mass changes and rates of magma emplacement or recharge. While the long-term micro-gravity, ground deformation and SO2 flux data collected at Masaya volcano (Nicaragua) since 1993 has identified significant multi-year cycles of activity, recent continuous micro-gravity surveys have shown significant short-term signals. Current activity is characterised by repeated periods of significant passive degassing (>2000 t/d SO2) with infrequent vent-clearing eruptions of negligible amounts of juvenile material. The multi-year gravity and gas flux variations are most likely due to gas accumulation in the shallow substructure controlled by the convective overturn of shallow degassed, cooled and dense magma replaced periodically by lower density, hot, gas-rich magma from depth. However, the presence of a small hydrothermal system can act as a physical and chemical buffer to the volcanic system while the essentially 'open' nature of Masaya's substructure can obscure short-term variations. In order to investigate the origin and importance of these short-term variations, continuous gravity and deformation surveys were made in March 2006 and 2007 in the summit craters of Masaya. During this period, residual gravity variations of ~60 µGal were measured with wavelengths of ~20 hours. These variations may be due in part to rapid changes in the hydrothermal system, occilations in height of the magma column or changes in vesiculation due to varying rates of gas flux. Concurrent detailed self-potential (SP) mapping, with soil CO2 and temperature measurements, delineated the shape and position of a small hydrothermal system centered on the main Nindiri crater. Processing of the SP data by continuous wavelet transform indicates that the principal hydrothermal cell is less than 200 m below the surface. Future SP surveys will thus allow for monitoring of any variations in depth of the hydrothermal system. Near continuous SO2 flux measurements were also made in order to investigate whether rapid changes in vesiculation of the shallow magma could be responsible for the observed gravity changes. While long-term multi-component monitoring has proven valuable in explaining the observed multi-year changes, notably, the importance of gas-driven convection, a combination of self-potential, continuous gravity, deformation and gas flux measurements are necessary to explain the short-term variations in magmatic activity.
V42A-02 INVITED
Magma plumbing processes for persistent activity at Poas Volcano, Costa Rica
Microgravity data from the active crater of Poas volcano, Costa Rica, collected between 1986 and 2004 extends the existing dataset to provide a unique time series. These data show that gravity has decreased monotonically in the north and east of the crater over the last 5 years, whilst it has increased to the west and remained approximately constant in the south. These changes are interpreted in terms of convective recharge within dendritic intrusions beneath the crater, with overall down-welling in the north and up-welling in the west. The data reveal a 5-10 year periodicity in sub-crater mass movement, but overall, the upper part of the conduit system appears to have maintained a state of mass equilibrium. The intrusion episodes in 1953-4 and the 1980s at Poas have resulted in considerable local environmental destruction and hazard to the population. Here we present data which suggest that a further intrusive episode may be occurring in the west of the lake whilst magma withdrawal is ongoing to the northeast.
V42A-03
Geochemical and Geophysical Signatures of Poas Volcano, Costa Rica
Among many research fields in volcanology, prediction of eruptions is the most important from the hazard- mitigation point of view. Most geophysicists have sought for the best physical parameters for this objective: various kinds of wave signals and geodesic data are two of such parameters. Being able to be remotely monitored gives them advantage over many other practical methods for volcano monitoring. On the other hand, increasing volcanic activity is always accompanied by mass transfer. The most swiftly-moving materials are volcanic gases which are the target geochemists have intensively studied although monitoring gases is rather tedious and limited for active volcanoes hosting crater lakes. A Japanese group lead by Bokuichiro Takano has recently developed an indirect method for monitoring gas injection into volcanic crater lakes. Polythionates are formed when SO2 and H2S are injected into the lake from subaqueous fumaroles. Such polythionates consist of chains of 4 to 6 sulphur atoms, the terminal ones of which are bonded with three oxygen atoms. The general formula for these anions is SxO62- (x= 4 to 6). Important to note is that SO2 input into the lake also depends upon the plumbing system of the volcanoes: conduits, cracks and hydrothermal reservoirs beneath the lake that usually differ from volcano to volcano. Despite such site-specific characters some general statements can be made on the behaviour of these chemical species. For example, at low volcanic activity S6O62- predominates while S4O62- and S5O62- become predominant with increasing SO2 that increases with volcanic activity. At higher SO2 input and high temperature polythionates disappear in the lake through interaction with aqueous SO2 (sulfitolysis). Thus, the ratios of the three polythionates or their absence serve as an indicator for various stages of volcanic activity. Monitoring polythionates is an independent method that can be compared with results from geophysical methods. However, it still remains open to a question if they will play a helpful role as a geophysical monitoring tool in determining the commencement of volcanic events. A range of Poas volcano geochemical and geophysical parameters have been combined to envisage relationships between them and to gain a better insight in the understanding of the Poas volcanic system.
V42A-04
Volcanic Processes, and Possible Precursors of Eruptions at Etna and Stromboli Volcanoes Revealed by Thermal Surveys
Thermal imaging has recently been introduced in volcanology to analyze a number of different volcanic
processes. This system allows us to detect magma movements within the summit conduits of volcanoes, and
then to reveal volcanic activity within the craters even through the thick curtain of gases usually released by active
volcanoes such as Mt Etna and Stromboli. Thermal mapping is essential during effusive eruptions, since it
distinguishes lava flows of different age and concealed lava tubes' path, improving hazard evaluation. Recently,
thermal imaging has also been applied to reveal failure planes and instability on the flanks of active volcanoes.
Excellent results have been obtained in terms of volcanic prediction during the eruptions of Mt Etna and Stromboli
occurred in 2002-2003. On Etna, thermal images monthly recorded on the summit of the volcano revealed the
opening of fissure systems several months in advance. At Stromboli, helicopter-borne thermal surveys allowed
us to recognize the opening of fractures one hour before the large failure that caused severe destruction on the
island on 30 December 2002. The INGV - Sezione di Catania started in 2001 to monitor active volcanoes using a
hand-held thermal camera. This instrument was used in field and from helicopter to detect any thermal anomaly
recorded on the surface of active volcanoes, and has since been applied to a number of eruptions and eruptive
processes. After the two major eruptions at Etna and Stromboli, fixed thermal cameras have been installed on
Stromboli, Etna and Vulcano, allowing us to keep under control the eruptive activity, flank stability and ash
emission. On Etna, we have monitored the 2002-03, 2004-05, July 2006 and August-December 2006 eruptions.
On Stromboli, thermal surveys from helicopter allowed us to follow the propagation of ephemeral vents and thus
the path of hidden lava tubes, as well as the stages of inflation and deflation of the upper lava flow field. Thermal
cameras have also been used to calculate the effusion rate, the most important parameter to estimate maximum
lava flow length, and also to detect ash plumes on Etna in good weather conditions. However, the three most
recent eruptions on Etna, occurred on 2004-05, July 2006 and August-December 2006, did not show evident
thermal anomalies on the summit craters before the opening of eruptive fissures. Thus, the role of thermal
anomalies and their meaning should be compared to and discussed with other geophysical data in order to
understand when and if these data can be used to forecast eruptions.
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V42A-05
Magma Stagnation and Ascent at Popocatepetl Volcano, Mexico during the last 10 years
Small eruptions (VEI 1-3) at Popocatepetl have been producing magnesian andesites to dacites (SiO2 56-65; MgO 7-2 wt. percent). since 1994. Since then, continuous multiparameter geophysical and geochemical monitoring (ash sampling and analyses, magnetic recording and processing, spring water analyses and correlation with seismicity) has been carried out. The time series shows that the mafic end-member is a small volume hydrous andesitic melt from the mantle which mixes with an evolved end-member at depths between 4- 13km. The stagnation level appears to be a series of interconnected dykes not a large magma chamber, since during the 10 years of monitoring no time-progressive compositional trend in the ejecta has been observed, therefore implying a steady influx and outflux of melts through the Popo plumbing system in a short timescale. If the reservoir was replenished periodically with mafic melt, a trend towards more mafic compositions would be expected if the evolved melt was to be gradually exhausted. Negative magnetic anomalies (2 to 6nT) correlate with periods of harmonic tremor and decreasing springwater pH, related with successive ascending magma batches. Larger eruptions are associated with influx of deeper magma.
V42A-06
Co-eruptive Strain Changes and Constraints on Magma Reservoir Incompressibility: Examples from Hekla and Montserrat
Borehole strainmeters have the capability of providing continuous (50 sps) monitoring of deformation with a sensitivity that, for durations up to many days, is much greater than any other measurement type currently available (e.g. roughly 1000 times that for GPS observations). Thus, by installing small arrays of such instruments close enough to an active volcano, it is possible to record deformation due to subsurface magma movement. We examine here data recorded during the 2000 eruption of Hekla, Iceland and a small explosion at Soufriere Hills, Montserrat. In both cases a small network of Sacks-Evertson borehole dilatometers recorded strain changes before and following the surface manifestation of the activity; from these changes we constrain the subsurface geometry of the magmatic system including the reservoir and conduit or dike that forms to connect the reservoir to the surface. Additionally, for the interval preceding surface release of material, we can apply conservation rules for a closed system: we use the relation dV/V= -dP/K (dV is volume change, V is volume, dP pressure change, K is incompressibility). From the data we can estimate values for the dike (or conduit) volume which is also (assuming constant density) the change in reservoir volume and the product dP*V for the reservoir. This allows calculation of the bulk modulus (more correctly a lower limit). Our results indicate that for Montserrat the bulk modulus of the reservoir is quite low (implying a few volume percent of gaseous phases); for Hekla the modulus is very high implying a gas poor (free?) reservoir.
V42A-07
Numerical Models of Dike Propagation Near the Surface
We have modelled dike propagation using two different hydrofracture models and a discrete element model. One of the hydrofracture models was driven by a compressible fluid; other magmas were incompressible. With an incompressible hydrofracture code (NPHF2D), the crack tip accelerates markedly as the surface is approached and the open cavity between the crack tip and the magma front may be hundreds of meters long. This is consistent with the several hours lapse between the appearance of a surface crack at Paricutin and continuous eruptions. By concatenating incompressible results, we show the surface instability is enhanced if the magma is expanding. EMSA, a near-surface, compressible fluid hydrofracture code, has magma density in equilibrium with pressure. It shows the near-surface acceleration of the tip is even greater than predicted with NPHF2D. EMSA also models vertical variations in lateral confining stress. This feature shows that an increase in lateral confining stresses due to radioactive heat in the first millennium after waste is placed in the proposed Yucca Mountain nuclear waste repository in Nevada,USA, will not prevent a dike from penetrating the repository; neither will it cause a sill below the repository. This was confirmed by a UDEC model with a mass of randomly oriented polygons filling the upper part of the model. This allowed the magma to seek its own path uninfluenced by preferred orientation of cracks. This produced the same result regarding thermal stresses at Yucca Mountain as EMSA. It also showed the dike had a tendency to bifurcate in the upper 100 m. This agrees with field observations of exhumed Miocene basaltic sites in southern Nevada, USA, where it is reported that dikes less than 5 meters wide at depths of 250 m began to bifurcate and splay into a zone 15 to 25 meters wide at depths about 100 meters and opened into conduits as wide as 100 m in diameter at the surface. We also used UDEC to confirm an approximate analytic solution showing that magma can be diverted more easily into a fault if the fault angle is steep or if the intersection is shallow. Together, these results suggest a complex variety of phenomena may be expected as a dike approaches the surface and confining stresses diminish.
V42A-08
The Stability of Dike-fed Eruptions Based on Magma Transport Modeling
Finite volume calculations of magma flow in dikes are presented that tie observational markers available during effusive eruptions to the stability of the event. We use a new model for heat and mass transfer of a melt with a temperature dependent viscosity and the potential to undergo phase change. Magma driving pressures are coupled with dike geometries that satisfy simple elastic considerations for the host rock. Different regimes are identified that dictate a volcanic system's tendency towards eruption cessation due to thermal close off of the dike, steady eruption, or growth of the eruption. These fields are characterized as a function of dike size, magma overpressure, and variations in magma flux. We verify our model by looking at the applicability of the identified trends to modern systems.