Active Crater Research Articles

Groundwater transport of crater-lake brine at Poa´s Volcano, Costa Rica

Poa´s Volcano is an active stratovolcano in Costa Rica that has a lake in its active crater. The crater lake has high temperatures (50–90 °C), high acidity (pH ≈ 0.0), and a high dissolved-solids content (100 g/kg). The volcano has numerous freshwater springs on its flanks, but a few on the northwestern flank are highly acidic (pH = 1.6–2.5) and have high dissolved-solids concentrations (2–22 g/kg). This study analyzes the regional groundwater system at Poa´s and demonstrates the likelihood that the water discharging from the acidic springs in the Rio Agrio watershed originates at the acidic crater lake. Both heat and solute transport are analyzed on a regional scale through numerical simulations using the HST3D finite-difference model, which solves the coupled equations for fluid flow, heat transport, and solute transport. The code allows fluid viscosity and density to be functions of both temperature and solute concentration. The simulations use estimates for recharge to the mountain and a range of values and various distributions of permeability and porosity. Several sensitivity analyses are performed to test how the uncertainty in many of the model parameters affects the simulation results. These uncertainties yield an estimated range of travel times from the crater lake to the Rio Agrio springs of 1–30 years, which is in close agreement with the results of tritium analyses of the springs. Calculated groundwater fluxes into and out of the crater lake are both about several hundred kg/s. These fluxes must be accounted for in water budgets of the crater lake.

Seismic assessment of phreatic-explosion hazard at ?La Fossa? Volcano (Island of Vulcano, Italy)

Active volcanic areas are sometimes affected by phreatic eruptions, which are explosions due to the outbreak of a confined pocket of steam and gas without the direct involvement of molten magma. Eruptive activity at ‘La Fossa’ Volcano, Island of Vulcano (Italy), typically starts with a phreatic explosion, continues as phreatomagmatic and turns into a late magmatic stage, depending on the reduction in the efficiency of magma-water interaction. The present risk is mainly related to a village located at the very foot of the active volcanic cone, which in summer seasons is inhabited by more than 10,000 people. Because the last eruptive episode occurred in 1888–1890, when no local instrumental control was performed, the scientific community does not have any information about the seismic phenomena which could precede the initial phreatic blast. Inferences from similar monitored volcanoes, which awakened in the last few years after more or less long quiescence, are therefore made to tentatively depict possible pre-eruptive seismic scenarios and also to evaluate the most appropriate surveillance and alert criteria to adopt. Appreciable signs of volcanic unrest have been detected in recent years, mainly related to modifications in the physical and chemical features of fumarolic fluids discharged at the active crater. Following the classification of local seismic shocks, the recording oflong-period events (peak frequency ≤2 Hz) should be taken into account as a realistic seismic warning of volcanic hazard enhancement. A likely short-term seismic precursor may be finally given by the appearance of the so-calledbanded tremor, a seismic signal likely produced by intermittent hydrothermal boiling accompanying the hydraulic fracturing of rocks at the top of a pressurized aquifer. Real-time recognition of tremor can be therefore very useful to avoid, at least, injuries to volcanologists and visitors working or staying on the most dangerous sectors of the volcano.

Hollow natrocarbonatite lapilli from the 1992 eruption of Oldoinyo Lengai, Tanzania

A new style of layered tuff deposit in the SW of Oldoinyo Lengai’s active crater consists of bedded spherical lapilli 8–12 mm in diameter with hollow cores and vesicular rims. Lapilli of this type have not previously been reported from extrusive carbonatites and similar deposits are apparently unreported amongst siliceous pyroclastics. Some lapilli are solid and are either vesicular throughout or contain vitreous crystal cores. A mechanism for the eruption and a sequential model for the formation of hollow lapilli are suggested.

Oxygen, hydrogen, and sulfur isotope systematics of the crater lake system of Poas Volcano, Costa Rica.

Oxygen, hydrogen, and sulfur isotope data for fluids and minerals associated with the crater lake of Poas Volcano, Costa Rica, are interpreted in the context of the chemical and hydrologic structure of the volcano. Oxygen and hydrogen isotope data were obtained for rain, spring, and river water, low-temperature fumarole condensates, and acid brines collected from the hot crater lake before its disappearance in April 1989. Flank river and spring waters whose solute compositions have been modified by volcanic and hydrothermal activity have, with one exception, isotopic compositions similar to local meteoric water. Acid chloride-sulfate brines of the summit crater lake are extremely enriched in 18O with respect to local meteoric water; in the most enriched brines 18O shifts are greater than 20‰. The 18O shift is related to a kinetic isotope effect associated with the intense evaporation at the surface of the lake. These same brines exhibit only minimal shifts in their D/H ratios. The apparent lack of deuterium fractionation in the brines is attributed to an increase in the flux of isotopically light steam into the crater lake and/or a decrease in the deuterium fractionation factor for evaporation that occurs at the surface of the lake. The decrease in deuterium fractionation is correlated with large increases in lake-brine acidity and dissolved solids concentration that preceded the disappearance of the lake. Sulfur isotope data are presented for H2S and SO2 gas collected from low temperature fumaroles; dissolved sulfate in spring, river, and crater lake waters; and native sulfur and gypsum found in the acid lake and active crater area. ΔSO2-H2S for low temperature gases is approximately 24‰ indicating an equilibration temperature of 165°C. ΔSO2-H2S for low temperature H2S and lake brine sulfate is approximately 23‰, all indicating subsurface equilibration occurred at 265°C. The H2S and native sulfur are both highly depleted in 34S (δ34S = –8 to –11‰). δ34S values of 34S-depleted H2S and 34S-enriched sulfate in lake brine are produced by disproportionation of SO2 released by the shallow magma body. Native sulfur is formed by the oxidation of 34S-depleted H2S by non-sulfur-bearing oxidants such as atmospheric oxygen and ferric iron. Mass-balance calculations indicate that sulfitolysis of polythionic acids could also result in the deposition of significant quantities of native sulfur. Implications of the isotopic composition of present-day fluids observed at Poas Volcano with respect to the isotope systematics of acid-sulfate ore deposits are considered.

B-type volcanic earthquakes at White Island volcano, New Zealand

B-type volcanic earthquakes at White Island have almost all their energy in the band 0.5–4 Hz, around a dominant spectral peak of about 2 Hz. A high degree of similarity in the waveforms suggests that the earthquakes originate from a volume of about 100 m 3, and that the process that generates them is repetitive. At a distance of 1 km from the active crater the particle motion of B-type events suggests an initial P-wave, followed 0.5 seconds later by a horizontally polarised S-wave and then a wave of unknown type. The exact mechanism for B-type events is unknown, however, the absence of higher harmonics implies they cannot be caused by the resonance of gas or magma within a pipe-like structure, and the P- and S-waves suggest a simple shear-induced rock fracture in a hot ductile medium.

Galeras Volcano: International Workshop and Eruption

Galeras, a 4270‐m high andesitic stratovolcano in southwestern Colombia near the Ecuadorian border (Figure 1), gradually reawoke in 1988 after more than 40 years of dormancy. In 1991, after a request from the Geological Survey of Colombia (INGEOMIAS) and the National Disaster Prevention Office (ONAD), Galeras was named a “Decade Volcano” by the International Association of Volcanology and Chemistry of the Earth's Interior (IAVCEI) as part of the United Nations' International Decade for Natural Disaster Reduction (IDNDR) program. A workshop was held in 1993 to research, monitor, and mitigate the hazards of the volcano.Galeras lies at latitude 1°14′N, longitude 77°22′W (see Figure 2), and its active cone rises about 150 m above the floor of a small summit caldera that is open to the west [Calvache and Williams, 1992]. The active crater is located about 6 km west of Pasto, a city of about 300,000 inhabitants. At least six major Galeras eruptions have been identified during the past 4500 years. These eruptions were mainly vulcanian, with inferred low‐altitude eruption columns (<10 km) that produced small‐volume pyroclastic flow deposits containing a high proportion of nonjuvenile material and lava flow fragments [Calvache and Williams, 1992]. During the last 500 years, eruptions have been characterized by gas and ash emissions, small lava flows, and explosive eruptions producing pyroclastic flows that have traveled up to 15 km from the crater [Calvache, 1990; Cepeda, 1993].

Eruption mechanism as inferred from geomagnetic changes with special attention to the 1989–1990 activity of Aso volcano

Abstract Geomagnetic changes associated with the volcanic activity of Aso volcano were detected with a dense network of continuously recording proton-precession magnetometers during the period from June 1989 to June 1990. Magnetic date clearly indicate that changes in the magnetization within the volcano are most probably caused by temperature changes. This activity can be divided into five stages, which are characterized by magnetization and demagnetization of the volcano. These magnetic changes with durations of a few months are definitely correlated with some typical volcanic events at the crater as well as the volcanic tremor activity. The demagnetization stage appears when the vent is covered by a water pool or the volcanic activity is enhanced. The magnetization stage follows the opening of a vent and several large explosions which make the vent permeable. The source of magnetic changes lies at a depth of about 200 m below the crater rim in the southwestern part of the active crater. Magnetic moments responsible for observed magnetic changes at stages 1, 2 and 4 are 3.4, −5.2 and −2.2 Wbm, respectively. The corresponding source volume is estimated as a single sphere of radius 40–50 m or a spherical shell of 100 m or so. An effective mechanism of rapid heating/cooling exists, which is ascribed to the interaction of groundwater and superheated vapor, i.e., a shallow hydrothermal system. This hydrothermal system driven by the surface cap of the vent, controls every feature of the eruptions at the final outlet of Aso volcano.

Inversion of aeromagnetic data using digital terrain models

Although draped magnetic surveys contain more information about the magnetization of the rocks near the surface of the earth than surveys at constant elevation, allowance for the effects of the terrain is critical for their correct interpretation. A new method for calculating the magnetic effect of the topography from a digital terrain model by integrating analytically in the vertical direction and then numerically in the horizontal plane is presented. This method lends itself to the calculation of anomalies when the magnetization of the rocks varies with position and thus is well suited to the inversion of draped aeromagnetic surveys to obtain the apparent magnetization of the surficial rocks. This inversion is achieved by repeated use of an approximate inverse function in the form of a two‐dimensional (2-D) filter that is applied to gridded data. An example, using draped magnetic data collected over White Island, an active volcanic island of high relief, shows that although the anomaly pattern is dominated by topographic effects, the distribution of near‐surface magnetic bodies can be determined by a joint inversion of the data and the topography. One of the highly magnetized areas of White Island is interestingly in the vicinity of the active crater, with another near the inner wall of the caldera where there are numerous fumaroles. It may be expected that the higher temperatures in these areas would reduce the magnetization. However, it appears that an explanation for the higher magnetization can be found in the stability field of the mineral magnetite.

Mt. Etna: a model for the 1989 eruption

During the 1989 eruption of Mt. Etna, one of the most important in the last 20 years in terms of effusion rate, a pair of fractures formed on the slopes of the Southeast Crater cone and propagated in a few days trending ca. northeast and southeast, with large quantities of magma being drained from the northern fracture. In contrast the southern fracture, after a short initial eruptive phase, grew by 5 km in five days without eruptive or seismic activity. When the fracture system crossed the southern scarp of the Valle del Bove wall, some 6 km SE of the active crater, an intense seismic swarm started in the same area. This lasted about four days and consisted of several hundred events clustering in a small focal volume. Although such a sequence of failure events contains elements of compatibility with a shallow dyke intrusion, it did not lead to the expected flank eruption which would have threatened settlements just a few kilometres from the distal end of the fracture. The question arises, therefore, whether or not the fracture system was dyke-related, and if so to what extent. We attempt here to answer such a question by integration and interpretation of the available field observation and geophysical data. Indeed, field observation constraints on the southern fracture system and the typical depth (> 2 km) of the swarm do not contain evidence for shallow dyke emplacement beneath the lowermost part of the SSE fracture. Conversely, the inferred direction and shape of the tremor source, the location and extent of a gravity anomaly observed a few months before the eruption, the near-horizontal migration of the seismic focii during the swarm, and the change with time of related focal mechanisms, lead us to hypothesize that the extreme development of the fracture system at its south was due to a deeper intrusion beneath the southern slope of the volcano.

Modelling of volcanic tremor wave fields

The comparison of the temporal variations of the volcanic tremor for various eruptions shows stable connection between tremor intensity and volcanic activity. We have determined the wave pattern of seismic signals produced by individual explosions in the crater during eruption of the Tolbachik volcano (Kamchatka). Considering an explosion in the crater as the source for formation of the seismic wave field, we determined the relation between the spectrum of the source and volcanic tremor spectra. Theoretical seismograms for the unit source of explosion type in various medium models correlate well with the real volcanic tremor seismograms and explain the formation of the volcanic tremor wave fields in real conditions. Volcanic tremor (frequency range of 0.6–5 Hz) forms as a result of disturbances near the surface in the active crater (explosions, pressure variations at sustained flow, etc.) and consist mostly of surface waves. A comparative estimation of transfer functions for the system “explosion in the crater-volcanic tremor” and for theoretical models with the isolated source of explosion type has been done.

Thermal survey of Mount Etna Volcano from space

Surveys of ground thermal anomalies and the monitoring of their evolution are of great importance in the study of volcanoes. Thermal monitoring techniques could be used in conjunction with classical monitoring tools (i.e. seismological and deformation networks), to give better predictions of the onset of a volcanic event. In order to detect and emphasize small anomalies in the surface temperature of the ground, we have developed a new method based on the joint use of two satellite radiometers: the NOAA‐Advanced Very High Resolution Radiometer (AVHRR) for the quality of its thermal band calibration and the Landsat Thematic Mapper (TM) for its high spatial resolution. This method is applied to the Mount Etna volcano, Sicily, and reveals several thermal anomalies. One anomaly is already known, and is associated with the permanently active craters of the summit zone. The second is a larger area where an eruption occurred one week after the data acquisition (10/23/86). The general trend of the thermal anomalies leads to recognition of a large, semi‐circular intrusive zone corresponding to the borders of the well‐known Valle del Bove.

Isotope composition of rain water, well water and fumarole steam on the island of Vulcano, and their implications for volcanic surveillance

Abstract Water samples from eight wells in the Vulcano Porto area and fumarole steam from the crater of Vulcano (Southern Italy) were collected at monthly intervals over a two-year period from February 1987 to January 1989. Samples of bulk precipitation were also collected in three points at Vulcano Porto and Vulcano Piano (island of Vulcano) and Castroreale (Tyrrhenian coast of Sicily), again at monthly intervals. The weighted mean isotopic composition of rain water on Vulcano is δ D =−32‰ , and δ D 18 O =−6.4‰ . Results for well waters tend to support the existence of at least two water bodies: one of purely meteoric water and the other of brackish thermal water. An intermediate water body, probably the result of mixing of the other two, was also recognized. The isotope composition of fumarolic steam from the crater suggests the mixed contribution of both sea and rain waters, although mixing is not the only process occurring. The existence of a deep marine-brine body is inferred, characterized by δD and δ18O values more positive than those of Mediterranean sea water. A close relationship, indicating common source effects, is shown by temporal isotopic variations at some wells and the fumarolic steam, wich may be used for volcano survey at distance from the active crater.

Volcanic hazard assessment of Guagua Pichincha (Ecuador) based on past behaviour and numerical models

Abstract The active crater of Guagua Pichincha volcano is located only about 10 km west of Quito, the capital city of Ecuador. Archeological and historical records confirm that, in the last few millennia, human communities were repeatedly affected by volcanic eruptions in this region. Tephrostratigraphic studies and 14 C datings indicate that the eruptive history of Pichincha in the last 40,000 years has been characterized mostly be explosive eruptions with an average frequency of one event every few centuries. These eruptions produced relatively small volumes of dacitic pumice fallout and pyroclastic surge and flow. Eruptions of the last 10,000 years has also included lava dome growth and lateral blasts by dome explosions. Based upon the history of the volcano, the explosive eruptions of 970 and 1660 A.D. were selected as those representing, in the case of reactivation, the maximum phenomena to be expected. The eruption behaviour (ash fallout and pyroclastic surge) has been simulated by physical-numerical models on digitized topographic map, and the results have been used to evaluate the harzardous areas.

Petrogenesis of andesites and dacites of White Island volcano, Bay of Plenty, New Zealand, in the light of new geochemical and isotopic data

Abstract White Island volcano comprises three main lava types: (1) silicic andesite, forming Western Cone and a tholoid at Troup Head, (2) dacite forming the Central Cone, and (3) mafic‐silicic andesite erupted in March 1977 from the currently active crater. All lavas are calc‐alkalic and medium‐K orogenic types. The older andesites of Western Cone and Troup Head were probably formed from chemically dissimilar parental magmas by processes of assimilation and fractional crystallisation (AFC). Andesite blocks and bombs ejected during phreatomagmatic activity in March 1977 are geo‐chemically primitive having high Mg‐numbers, high Cr and Ni contents, and containing forsteritic olivine. They cannot be derived from magmatic compositions similar to known basaltic lavas of Taupo Volcanic Zone, and it is possible that the blocks are hybrid magmas resulting from mixing of a high‐Mg basalt parent and Central Cone dacite. The bombs appear to be fractionated derivatives. Central Cone dacites may also be AFC derivative...

Fractal properties of tremor and gas piston events observed at Kilauea Volcano, Hawaii

We study the fractal properties of shallow volcanic tremor and gas piston events associated with magma degassing at Kilauea Volcano, Hawaii, using data from two dense short‐baseline arrays of seismographs deployed near the active crater of Puu Oo on the east rift of the volcano. We found an upper bound on the fractal dimension of a strange attractor common to phase portraits of both tremor and gas piston activities in the range of 3.1–4.5 with a mean of 3.75 over a scale length of the wave field of the order of 100 m. The dimension of the attractor fluctuates over the course of tremor and gas piston episodes within a range comparable to the spatial variations of fractal dimensions found in the wave field. The existence of a categorically stable attractor characterizing both types of activities strongly suggests that the excitation mechanism of tremor is similar to that of gas piston events, which in turn are correlated with visual observations at the volcanic vent. The low value derived for the dimension of the attractor in phase space points to significant self‐organization in the process of generation of tremor and offers general constraints on the dimensionality of attractors derived from models of acoustic emission associated with magma flow, vesiculation, and degassing.

Soil gas emanations as precursory indicators of volcanic eruptions

Field measurements conducted on several active volcanoes in Italy, the Lesser Antilles, and Indonesia demonstrate the common Occurrence of diffuse soil gas emanations from the volcanic piles, at distances from active craters or fumarolic zones. These emanations consist essentially of carbon dioxide and rare gases and their genetic link with crater fumaroles and/or magma degassing at depth can be verified both chemically and isotopically. We emphasize here the potential use of these fluids for continuous volcano monitoring and eruption forecasting.

Diffuse volcanic emissions of carbon dioxide from Vulcano Island, Italy

RECENT investigations on Mount Etna (Sicily)(1-3) have revealed that volcanoes may release abundant carbon dioxide not only from their active craters, but also from their flanks, as diffuse soil emanations. Here we present analyses of soil gases and air in water wells on Vulcano Island which provide further evidence of such lateral degassing. Nearly pure carbon dioxide, enriched in helium and radon, escapes from the slopes of the Fossa active cone, adding a total output of 30 tonnes per day to the fumarolic crater discharge ( 180 tonnes CO(2) per day). This emanation has similar He/CO(2) and (13)C/(12)C ratios to those of the crater fumaroles (300%ndash;500 degrees C) and therefore a similar volcanic origin. Gases rich in carbon dioxide also escape at sea level along the isthmus between the Fossa and Vulcanello volcanic cones, but their depletion in both He and (13)C suggests a distinct source. Diffuse volcanic gas emanations, once their genetic link with central fumarole degassing has been demonstrated, can be used for continuous volcano monitoring, at safe distances from active craters. Such monitoring has been initiated at Vulcano, where soil and well emanations of nearly pure CO(2) themselves represent a threat to the local population.

Observations on changes in the active crater of Ol Doinyo Lengai from 1960 to 1988

Drastic changes in the morphology of the summit crater of Ol Doinyo Lengai volcano have occurred over the last 3 decades. The wide flat-floored crater that persisted between 1960 and mid 1966 was filled in with volcanic ash during the explosive eruption of August–September 1966. Soon after, a deep narrow pit crater was formed, probably by collapse along circular fractures. This remained essentially unchanged until early 1983, when eruptive activity was resumed in its floor. Since then, frequent episodes of activity have occurred, with the eruption of highly fluid lava that has built up the level of the crater floor. The crater in 1988 was wide and relatively shallow, while small scale activity continued to take place.

Monitoring of volcanic eruptions at Yugama crater lake by aqueous sulfur oxyanions

Abstract Variations of polythionates (sulfane disulfonates) and sulfate in the Yugama crater lake, Japan, have been monitored for more than 25 years. Just before the 1982 eruption at the crater lake, polythionate ions decreased to zero from the normal level of about 2000 ppm and sulfate ions increased from 2500 to 5000 ppm. During the 1982 eruption polythionate and sulfate ions varied inversely in concentration and the variations exactly coincided with the frequency of volcanic earthquakes and subsequent explosions. These observations are interpreted in terms of aqueous reactions of fumarolic SO2-H2S gases, resulting in precipitation of alunite. The behavior of polythionate and sulfate ions strongly suggests that they are useful indicator for prediction of impending volcanic hazards from active crater lakes.

The primitive volcano of Mount Pelée: its construction and partial destruction by flank collapse

Abstract The geology of Mount Pelee, the active volcano of Martinique (Lesser Antilles), has been actively studied during the past decade. This paper reviews the current knowledge about the oldest, least well-known part of the volcano (here called Paleo-Pelee) and the relations between Paleo-Pelee and the Mont Conil volcanic complex. Paleo-Pelee consists mostly of volcanic breccias with a few lava flows. It differs in this respect from neighbouring active volcanoes such as the Grande Decouverte (Soufriere) in Guadeloupe and the Soufriere in St. Vincent, the oldest products of which are mainly lava flows. Paleo-Pelee initially formed a regular cone which was comparable in size with the present edifice. It is now largely eroded and its products form prominent radial ridges on the western flank of Mount Pelee. These are buried under younger deposits on the eastern flank. The uniformly andesitic breccias of Paleo-Pelee are, for the most part, brecciated lava flows. It is inferred that the activity of Mount Pelee was less explosive at this stage than during following periods (Neo-Pelee). The Paleo-Pelee formations overlap those of Mont Conil. The geology of Mont Conil is poorly known. However, its lowest exposed products are submarine andesitic breccias cut by many dikes. This contrasts with Paleo-Pelee where all breccias are of subaerial origin. Pleistocene limestones are interbedded within the Mont Conil breccias between 120 and 150 m above sea level. Though Mont Conil seems to be closely linked to Mount Pelee (on the basis of available petrological data), a major uplift must have occurred sometime during or after the construction of Mont Conil and forms a recognizable geological boundary between the two volcanic systems. The Paleo-Pelee edifice was partially destroyed by a flank-collapse event that occurred on the southwest flank during the early stage (> 40,000-20,000 y.B.P.) of Neo-Pelee. This event is evidenced by a large (6 × 2.5 km), horseshoe-shaped structure which is limited by well-preserved morphological scarps on land and by a 300-m offshore scarp, as shown by Seabeam bathymetry. Paleo-Pelee breccias and lava flows are truncated by the boundary of the collapse structure. Lava domes occupied the upslope part of the structure at a later time. These domes are intersected by the present crater (“Etang Sec”), which has probably been the active vent for at least 2000 years. The active crater is located at the intersection between the flank-failure depression and an older caldera formed at the Paleo-Pelee stage but probably enlarged later. The flank-failure structure appears as a major structure of the volcano and creates a strong structural asymmetry in the edifice. This fact should be considered when dealing with such various aspects as the hydrogeology of the volcano, the structural control of past and future eruptions, and the volcanic hazards.