AB - ABSTRACT: Diffuse degassing at Galeras volcano, Colombia, was studied during three consecutive field seasons from 1993 to 1995. Measurements of (super 222) Rn and CO (sub 2) were made at 30 stations which were distributed on the volcano and on regional faults intersecting the edifice. Time series data show a decline of radon soil gas of up to 50% prior to a M 2.8 earthquake on 12 August 1993 at stations located near the epicenter and on the volcano near the location of earthquake swarms which occurred in April 1993, November-December 1993 and March 1995. The onset of volcanic seismic activity ("tornillos") on 9 August 1994 was preceded by anomalous soil gas increases at six stations located on the flanks of the volcano. On the southwestern flank, radon increased from 51 to 130 pCi/l between 7 and 14 August, while on the northern flank, radon concentrations began to increase 19 days before the appearance of tornillos. In general, stations close to the crater showed the largest radon increases. Soil gas distributions and carbon isotope data suggest that diffuse degassing on the volcano is structurally controlled and that the abundance of CO (sub 2) in soil gas on the edifice cannot be taken as an indicator for the presence of magmatic gases. Radon soil gas concentrations and the (super 222) Rn emanating (super 226) Ra concentration increase near faults, whereas CO (sub 2) concentrations are more variable but commonly are higher on the volcano than near faults. delta (super 13) C values in soil CO (sub 2) vary between -8.5 and -23.2 per mil, with delta (super 13) C values more enriched than -15 per mil found only in the vicinity of faults or sites prone to earthquake swarms. This suggests a magmatic origin of CO (sub 2) soil gas only near faults and an almost impermeable edifice in unfractured areas. The observed correlations between seismic activity and soil degassing provide further evidence that soil gas studies, especially when correlated to other methods of volcano surveillance such as seismicity and deformation, may be useful in forecasting volcanic and seismic events.
AB - ABSTRACT: We have measured (super 3) He/ (super 4) He, (super 4) He/ (super 20) Ne and CO (sub 2) / (super 4) He ratios, and delta (super 13) C values of CO (sub 2) in eighteen samples from crater fumaroles of Galeras volcano, Colombia, and seven samples of bubbling gases of the Pandiaco mineral spring located about 8 km east of the volcano from April 1988 to July 1995. The (super 3) He/ (super 4) He ratios of crater fumaroles, corrected for atmospheric contamination, increased significantly from 5.47 R (sub atm) in December 1989 to 7.93 R (sub atm) in August 1990 and reached a maximum of 8.84 R (sub atm) in May 1992. The ratio then decreased gradually until July 1995. Overall variation of helium isotopes appears to be related to the current activity of the volcano. The (super 3) He/ (super 4) He ratios of Pandiaco mineral spring were constant between April 1988 and November 1991 and increased in April 1993, although experimental error is large. The delta (super 13) C values of CO (sub 2) of crater fumaroles show negative correlation to the helium isotopes, while those of Pandiaco are relatively constant. Based on the delta (super 13) C values and CO (sub 2) / (super 3) He ratios, it is possible to estimate fractions of upper mantle carbon in these samples. Calculated mantle fractions are 3-10% in crater fumaroles, while those in Pandiaco spring are negligibly small.
AB - ABSTRACT: Galeras fumarole discharges have been collected since its reactivation, in 1988, through December 1995. The gases are dominated by H (sub 2) O, CO (sub 2) , S (as SO (sub 2) and H (sub 2) S) and HCl. The relative proportions of these gases classify them as "magmatic". Thermodynamic equilibrium temperatures of the gases range from 260 to >600 degrees C. The relative abundance of inert gases, N (sub 2) , Ar and He, can be used as "tracers" to identify the source of the fumarole discharges. At Galeras the majority of the samples have a composition characteristic of gases originating from arc-related magmas, with relatively high N (sub 2) contents and minor He and Ar. During 1993, the year of frequent eruptions, the gas composition changed to basaltic or "mantle-derived" gases, with significantly higher He contents. This is interpreted to be the result of injection of volatiles from a basaltic magma body at depth prior to and during the increased eruptive activity of 1993. The delta (super 13) C values for CO (sub 2) in fumarole discharges are typical of andesitic volcanoes and may indicate addition of MORB-derived CO (sub 2) . The delta (super 15) N values for N (sub 2) may indicate significant contribution of N (sub 2) from marine sediments and only minor contribution of MORB-derived N (sub 2) . The delta D and delta (super 18) O values of the discharging steam lie on a mixing trend between the isotopic composition of "arc-related" magmatic water and (super 18) O-shifted meteoric water. The most magmatic discharges have delta D values of -30 to -35 per mil; while the most meteoric discharges have values of -70 to -75 per mil, similar to Galeras thermal spring waters. Galeras thermal water discharges consist of acid sulfate and bicarbonate waters. S/Cl ratios in the acid sulfate waters are similar to fumarole ratios, suggesting direct absorption of magmatic gases into shallow ground waters. This is supported by the essentially meteoric delta D and delta (super 18) O values of the discharges and by elevated (super 3) He/ (super 4) He ratios of thermal spring waters. The absorption of acid S- and Cl-rich gases yield acid waters which are capable of dissolving rocks. The thermal waters, however, are far from equilibrium with Galeras lavas and pyroclastic rocks, providing evidence of the immaturity of the Galeras hydrothermal system. The SO (sub 4) and Cl content, as well as the O and H isotopic composition of Galeras thermal springs vary with the activity of the volcano. The 7-year sampling program at Galeras revealed intriguing results concerning the activity of Galeras, its magmatic-hydrothermal system and the origin of the volatiles. Despite decreasing outlet temperatures since 1992, deep temperatures remain high, implying continued unrest in the Galeras magmatic system.
AB - ABSTRACT: Characterization of acid gases discharged by Galeras Volcano have allowed us to define a clear change in the pattern of degassing during 1989 through 1994, probably associated with pressure buildup that ended with the destruction of the dome by an explosive eruption on July 16, 1992. Using Japanese boxes for sampling, it was possible to establish characteristic discharge (absorption) levels for the period of study and to conclude that the absolute discharge of acid gases was declining at least until the middle of 1991. A staggered temporal response in gas compositions, particularly CO (sub 2) and St, for the different fumaroles suggests at least three possibilities: (1) magma resurgence, which would increase relative CO (sub 2) discharge as a result of new gas contribution and its relatively low solubility in magma; (2) solidification and/or crystallization of magma, first in a zone of higher thermal gradient where there is a greater interaction with the hydrothermal system (Calvache fumarole), and second toward the volcanic-magmatic fumaroles (Deformes and Besolima); and (3) contribution of the hydrothermal system, produced by water saturation and/or temperature increase that would reduce CO (sub 2) solubility in water. The similar behavior of the Calvache fumarole and Japanese Box 3, as well as the existence of La Trucha lake in the northeastern moat between the active cone and the amphitheater margin, help us to define the location of the principal hydrothermal system. The abrupt change toward a hydrothermal component in Deformes fumaroles in August 1990, and the variations in chloride concentrations and other cation concentrations in condensate samples, allow us to observe very large changes over short periods in the composition of the fluid phase of the volcanic system. This suggests that the density of sampling should be increased, together with the development of alternative techniques that could reduce risks and augment the resolution in the observation of the chemical changes. On the other hand, the dependence of gas absorption (e.g., CO (sub 2) ) on the sampling period, seen at the Japanese boxes, suggests that fixed sampling intervals would improve this method by reduction of sampling variables.
AB - ABSTRACT: Galeras volcano has emitted a minimum of 1.9 Tg SO (sub 2) during the past seven-year period of re-activation from 1989 to 1995. Emissions from the volcano are almost exclusively by passive degassing and represent at least 1.4% of the annual volcanic output of SO (sub 2) . The volcano was characterized by high SO (sub 2) flux during 1989-1990, frequently exceeding 2000 metric tons/day. Fluxes decreased to less than 1700 tons/day in October 1990. A further decrease to less than 1000 tons/day occurred in late 1991 after emplacement of a lava dome. Currently, SO (sub 2) fluxes are less than 500 tons/day. The general decrease of the flux with time is interpreted as: (1) progressive degassing of a single batch of magma; and (2) obstruction of the conduit by emplacement of the dome. During 1992-1993, six explosive eruptions occurred which destroyed the dome and excavated the crater. Three of the largest eruptions were immediately followed by intense long-period seismicity (up to 600 events/day) and by comparatively large SO (sub 2) fluxes. This high seismicity and SO (sub 2) flux are interpreted as rapid degassing of partially solidified magma that had been unroofed by the eruptions. Although the volcano is currently in a comparatively degassed state, the six recent eruptions demonstrate that there is still residual magmatic gas which is unable to be released freely. This can lead to pressurization, resulting in short, periodic eruptions or gas emissions that rapidly dissipate the pressure buildup.
AB - ABSTRACT: Events with slowly decaying coda waves are observed in active andesitic volcanoes associated mainly with vulcanian-type eruptions or large gas emissions. However, these signals are also recorded in some quiescent volcanoes. These unusual signals are considered to be related to magmatic activity and generally occur beneath the active crater. The signals have been observed to be a short-term precursor (Galeras volcano, Colombia, 1992-1993; Asama-yama volcano, Japan, 1983), after eruption (Tokachi-dake volcano, Japan, 1989), during seismic swarms (Meakan-dake volcano, Japan, 1982) and during quiescence (Purace volcano, Colombia, 1994-1995; Tarumai volcano, Japan, 1970-1971, 1975). Spectral analysis reveals common characteristics for this type of signal. The spectrum is characterized by one or several sharp frequency peaks. Fundamental frequencies are not affected by epicentral distance, azimuth or travel time, indicating a source effect. The damping coefficient for coda waves ranges from 0.002 to 0.02 and is related to large values of the Quality factor (Q (sub c) ) ranging from 250 to 25, respectively. These parameters may be the result of large amounts of gas bubbles in the magma body.
AB - ABSTRACT: Unusual low-frequency seismic events, called "tornillos" ("screws") at the Observatorio Vulcanologico y Sismologico de Pasto (OVSP), have been observed at Galeras volcano during 1992-1993. Of six eruptions that occurred between July 1992 and June 1993, five were preceded by episodes of tornillo signals. These signals are characterized by (1) a waveform having a homogeneous distribution of frequencies, (2) a long coda lasting up to several minutes, (3) a small amplitude compared to the duration, and (4) a slow decay of the coda. The tornillo signals have been grouped into thirteen principal forms. The 1992-1993 tornillos showed four main periods of occurrence, each of which was followed by an eruption. Episode I occurred from 11 to 16 July 1992 and consisted of nine events. Episode II lasted from 23 December 1992 to 14 January 1993, with twenty events. Episode III was observed from 13 February to 23 March 1993, with 74 events. Episode IV lasted from 10 April to 7 June 1993, with 109 events. The seismic activity at Galeras in April-May 1993 was characterized mainly by the occurrence of tornillo signals. The behavior of these signals was similar to that before previous episodes ending with eruptions; these observations permitted us to forecast the 7 June 1993 eruption several days to weeks beforehand. The occurrence of tornillos is the most important criterion for determining the probability of an eruption at Galeras in the current period of reactivation. The maximum daily number of tornillos and the longest duration of individual events occur near the end of each episode. Within an individual episode, the durations of single events increase progressively near to the point of eruption. A positive correlation is observed between the total number of pre-eruptive tornillo signals during an episode and the volume of material ejected by the eruption. These observations may suggest that the presence of tornillos is an indication of variations in the physical conditions between the fluid and the surrounding solid material within the volcano, and they constitute an important tool for forecasting future eruptions at Galeras.