Report on Galeras (Colombia) — March 1989
Scientific Event Alert Network Bulletin, vol. 14, no. 3 (March 1989)
Managing Editor: Lindsay McClelland.
Galeras (Colombia) Small phreatic explosions; seismicity
Please cite this report as:
Global Volcanism Program, 1989. Report on Galeras (Colombia) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 14:3. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198903-351080.
1.22°N, 77.37°W; summit elev. 4276 m
All times are local (unless otherwise noted)
Most of the following is from a draft report by Hector Cepeda and others.
Eruptive activity. Increased fumarolic activity accompanied by minor ash emission and seismicity began in mid-February. A summit-area explosion was felt on 19 February before 1700. Weather over the cone cleared at 1700, and strong fumarolic activity from one of the secondary craters was visible from the army/police communications station on the edge of the caldera. The height of the column varied between 100 and 200 m. This information triggered the mobilization of the Comité Regional de Emergencia de Nariño (CREN) which requested help from higher authorities.
During a 25 February summit climb, a column of water vapor, sulfur compounds, and sulfuric acid was being vigorously emitted from a vent (named El Pinta) immediately E of the main crater. The area around the active crater was covered by recently ejected pyroclastic material, consisting of very fine, gray, wet ash and lithic fragments of centimeters (occasionally to tens of centimeters) in size. No fresh magma was found in the tephra.
Before 19 February, El Pinta crater had shown no fumarolic activity. After the onset of activity, the crater was elongate N-S, with horizontal dimensions of 40 x 25 m and a depth of >30 m. A chain of small fumaroles within the crater emitted gases at high pressure and had deposited sulfur. Resonance effects produced a sound like a jet aircraft. Fumarolic activity at other vents had also increased substantially.
As activity continued, ejecta from El Pinta were dispersed nearly 1 km to the WSW. Maximum tephra thicknesses were ~5 cm within 5 m of the crater rim (apparently by the 20 March report date). Impacts of the largest blocks left visible marks. The direction in which pyroclastics had been ejected by the phreatic activity and the behavior of the gas jet suggested that the conduit of El Pinta was inclined to the E.
On 12 March, the staff of the nearby police post reported that another old secondary crater (El Peladito) had reopened on the edge of the main crater, 50 m S of El Pinta. This crater did not show any fumarolic activity. A pre-1964 collapse of the wall that formerly separated it from the main crater left it perched on the main crater's inner wall. The bottom of El Peladito was not visible, but was more than 50 m below the main crater's rim.
Ash was emitted for 4 hours on 27 March, lightly dusting Pasto, at the E foot of the volcano. A tremor-like signal accompanied the ash emission, without a precursory swarm or eruption earthquakes. In response, INGEOMINAS elevated the alert level from "watchfulness" to "white." A strong swarm on 5 April at 1000 prompted an increase to "yellow" alert [see also 14:04], which had not changed as of 12 April. The Nariño seismograph, 5 km from the crater, was saturated by 6-7 of the earthquakes, but no ash was emitted.
Two small lakes were present in the summit complex, one (La Trucha) between the base of the active cone and the smaller caldera, the second (Rafa) in the bottom of the main crater. Levels of these lakes were higher than usual because of the recent unusually heavy rains, and geologists suggested that the additional water in the volcano's hydrothermal system may have influenced its activity.
Since 1988, INGEOMINAS and the Univ Nacional (Manizales campus) have monitored several fumaroles at Galeras, measuring H2S, SO2, HCl, CO2, and total S. Only small temperature increases were noted, reaching roughly 10°C at one fumarole (Calvache) on the NNE flank of the active cone, which had a temperature of 100.6°C on 15 March. COSPEC measurements on 19 and 20 March yielded values of only 30 and 40 t/d.
Deformation. Baseline deformation data was limited to routine geodetic work by the Instituto Geográfico Agustín Codazzi. A series of proposed level lines and dry tilt stations have been identified, and initial measurements were made 2 March on the upper E flank. Remeasurements since then have identified no changes that exceeded instrumental error.
Seismicity. A WWSSN seismic station ~5 km SE of the crater has been collecting data for ~15 years. At the request of the CREN and the Oficina Nacional para la Atención de Desastres, personnel from the Observatorio Vulcanológico de Colombia installed three additional seismic stations on the volcano on 28 February. Early February seismicity had consisted of low-frequency, small magnitude (M < 0.5) events, at rates of no more than 10/day. The newly installed seismic net recorded a gradual increase in the number of events/day, mostly B-types (see figure 1). Energy release generally remained low, with most events of M<1, although high-frequency shocks on 17 and 20 March reached magnitudes of 1.5 and 2.2.
A small number of A-type events were recorded. Very shallow focal depths were indicated by the small separations between P- and S-wave arrivals at stations (Crater 1 and 2) 0.5 and 1 km from the active vent, and attenuation was strong (at Nariño 1) 5.5 km away. Many of the shallow high-frequency events were preceded by small bursts of signals characteristic of gas emission. A very few (4 or 5) high-frequency events from a different source were detected, with S-P of 1.5 seconds at Crater 1 station. These decreased after 8 March, and seismicity was dominated by low-frequency events. Long-period events were a little less numerous but of about the same magnitude as the high-frequency shocks. These were also believed to be very shallow and were greatly attenuated at Nariño 1. No felt shocks were reported until 20 March at 0033 when persons in the Telecom communications station 1 km from the crater noticed an A-type event.
A signal similar to spasmodic tremor began to be observed on 8 February, and the next day there were a series of events that looked like long-period seismicity. Tremor was clearer after 8 March, in brief pulses that had a more spasmodic than harmonic character. Energy was relatively low, although amplitudes occasionally reached moderate to high levels near the source of the seismicity (on stations Crater 1 and 2). The predominant frequency was 4-5 Hz, with some 2-Hz components. The most conspicuous pulse occurred on 11 March at 1803, possibly related to the reopening of El Peladito crater, reported 12 March (see above). Low-frequency events and the short pulses of tremor could be correlated with small changes in fumarolic activity.
Geologic Background. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large horseshoe-shaped caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate historical eruptions since the time of the Spanish conquistadors.
Information Contacts: H. Cepeda and B. Pulgarin, INGEOMINAS, Popayán; F. Muñoz, A. Acevedo, F. Gil, A. Nieto, M. Calvache, H. Mora, and C. Carvajal, INGEOMINAS, Manizales; A. Londoño, Univ Nacional, Manizales; I. Mejía, INGEOMINAS, Medellín; N. Banks, J. Ewert, and M. Doukas, USGS.