Logo link to homepage

Report on Galeras (Colombia) — April 1991

Bulletin of the Global Volcanism Network, vol. 16, no. 4 (April 1991)
Managing Editor: Lindsay McClelland.

Galeras (Colombia) Frequent ash emission and seismicity

Please cite this report as:

Global Volcanism Program, 1991. Report on Galeras (Colombia). In: McClelland, L. (ed.), Bulletin of the Global Volcanism Network, 16:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199104-351080.

Volcano Profile |  Complete Bulletin


Galeras

Colombia

1.22°N, 77.37°W; summit elev. 4276 m

All times are local (unless otherwise noted)


Following the pattern begun in March, activity continued to increase during April, when ash emissions from the main crater and associated seismicity were very frequent (table 5). Fieldwork revealed new fissures and vents on the crater's W wall, increases in the area of incandescence, and slumping of loose material. Analyses of gas samples from Deformes and Besolima fissure fumaroles suggest an increasingly magmatic composition. At Calvache fumarole, the ratio of CO2/SO2 has increased steadily (figure 36), while H2S and HCl have shown no significant variations. Besolima fissure fumarole temperatures continued to decline, from 514°C in March to 468°C on 2 April.

Table 5. Eruptive activity and associated seismicity at Galeras, 1-19 April 1991. Atmospheric conditions prevented direct observations 20-30 April. "Inc" means increased, column heights are in meters, and durations are in seconds.

Date Time Activity Column height Signal Type Signal Duration
01 Apr 1991 0640 Ash emission -- Long-period 34 s
01 Apr 1991 0905 Inc sulfur odor -- Tremor 1800 s
02 Apr 1991 0620 Inc column size 300 m Tremor 159 s
02 Apr 1991 0711 Ash emission 900 m Tremor 275 s
02 Apr 1991 1014 Ash emission -- Tremor 116 s
02 Apr 1991 1029 Ash emission -- Long-period 42 s
03 Apr 1991 0741 Ash emission -- Tremor 89 s
05 Apr 1991 0500 Inc noise -- Tremor 475 s
06 Apr 1991 0002 Inc incandescence -- Tremor 182 s
07 Apr 1991 1757 Ash emission 700 m Long-period 52 s
07 Apr 1991 1823 Ash emission 500 m Tremor 140 s
08 Apr 1991 1717 Ash emission -- Tremor 135 s
09 Apr 1991 1827 Ash emission 400 m Tremor 130 s
10 Apr 1991 0608 Ash emission 1100 m Tremor 89 s
10 Apr 1991 0644 Ash emission 200 m Tremor 71 s
10 Apr 1991 1010 Ash emission 700 m Tremor 230 s
10 Apr 1991 1643 Inc noise -- Tremor 110 s
10 Apr 1991 1820 Ash emission -- Long-period 50 s
10 Apr 1991 1820 Inc noise -- Long-period 61 s
10 Apr 1991 1820 Inc incandescence -- Tremor 165 s
10 Apr 1991 1916 Ash emission -- Long-period 30 s
11 Apr 1991 0320 Ash emission, inc incandescence -- Tremor 170 s
11 Apr 1991 0324 Ash emission -- Long-period 17 s
11 Apr 1991 0324 Inc incandescence -- Long-period 29 s
11 Apr 1991 0605 Ash emission 200 m Long-period 44 s
11 Apr 1991 0611 Ash emission 400 m Long-period 58 s
11 Apr 1991 1508 Ash emission -- Tremor 131 s
11 Apr 1991 1758 Ash emission 1700 m Tremor 120 s
11 Apr 1991 1836 Ash emission 200 m Long-period 26 s
11 Apr 1991 1841 Ash emission 800 m Tremor 115 s
12 Apr 1991 0806 Ash emission -- Tremor 295 s
12 Apr 1991 0826 Ash emission -- Tremor 250 s
12 Apr 1991 0854 Ash emission -- Long-period 46 s
13 Apr 1991 0359 Ash emission -- Tremor 625 s
13 Apr 1991 0555 Inc column size 500 m Tremor 260 s
13 Apr 1991 0622 Inc column size 400 m Long-period 20 s
13 Apr 1991 0658 Ash emission 400 m Long-period 50 s
13 Apr 1991 0958 Ash emission, inc noise -- Tremor 91 s
14 Apr 1991 0632 Ash emission 800 m Tremor 83 s
14 Apr 1991 0735 Ash emission 1100 m Tremor 130 s
14 Apr 1991 0808 Ash emission 700 m Long-period 56 s
14 Apr 1991 0845 Ash emission, explosions, inc sulfur odor 1500 m Tremor 179 s
15 Apr 1991 0757 Ash emission 1500 m Tremor 137 s
15 Apr 1991 1355 Ash emission, explosions -- Long-period; tremor 380 s
15 Apr 1991 1509 Ash emission, explosions -- Tremor 82 s
15 Apr 1991 1921 Ash emission, inc incandescence -- Tremor 130 s
16 Apr 1991 0559 Ash emission -- Tremor 111 s
16 Apr 1991 0711 Ash emission -- Long-period 40 s
16 Apr 1991 0815 Ash emission 800 m Long-period 34 s
16 Apr 1991 0835 Ash emission 1500 m Tremor 600 s
16 Apr 1991 1004 Ash emission 1500 m Tremor 171 s
16 Apr 1991 1107 Ash emission -- Tremor 145 s
17 Apr 1991 0711 Ash emission -- Long-period 47 s
17 Apr 1991 0740 Ash emission -- Long-period 57 s
17 Apr 1991 0752 Ash emission -- Tremor 122 s
17 Apr 1991 1742 Ash emission -- Tremor 205 s
17 Apr 1991 1802 Ash emission -- Tremor 370 s
17 Apr 1991 1948 Ash emission -- Tremor 1500 s
18 Apr 1991 0706 Ash emission -- Tremor 190 s
18 Apr 1991 0918 Ash emission -- Long-period 70 s
19 Apr 1991 0627 Ash emission -- Long-period 21 s
19 Apr 1991 0728 Ash emission -- Tremor 76 s
19 Apr 1991 0855 Ash emission -- Tremor 180 s
Figure (see Caption) Figure 36. Concentration of CO2 (squares) and SO2 (circles) in Calvache fumarole gas at Galeras, April 1988-early April 1991. Courtesy of INGEOMINAS.

A significant increase in high-frequency seismicity was recorded during the second half of April, including swarms of events on the 18th and 29th. The earthquakes (M<=2.9) were mostly located SSW of the crater at 1-5 km depth (figure 37). Long-period seismicity was at high levels, and the daily reduced displacement on 13 April was the highest recorded since monitoring began in February 1989 (figure 38). The amplitudes and durations of tremor pulses fluctuated; deep tremor and low-frequency, modulating tremor were also recorded.

Figure (see Caption) Figure 37. Epicenter map of 36 high-frequency earthquakes at Galeras, April 1991. Courtesy of INGEOMINAS.
Figure (see Caption) Figure 38. Daily reduced displacement of long-period earthquakes at Galeras, April 1991. Courtesy of INGEOMINAS.

The electronic tiltmeter 0.9 km E of the crater (at "Crater" station) showed continued inflation, with 85 and 48 µrad of accumulated tangential and radial inflation, respectively, since September 1990 (figure 39). Three km E of the crater, dry tilt (El Pintado station) showed very low, but consistent inflation. Geologists interpreted the inflation as volcanic deformation or neotectonic tilt along the Buesaco fault.

Figure (see Caption) Figure 39. Tangential (top curve) and radial (bottom curve) deformation 0.9 km E of the crater ("Crater" electronic tiltmeter) at Galeras, May 1990-April 1991. Courtesy of INGEOMINAS.

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: INGEOMINAS-OVP.