Global Volcanism Program | Report on Galeras (Colombia)

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

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 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 37. Epicenter map of 36 high-frequency earthquakes at Galeras, April 1991. Courtesy of INGEOMINAS.

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 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.

Geological Summary. 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 open 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 eruptions since the time of the Spanish conquistadors.

Information Contacts: INGEOMINAS-OVP.