Geologic Background:
Galeras located immediately west of the city of
Pasto, is one of Colombia's most frequently active
volcanoes. The Galeras complex consists of at least 2
calderas, one preglacial, the younger probably less than
5000 years old. Major explosive eruptions since the mid
Holocene have produced widespread tephra deposits and
pyroclastic flows that swept all but the southern flanks.
The active cone, with a crater about 120 m across and 80
m deep, has grown in the center of the younger caldera
(2.2 km in diameter) and on the east edge of the older
(4.8 km in diameter).
Historic Activity:
- Galeras has erupted at least 16 times since 1535 with
many periods of minor explosivity lasting for years.
- A central cone slightly lower than the caldera rim
has been the site of numerous historical eruptions since
the time of the Spanish conquistadors.
- A strong eruption in 1865 ejected a nearly 9-km plume
and a 5-km lava flow. The volcano is breached to the west
in the direction of many small towns.
Recent Activity:
- Last activity had been in 1979.
- Increased fumarolic activity was noted in
mid-February 1989. Eruption of ash from May 4 to May 9,
1989.
- Small ash emissions began in February 1990 and
continued thru March. Small ash emissions occurred on
July 11, 21, 27, August 2 and 15.
- Ash emissions and tremor episodes were frequent in
March 1991 with continued activity through August. The
increased tephra emission, seismicity, and deformation
culminated in the extrusion of a summit-crater lava dome
in mid October. The dome was continuing to grow in to
mid-November.
- Occassional minor ash emission into May 1992. During
June 1992 the northwest portion of the 1991 lava dome
collapsed and explosions and ash emissions occurred from
the collapse area. An explosion on July 16, 1992
destroyed more than 90% of the dome at the bottom of the
crater. Blocks 30 to 40 cm in diameter fell as much as
2.3 km away; and some to 1 m in diameter reached 1.3 km
distance creating impact craters 3 m across and 1 m deep;
fragments 3.5 m across were found 400 m from the crater
rim. Incandescent blocks started forest fires on the NE
flank, 2.3 km from the crater. The eruption column rose
about 4 km. Ash emission on Aug. 11, 1992.
- An explosion on January 14, 1993 killed 9 people
including 6 volcanologists. There were no immediate
precursors in the hours before the eruption. The eruption
lasted 15 minutes. The volcanologists were attending the
Galeras Volcano International Workshop and were on a
fieldtrip gathering data. Additional eruptions occurred
on March 23, April 4, April 13 and June 7th.
- A 4.7 magnitude quake occurred on March 4, 1995 and
killed six people. Earthquake swarms occurred in March
and April.
- Continuing low-level seismic activity in late
1999.
- Two small eruptions in March and April 2000.
- Small eruptions in April and May 2001,
volcano-tectonic earthquakes during July to late
Sept.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 14, No. 2 to V. 14,
No. 11)
- Global Volcanism Network (V. 15, No. 1 to V. 26, No.
10).
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