AB - ABSTRACT: Since its reactivation in 1988 the principal eruptions of Galeras Volcano occurred on May 4-9, 1989, July 16, 1992, and January 14, March 23, April 3, April 14 and June 7, 1993. The initial eruption was a phreatic event which clearly marked a new period of activity. A lava dome was extruded within the main crater in October 1991 and subsequently destroyed in an explosive eruption on July 16, 1992. The eruptions that followed were all vulcanian-type explosions. The seismicity accompanying the emplacement, extrusion, and destruction of the lava dome was dominated by a mix of long-period (LP) events and tremor displaying a variety of waveforms. Repetitive LP events with dominant periods in the range 0.2-1 s were observed in October and November 1991 and visually correlated with short energetic pulses of gas venting through a crack bisecting the dome surface. Each LP event was characterized by a weak precursory signal with dominant periods in the range 0.05-0.1 s lasting roughly 7 s. Using the fluid-driven crack model of Chouet (1988, 1992), we infer that two distinct cracks may have acted as sources for the LP and precursor signals. Spectral analyses of the data yield the following parameters for the LP source: crack length, 240-360 m; crack width, 130-150 m; crack aperture, 0.5-3.4 mm; crack stiffness, 100-500; sound speed of fluid, 880 m/s; and excess pressure, 0.01-0.19 MPa. Similar analyses yield the parameters of the precursor source: crack length, 20-30 m; crack width, 15-25 m; crack aperture, 2.3-8.7 mm; crack stiffness, 5-15; sound speed of fluid, 140 m/s; and excess pressure, 0.06-0.15 MPa. Combined with geologic and thermodynamic constraints obtained from field observations, these seismic parameters suggest a gas-release mechanism in which the episodic collapse of a foam layer trapped at the top of the magma column subjacent to the dome releases a slug of pressurized gas which escapes to the surface while dilating a preexisting system of cracks in the dome structure. Accordingly, the fracture observed on the crystallized dome body is the surface extension of the LP-source crack, where LP activity is induced by the rapid emission and expansion of gas flowing through this conduit. The width and aperture of the crack estimated in the model are in good agreement with the length and aperture of the fracture estimated from visual observations. The source parameters of the precursor signal are suggestive of a nozzle-like conduit connecting the LP-source crack to the underlying magma reservoir. Excitation of this conduit segment is attributed to the rapid emission and acceleration of the frothy fluid resulting from the collapse of the foam layer at the top of the reservoir. The calculated periodicity of foam collapse events is in agreement with the observed average rate of thirteen LP events per hour.
AB - ABSTRACT: During 1989-1992, two electronic tilt meters named Crater and Peladitos, and a short level line named Las Piedras, were used for monitoring Galeras volcano. The two tilt meters detected considerable changes of uplift mainly from June to November 1991. The Crater station showed cumulative values of more than 600 microradians, while the Peladitos station exhibited values of more than 100 microradians. The short level line showed inflationary tilt of more than 20 microradians in 1989. This behavior was related to increases in the daily occurrence of long-period seismic events during July-November 1991 caused by the rise and extrusion of a lava dome, which was subsequently destroyed in July 1992. At the end of 1990, the deformation source was located at a depth of 2-3 km. By December 1991, the source had migrated to less than 500 m in depth. The magma ascent rate was 1-4 m/day during this period. The dome emplaced in the crater had a volume between 300,000 to 400,000 m (super 3) . Using a hydrodynamic model and the tilt changes registered by the Peladitos station, the volume of the magma body is estimated at approximately 2.8X10 (super 6) m (super 3) .
AB - ABSTRACT: The current period of re-activation since 1988 at Galeras volcano, Colombia, has been characterized mainly by the following events: (1) a semi-continuous series of Vulcanian eruptions during 5-9 May 1989; (2) emplacement of an andesitic lava dome at the bottom of the main crater in October-November 1991; (3) six vulcanian eruptions during 1992-1993, the first of which destroyed most of the dome on 16 July 1992; and (4) three volcano-tectonic seismic crises in April 1993, November-December 1993 and March 1995. During much of this seven-year period, several small ash and gas emissions also have taken place. The 4-9 May 1989 eruptions originated from the secondary crater El Pinta and deposited ash, lapilli and blocks in the crater area. The 1992-1993 eruptions originated from the main crater and were associated with obstruction of the conduit by magma from dome emplacement in late 1991, causing overpressurization of the system. For the 1992-1993 eruptions, pre-eruptive seismicity, deformation and SO (sub 2) flux all exhibited very low levels. The eruptions were characterized by their sudden initiation, low intensity (VEI = 1), small eruption columns, and small volumes of erupted material. The source of the volcano-tectonic seismic crises is located approximately 3 km north and northeast of the crater. Some of these events were felt in Pasto and other towns located around the volcano, on one occasion causing loss of life, injuries and damage to buildings.
AB - ABSTRACT: Galeras is an andesitic stratovolcano whose activity is generally characterized by vulcanian eruptions. Based on (super 14) C dating, the active cone is considered to be 5000 years old. During that time, six major periods of eruptive activity have been identified, together with 45 individual events of pyroclastic flows, tephra falls, lava flows and lahars, excluding those reported in historic time (<500 years) and those which have occurred since the first signs of the current activity (1988). The stratigraphy of the past 5000 years reveals that the most common events are pyroclastic flows, constituting the principal volcanic hazard of Galeras. Additionally, lava extrusion, tephra falls, and lahars have also occurred, but are lesser hazards. The two previous hazard maps of Galeras did not account for the studies mentioned above. Methodology for this revision of the volcanic hazard map of Galeras is based on (1) probabilistic zoning of phenomena associated with eruptions that have occurred during the past 5000 years and (2) modeling of other phenomena which did not appear in the geologic record, such as shock waves and ballistic projectiles. The map consists of three categories of hazard zones: high, medium and low. The zone of high hazard is defined principally by the distribution of pyroclastic flow deposits, while the zone of low hazard is defined by tephra fall deposits. The medium hazard is a transition zone and ideally, if based on the geological record, does not include pyroclastic flows; however, it is possible that a comparatively large pyroclastic flow could extend into this zone. Additionally, this zone defines the possible trajectories of secondary lahars.
AB - ABSTRACT: The morphology of the active cone of Galeras volcano is a consequence of the complex interplay between effusive and explosive processes, and erosion. The main crater, several secondary craters, and zones of fumarolic activity were formed on the active cone as a result of these processes. Analysis of detailed descriptions, from observations that began in 1989, indicates that volcanic activity led to the formation of many fissures in the main crater and on the active cone which appear to show an important relationship with the volcano's structure. The fractures and fissures are associated with a tangential system generally aligned with the crater rim and a radial system that tends to intersect the crater rim at high angles. The intersections of the tangential and radial systems have generated zones of weakness, some of which have developed into eruptive centers. Thus, certain fumaroles have evolved into small craters, and these craters have contributed to the growth of the main crater on the active cone.
AB - ABSTRACT: Since the reactivation of Galeras volcano in 1988, the emplacement of a lava dome in the main crater of the volcano has been its most significant magmatic activity. Changes leading to the emplacement of the dome were present as early as June-July 1990, when the trend of degassing for the major species (mainly SO (sub 2) and CO (sub 2) ) in the summit fumaroles changed drastically, and small vulcanian explosions become more frequent. Moreover, after August 1990, an inflationary trend as measured by an electronic tiltmeter was observed to increase over background levels, and the temperature of a newly formed fumarole (Besolima) increased from 130 to 738 degrees C within a month. The dome was seen for the first time in early October 1991 at the base of the main crater. The dome grew for about a month to a total volume of nearly equal 4X10 (super 5) m (super 3) . During this time, the growth rate was calculated to be 12X10 (super 3) m (super 3) /day or 0.14 m (super 3) /s. An explosive eruption on 16 July 1992 destroyed most of the dome. The chemical composition of the dome ranges from 59.4 to 61.1 wt.% SiO (sub 2) (anhydrous). The presence of (1) large plagioclase phenocrysts with strongly inversely zoned rims and (2) small euhedral plagioclase crystals with normal zoning and Ca-rich cores indicates that a magma mixing event occurred early in the history of dome emplacement. Large amounts of crystal fractionation then took place. About 70% fractionation can explain the mineral suite and the major- and trace-element compositions of the glasses. The uppermost part of the dome, degassing freely at the surface for ten months, was able to isolate the magmatic system, producing a highly evolved melt in the dome interior. The July 1992 eruption tapped not only the uppermost part of the dome but also a deeper H (sub 2) O-saturated magma. Analyses from these samples show that most of the H (sub 2) O was degassed while Cl and B contents were higher than in the uppermost part of the dome. However, the F content is lower in this deeper part of the conduit, while it increases in the matrix glass of the degassed upper part of the dome.
RP - RESEARCH PROGRAM: USGSOP (Non-USGS publications with USGS authors)
CP - COUNTRY OF PUBLICATION: Netherlands
PY - PUBLICATION YEAR: 1997
LA - LANGUAGE: English
AB - ABSTRACT: Volcan Galeras is the southernmost Colombian volcano with well-recorded historic activity. The volcano is part of a large and complex volcanic center upon which 400,000 people live. Historic activity has centered on a small-volume cone inside the youngest of several large amphitheaters that breach the west flank of the volcano, away from the city of Pasto (population 300,000). Lava flows (SiO (sub 2) between 54.6 and 64.7 wt.%) have dominated activity for more than 1 Ma, but explosive events have also occurred. Joint studies by volcanologists from Colombia, Ecuador, Peru, Bolivia, Argentina, and the United States produced 24 new (super 14) C ages and more than 100 stratigraphic sections to interpret the past 50 ka of activity at Galeras, including sector collapse events. The youngest collapse event truncated 12.8 ka lava flows and may have occurred as recently as 8 to 10 ka. Tephra-fall material rapidly thins and becomes finer away from the vent area. The only widespread marker in the <10 ka section is a biotite-bearing tephra deposited between 4.1 and 4.5 ka from a source south of Galeras. It separates cryoturbated from largely undisturbed layers on Galeras, and thus dates a stratigraphic horizon which is useful in the interpretation of other volcanoes and geotectonics in the equatorial Andes. Pyroclastic flows during the past 50 ka have been small to moderate in volume, but they have left numerous thin deposits on the north and east flanks where lava flows have been impeded by crater and amphitheater walls. Many of the pyroclastic-flow deposits are lithic rich, with fines and clasts so strongly altered by hydrothermal action before eruption that they, as well as the sector collapse deposits, resemble waste dumps of leached cappings from disseminated sulfide deposits more than volcanogenic deposits. This evidence of a long-lived hydrothermal system indicates susceptibility to mass failure and explosive events higher than expected for a volcano built largely by lava flows and modest Vulcanian eruptions. Photographs, written accounts, and our study document historic north and east flank pyroclastic flows as far as 10 km from the summit; however, none have left recognizable deposits in Pasto for more than 40 ka.