Emplacement and petrological evolution of the andesitic dome of Galeras volcano, 1990-1992

Marta Lucia Calvache V., Stanley Williams

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29 Scopus citations


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 SO2 and CO2) 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°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 ≈ 4 × 105 m3. During this time, the growth rate was calculated to be 12 × 103 m3/day or 0.14 m3/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.% SiO2 (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 H2O-saturated magma. Analyses from these samples show that most of the H2O 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.

Original languageEnglish (US)
Pages (from-to)57-69
Number of pages13
JournalJournal of Volcanology and Geothermal Research
Issue number1-4
StatePublished - May 1997


  • Andesitic dome
  • Dome emplacement
  • Galeras volcano
  • Petrology

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology


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