Galeras, Columbia

Eruptive Style and History

The Galeras Volcanic Complex (GVC) has a long complex history. It has been the site of active magmatism for 1 Ma. The region has experienced two major caldera building events ~560 ka and 40+ ka. The first of these collapses is associated with 16 km3 of pyroclastic flows while the more recent collapse followed an extrusion of 2 km3. These calderas and the large volume deposits associated with them indicate Galeras is/was capable of very large eruptions. During the past 5000 years eruptions have been frequent but comparatively smaller. Sometime between 12-5 ka, during the post glacial Urcunina phase, the volcano reached ~4500 m in elevation before experiencing a massive summit collapse which has not been linked to an eruptive event. This collapse is attributed to the high relief and the extreme hydrothermal alteration which can be seen in the scar left by the collapse. 3-4 km3 of the edifice material disappeared. The youngest debris avalanche deposit has a volume of 3 km3 and extends 12 km to the west toward the town of Consaca. This avalanche is most likely the missing summit material. During the last 5000 years eruptions have been frequent but of a smaller scale.

1992-1993 eruptions: After dome emplacement during 1991, Galeras experienced six eruptions characterized by the explosive ejection of viscous blocks of incandescent lava (Vulcanian eruptions.) Five of these eruptions were preceded by long period monochromatic "tornillo" seismicity thought to accompany cyclic pressurization of magma. The quiet period between eruptions decreased with each event. Stix et al. (1997) propose a model in which gas was exsolved by the crystallization of anhydrous minerals. The predicted overpressures of 3-17 MPa could exceed the fracture criterion for a magma chamber [at what depth??]

Chemistry and proposed magma system

Both lavas and pyroclastic flows have been erupted from the GVC. The chemical composition has been rather consistent throughout its history. The erupted material is calc-alkaline, medium-potassium, high-silica andesite. The crystalline content of up to 50% and higher is quite high. Calvache et al. claim that it was only rheologically possible to erupt this material because of its high volatile content (H2O, Cl and F). These fluids are most likely derived from the subducting slab although it is possible that they were the result of crustal enrichment.

There is one exception to this stable chemistry. 166 ka, the Guaca cinder cone erupted a more alkaline basaltic-andesite (54% SiO2) with a low crystal content (10-20%). The result is a monogenic cinder cone which extruded 0.2 km3 of lava at the end of its activity. No other similar eruptions have been found at the complex. It is perhaps significant that the Guaca cone is located over 12 km SW from the current Galeras vent.

While overall chemistry is fairly constant, there is variation within individual erupted units. [May be addressed in Calvache et al??]

Andesites and basaltic-andesites may be produced by crystal fractionation around 12 kbar (36 km) of hydrated tholeiitic basalt melt. This produces amphibole, clinopyroxene, plagioclase etc. There is evidence that the melt then re-equilibrates at 10 km depth which crystallizes orthopyroxene and more plagioclase. Amphibole is no longer stable at this depth. The melt becomes more hydrous as the amphibole recrystallizes into oxides, pyroxine and plagioclase.

Local Seismicity

Regional seismicity appears low for a passive study.

Postscript version of plot.

Notes and Questions

• Basaltic-andesite to dacite in single eruptions but overall composition is quite constant (Calvache). Is this evidence of a long lived, perhaps large, magma chamber?
• Stratigraphy is complex and cross-cutting.
• What is the relationship between the peripheral location of the Guaca event and its more basic composition? Could the Guaca event have been off target and missed a magma chamber during its assent? Or, alternatively, could the melt have been heavier and hence unable to rise to the summit? It would be important to know the location of the summit at the time of the Guaca eruption to address this question.
• Could tomography help distinguish the future eruptive potential?

Selected References

JVGR, 77 (1997) Special issue on Galeras