A model of vulcanian eruptions at Galeras volcano, Colombia

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A series of six vulcanian eruptions at Galeras volcano, Colombia, occurred during 1992–1993. These eruptions followed the emplacement of a lava dome in late 1991, which was accompanied by cooling, crystallizing and partial solidification of the column of magma plugging the conduit. Five of the eruptions were preceded by episodes of monochromatic long-period seismicity, suggesting cyclic pressurization of the magma. The quiescent intervals between eruptions generally decreased with each successive eruption. We propose a model of gas release from magma into confined, stationary pore spaces by 5–10% crystallization of anhydrous minerals, causing repeated overpressurization of the magmatic system. Using initial lithostatic pressures of 20–100 MPa (800–3900 m depth), gas buildup by crystallization indicates overpressures of 3–17 MPa, which is the approximate fracture criterion for a magma chamber. Our Galeras data suggest two models of pressurization: (1) gas exsolution may have occurred at similar depths, but with progressively smaller overpressures being developed before each successive eruption. According to this model, the comparatively long quiescent periods between the first eruptions resulted in relatively large amounts of gas exsolution from the magma and accumulation in pore spaces, whereas the shorter quiescent periods for the later eruptions exsolved less gas. The decrease in overpressure with time may have resulted from progressive fracturing of partly solidified magma during the eruptions, which progressively decreased the tensile strength and increased the permeability of the magma. (2) Since overpressures are smaller at shallower depths for a given amount of crystallization, the first eruptions of 1992–1993 were caused by gas exsolution at shallow depths, while the last eruptions, having shorter quiescent intervals, resulted from a greater amount of gas exsolution at greater depths. In this case, the rate of gas exsolution varied as a function of depth. The two scenarios for pressurization may be combined into a more general model involving edifice weakening and the development of a shallow hydrothermal system from continued degassing of the magma plug. As weakening of the edifice formed an extensive system of fractures, the shallow hydrothermal system deepened due to meteoric water input and magmatic degassing. With time, a greater number of fractures were filled and partly sealed by hydrothermal minerals at progressively deeper levels. As a result, the source of the overpressure also deepened with time.