Shallow volcanic processes at persistently active volcanoes: multidisciplinary study at Poás volcano, Costa Rica

Fournier, Nicolas (2004). Shallow volcanic processes at persistently active volcanoes: multidisciplinary study at Poás volcano, Costa Rica. PhD thesis The Open University.



There is a discrepancy between the amount of gas emitted and the quantity of magma erupted at many persistently active volcanoes. The gas output suggests a larger magma supply than is observed to erupt. This is a major issue in contemporary volcanology but as yet it is poorly understood.

Calculations based on the micro-gravity data collected during the 1986-89 degassing crisis at Poas volcano, Costa Rica, and a new sulphur budget analysis suggest that approximately 28 x 109kg of magma was intruded beneath the Poas summit crater during this period, but much of the magma (at least 16 x 109kg) was recycled to depth. A new high-resolution Bouguer survey in the active crater also shows that a significant amount of magma has been intruded into the subsurface at Poas - probably during the last century (20% of it during the 1986-89 crisis) despite almost no magma being erupted in this period. This implies that the magma intruded had already degassed, otherwise major near surface exsolution of volatiles and eruptions would have followed intrusion. The magma reservoir at Poas must therefore be shallow enough in the edifice to allow volatile exsolution and segregation from the magma, and wide enough to allow magma convection. The stocky shape of the shallow intrusions at Poas and the apparent lack of ground deformation during their emplacement (both determined by geophysical studies) indicate that magma migrates mainly by assimilation of the surrounding material.

Hydrothermal systems seem to have a major role in controlling both the development of magma reservoirs high in the volcanic edifice and the subsurface intrusion of magma, facilitating assimilation by intruding magmas. Thus hydrothermal systems may have a much greater role in controlling the type of activity and the long-term evolution of stratovolcanoes than previously suggested.

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