Pressure, gas content and eruption periodicity of a shallow, crystallising magma chamber

Details Pressure, gas content and eruption periodicity of a shallow, crystallising magma chamber Pressure, gas content and eruption periodicity of a shallow, crystallising magma chamber

Feb 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989e%26psl..92..107t&link_type=abstract

Earth and Planetary Science Letters, Volume 92, Issue 1, p. 107-123.

Physics

121

Scientific paper

We present a model of the triggering of volcanic eruptions based on fractional crystallisation and oversaturation of volatile species in a shallow magma chamber. We calculate the overpressure in the chamber and consequent increase in its volume by deformation of the surrounding rocks as a function of the amount of crystallisation. When the overpressure reaches a value of twice the effective tensile strength of the volcanic edifice, eruption or emplacement of a dyke occurs, and the chamber returns to its original pressure and volume. We show the quantitative effects on the pressure history of the form of the solubility law (depending on the volatile species present), the crystallisation contraction and the presence of some initial mass of gas at t = 0. The most important of these is the solubility law. We show that, once saturated, the more soluble is the volatile species, the more important it is for the development of overpressure in the chamber. Only a few per cent fractional crystallization are required to cause overpressures equal to the fracture criterion for a pure H2O gas phase. A pure CO2 gas phase connot cause important overpressures because it is much less soluble. Only for the improbable case of a pure CO2 gas phase and a large crystallisation contraction do underpressures arise. We calculate the volumetric deformation of the surroundings and the erupted volume of lava as a fraction of the chamber volume. In the cases of Kilauea and Krafla volcanoes where the volume of the magma chamber is known approximately, our results are close to observations of both the amounts of tumescence and volumes of melt ejected from the chamber. In this model the repose time between eruptions is determined by the rate of crystallisation. Estimates for the time required to reach overpressures equal to the fracture criterion are on the order of a few years for basaltic melts and a few hundred years for more viscous systems. Over the course of many eruptions this model predicts an approximately constant output rate of lava. If the chamber is a closed system, after an eruption, the amount of liquid ejected is replaced by an equivalent volume of gas. The amount of gas builds up with each successive eruption and eventually the magma chamber roof can become unstable causing caldera collapse.

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