THE MECHANISM OF PHREATIC ERUPTIONS

We investigate the mechanism for initiating phreatic eruptions followi ng the emplacement of a shallow magmatic intrusion into water-saturate d permeable rock which contains subsidiary low-permeability crack netw orks and disconnected cracks. Heat from the intrusion causes the local groundwater to boil and ascend through the main permeable crack netwo rk. As the ascending superheated steam heats the overlying rock, the w ater in the subsidiary networks and disconnected cracks will boil. The pressure exerted by the vapor in the subsidiary and disconnected crac ks can lead to rapid horizontal crack propagation, resulting in an inc rease in crack length by more than an order of magnitude. According to the model, the eruption process starts near a free surface and migrat es rapidly along thermoelastic isostresses as a result of multiple bre akage of the thin surface layers above the cracks. For certain crack a nd rock parameters, however, the crack propagation mechanism, instead of leading to a dynamic eruption, may generate a highly cracked zone t hat may be removed later by fluid transport processes. The proposed me chanism gives rise to precursory phenomena observed in conjunction wit h many phreatic eruptions. According to the model developed here, phre atic eruptions are most likely to occur only for a rather restricted s et of rock parameters. For example, the country rock should not be too strong (sigma(t) congruent to 10 MPa) and should be characterized by two-scale permeability structure involving a main crack network of rel atively high permeability (greater than or similar to 10(-12) m(2)) an d a subsidiary crack network with much lower permeability (< 10(-17) m (2)). Moreover, the model works better if the mean crack aspect ratio is relatively large (beta similar to 10(-1)) and the crack concentrati on is not too low (Omega > 10(-2)). These restrictions may explain ind irectly why phreatic eruptions are not ubiquitous in volcanic regions.