The mechanism of phreatic eruptions (original) (raw)

1995, Journal of Geophysical Research

We investigate the mechanism for initiating phreatic eruptions following the emplacement of a shallow magmatic intrusion into water-saturated permeable rock which contains subsidiary low-permeability crack networks and disconnected cracks. Heat from the intrusion causes the local groundwater to boil and ascend through the main permeable crack network. As the ascending superheated steam heats the overlying rock, the water in the subsidiary networks and disconnected cracks will boil. The pressure exerted by the vapor in the subsidiary and disconnected cracks can lead to rapid horizontal crack propagation, resulting in an increase in crack length by more than an order of magnitude. According to the model, the eruption process starts near a free surface and migrates rapidly along thermoelastic isostresses as a result of multiple breakage of the thin surface layers above the cracks. For certain crack and rock parameters, however, the crack propagation mechanism, instead of leading to a dynamic eruption, may generate a highly cracked zone that may be removed later by fluid transport processes. The proposed mechanism gives rise to precursory phenomena observed in conjunction with many phreatic eruptions. According to the model developed here, phreatic eruptions are most likely to occur only for a rather restricted set of rock parameters. For example, the country rock should not be too strong (c• t • 10 MPa) and should be characterized by two-scale permeability structure involving a main crack network of relatively high permeability (•> 10 -•2 m 2) and a subsidiary crack network with much lower permeability (< 10 -•7 m2). Moreover, the model works better if the mean crack aspect ratio is relatively large (13 -10 -•) and the crack concentration is not too low (• > 10-2). These restrictions may explain indirectly why phreatic eruptions are not ubiquitous in volcanic regions. Introduction Hydrovolcanic phenomena vary greatly. At one end of a spectrum are hydrothermal eruptions [e.g., Lloyd, 1959; Hedenquist and Henley, 1985] that may be related to transient pressure changes in near-surface (< 300 m) regions of hydrothermal systems and that apparemly do not involve direct magmatic heating. At the other end are phreatomagmatic eruptions that involve fresh magma along with water, steam, and brecciated country rock. These eruptions apparently occur in connection with the influx of considerable quantities of water [Moore et at., 1966; Williams and McBirney, 1979; Shepherd and Sigurdsson, 1982]. Between these extremes are phreatic eruptions, which are generally thought to involve the transfer of magmatic heat to circulating groundwater and subsequent eruption of steam and country rock but, often, without the eruption of fresh magma [Oilier, 1974; Barberi et at., 1992]. This paper focuses on phreatic eruptions; however, the thermal crack propagation processes that we discuss below may be important in other types of hydrovolcanic phenomena. for the most part, the physical processes involved in initiating such eruptions are not thoroughly understood. Barberi et at. [1992] classify three broad types of phreatic eruptions: (1) fracturing phenomena following seismic events, (2) heating of shallow aquifers following magma rise, and (3) phreatic explosions at volcanoes with magma located high in the edifice. In this paper, we consider a phreatic eruption following the emplacement of a dike at shallow depths in the Earth's crest where groundwaters are free to circulate (type (2)). We estimate the rate at which superheated steam rising along the margins of the dike heats the overlying rock and then determine the conditions for the propagation of isolated, fluid-filled cracks in the heated country rock. We argue that sufficient crack propagation can lead to massive failure of the country rock near the dike. Following failure, excavation of a crater may occur in a manner analogous to the process of rock outbursts in mines [e.g., Khristianovich and Satganik, 1983], or the fractured material may be removed by fluidization processes as suggested by Holmes [ 1965]. This paper uses an order of magnitude analysis to address processes that lead up to the eruption but does not 8417 8418 GERMANOVICH AND LOWELL: PHREATIC ERUPTIONS address the details of removal of lithie material outside the eruption crater. Our analysis of the thermal boundary layer adjacent to the dike is similar to previous studies [e.g., Parmentier, 1979; Cheng and Verma, 1981]. Mechanical processes of crack propagation that result from the heating of isolated, fluid-filled cracks are also analyzed in a rather elementary way. The main contribution of this paper is the consideration of coupled thermal and mechanical processes. That is, we investigate a mechanism whereby thermal energy of magma, after being transferred to nearby groundwater, is converted into mechanical work in the shallow crust. Consequently, this paper treats three problems: (1) heating of an aquifer from below, (2) propagation of isolated cracks, and (3) failure of near-surface rocks to form a crater. The scenario we discuss would give rise to precursors such as anomalous seismicity, ground deformation, and changes in fumarolic and hot spring output that commonly occur from weeks to up to a few years prior to an eruption [Barberi et at., 1992]. The crack propagation and rock failure mechanism we propose is consistent with observed crater dimensions that range from tens to a few hundred meters in diameter and up to a few hundred meters deep [Muffler et at., 1971; Oilier, 1974; Kiente et at., 1980].