THERMO-HYDROLOGIC PROCESSES IN MAAR ERUPTIONS: THE ROLE OF VAPOR TRANSPORT AND CONDENSATION

The interaction of magma and groundwater often results in explosive eruptions due to processes of molten fuel-coolant interaction (MFCI). Explosivity of these eruptions requires mixing between liquid water (coolant) and magma (fuel). Most phreatomagmatic tuff-ring deposits are probably produced by explosions occurring above ~200 m depth, while deeper explosions rarely eject material at the surface. It is thus uncertain how phreatomagmatic eruptions occur in arid regions where the water table is deeper than 200 m. This study focuses on details of thermohydrologic processes relevant to maar eruptions to both test how MFCI occurs at maars with deep groundwater and investigate ways that water supply for MFCI may continue after the water adjacent to the magma system has been used and ejected. Conceptual models were developed for maar eruptions in systems with deep water tables based loosely on Colton Crater and Rattlesnake Crater, two maar volcanoes in the San Francisco Volcanic Field of northern Arizona. Hydrologic, structural, and stratigraphic data of the subsurface beneath each volcano are used to construct conceptual models of groundwater flow and heat transfer within each eruptive system, and relevant thermophysical flow processes are modeled using the TOUGH2 simulator. Results of simulations show that magma intruding into the aquifer quickly heats the water and vapor rises along fractures in the country rock, reaching the surface from depths of >300 m in hours to days. This vapor condenses to liquid in cooler areas bordering the fractures, saturating these rocks and providing abundant water for MFCI. Similar processes can explain massive outpourings of hot water and/or rises in the groundwater in wells during some eruptions. Our results suggest that, once a diatreme starts to form, the breccias in it provide an ideal environment for the development of a heat pipe, which then efficiently extracts water from the surrounding aquifer and sends it upward within the diatreme.