An investigation of the role of thermal conditions, hydrologic processes, and country-rock permeability in maar eruptions

Anderson, Emily S. (2017) An investigation of the role of thermal conditions, hydrologic processes, and country-rock permeability in maar eruptions. Masters thesis, Northern Arizona University.

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Abstract

The interaction of magma and groundwater often results in explosive eruptions due to processes of molten fuel-coolant interaction (MFCI). Explosivity of these eruptions is largely controlled by the extent of mixing between water (coolant) and magma (fuel) and the location in the conduit where interaction occurs. Recently published experiments and calculations show that most phreatomagmatic tuff-ring deposits are probably produced by explosions occurring above ~200 m depth, while those below this depth are rarely energetic enough to displace material at the ground surface. It is thus uncertain how phreatomagmatic eruptions occur where depth to the water table is far below this critical depth of 200 m, as it is in some semi-arid and arid environments. Even in locations where groundwater is above this depth, explosions using up available water can result in progressive drawdown of the water table, producing a cone of groundwater depression within the diatreme. In these cases, the question arises as to how explosions continue if the water source “dries up.” The idea of explosive transport, or water being thrown upward from explosions beneath the water table, has been described as one possible mechanism for providing shallow water for continued explosions; however, this process would not likely move the large quantities of water needed to sustain long-term phreatomagmatism. Few studies have focused on questions regarding the details of thermohydrologic processes that control these eruptions. To address this problem, I have investigated two maars, Colton Crater and Rattlesnake Crater, in the San Francisco Volcanic Field of northern Arizona. Hydrologic, structural, and stratigraphic data of the subsurface beneath each volcano have been used to construct conceptual models of groundwater flow and heat transfer within each eruptive system, and relevant thermophysical flow processes have been modeled using the TOUGH2 simulator. Colton Crater was formed by a prolonged, dry cinder-cone-building eruption with only a brief late period of intense phreatomagmatic activity, while Rattlesnake Crater’s deposits display characteristics of sustained magma-water interaction and an eventual drying-out. The continued phreatomagmatic explosions at Rattlesnake Crater may have resulted from large-scale vapor transport, driven by magmatic heat, through permeable country rock and vertical fractures prior to and during eruption. Water (from condensed vapor) held at a shallow depth could then interact with quickly ascending magma, producing explosions. These processes do not appear to have occurred to the same extent at Colton Crater, as the eruption only experienced a brief period of phreatomagmatism. Models indicate that some vapor transport and condensation could have occurred outside the conduit during the eruption, producing a small amount of available water for MFCI, but explosions could have also been driven by a limited volume of perched water present in the shallow limestone unit prior to eruption. Results of this study aim to provide an example of a modeling approach for quantitative analysis of complex non-isothermal two-phase systems undergoing vaporization and condensation driven by magma intrusion, which can potentially be used for hazard assessment and monitoring for future eruptions in active volcanic regions.

Item Type:	Thesis (Masters)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	maar; magma-water interaction; modeling; phreatomagmatic; San Francisco Volcanic Field; TOUGH2
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations
Date Deposited:	05 Sep 2018 21:07
URI:	http://openknowledge.nau.edu/id/eprint/5253

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