| This dissertation presents the results of unique numerical models of hydrothermal systems in unsaturated pyroclastic rocks. A simple 1-dimensional, radial conductive finite-difference heat-flow model is demonstrated to be an adequate tool for investigating possible thermal conditions in a limited contact metamorphic aureole that developed around a large mafic intrusion in unsaturated, silicic host rocks at Grants Ridge, New Mexico. A modified version of the Los Alamos National Laboratory code, FEHM, is presented that has a unique capability of simulating multiphase (air + H2O) flow under the high-temperature conditions near intrusive magma bodies or within hot volcanic deposits. The modified equations of state (EOS) for water and air are valid in the ranges 10 < T < 1500°C, 0.00123 < P < 1000 MPa, and 10 < T < 1500°C, 0.00123 < P < 22 MPa, respectively.;Two applications of the FEHM code are presented. A two-dimensional model of multiphase flow near a magmatic intrusion was constructed to characterize the cooling history of part of a late-Miocene mafic intrusive complex at Paiute Ridge, Nevada, USA. The results of the model were combined with paleomagnetic data to estimate that the rate of change of the transitional part of the geomagnetic field during a reversal was 0.06 to 0.13 degrees/year.;Multiphase cooling processes in cooling ash-flow tuffs (ignimbrites) were investigated using a set of FEHM models. The results of these models identify important factors in the cooling history, including the presence of a saturated zone in the shallow substrate, geometry of the substrate-ignimbrite interface (e.g., buried valleys), and welding zonation and fumarole structures within the ignimbrite. In addition, the models shed light on conditions that may give rise to the development of secondary explosions in ignimbrites. Finally, the model results indicate that superheated vapor from the boiling zone at the base of the ignimbrite, flowing upward through the core, may provide the necessary water mass in the upper zones of the ignimbrite to account for reported oxygen-isotope exchange in the Bishop and Chegem Tuffs, rather than requiring high meteoric infiltration on the surface of the ignimbrite, as previously proposed. |
