Intermittent Convective Overturn As a Geyser Eruption Mechanism

Abstract:

The mechanism of geyser eruption has long intrigued scientists and the public, yet it has remained a matter of debate. Three major categories of model have been proposed: One category focuses on the processes in the geyser conduit. The second focuses on the processes in an underground ‘bubble trap’ that connects to the surface through a conduit. The third considers the geyser system to consist of a vertical fracture zone of high permeability and compressibility, surrounded by rock matrix of low permeability and compressibility. Since the interior of geysers is largely inaccessible to direct observation, numerical simulation constrained by available observation may be an important tool for deciphering the mechanism of geyser eruption. Here we use numerical simulation to explore the mechanism of geyser eruption from a system that consists of a heated underground cavern, delineated by seismic tremor location near the Old Faithful Geyser, WY (Vandemeulebrouck, et al., 2013) and connects to the surface through a conduit. Differential equations representing conservations of mass, momentum and energy, constrained by appropriate initial and boundary conditions, are solved to simulate the coupled geyser processes.

Hydrothermal fluids seeping into the cavern heat up the bottom water to form an unstable thermal boundary layer near the base. Convection becomes intermittent at large Rayleigh number. Top-down boiling in the conduit releases pressure not only in the conduit but also in the cavern, triggering bubble nucleation and uprise from the base. Convective overturn of the thermal boundary layer leads to vigorous boiling in the cavern and geyser eruption through the conduit. Boiling in the cavern sustains geyser eruption until the thermal boundary layer is destroyed. Results suggest that the complex interaction between processes in the conduit and in the cavern, rather than either one in isolation, may control geyser eruption.