Transient multiphase processes during the explosive eruption of basalt through a geothermal borehole (Námafjall, Iceland, 1977) and implications for natural volcanic flows

Experimental and numerical studies have shown that vertical flows of gas–particle mixtures are characterized by transient behavior, with development of waves of high particle concentration separated by regions of relatively clean gas. In contrast, most models of explosive flow in volcanic conduits either treat the multiphase mixture as a single fluid (pseudo-fluid approximation) and/or assume steady flow, thereby eliminating the potential for time-dependent effects related to multiphase dynamics. The 8 September 1977 explosive eruption of basaltic tephra through a geothermal borehole (Námafjall, Iceland) provides a unique test case for multiphase volcanic processes, given that its vertical extent (∼ 1 km) is similar to that of natural volcanic conduits and its geometry is exactly known. We model this eruption by solving separate, time-dependent governing equations for conservation of mass, momentum, and energy of the gas and particle phases, allowing for drag and heat transfer between the phases. Model results are consistent with the development of transient waves of high particle concentration that propagate up the borehole, resulting in complex compressible flow phenomena along with ejection of particles in pulses in a manner that is consistent with observations at Námafjall. These transient processes occur even though the influx of gas and particles at the base of the borehole is treated as constant. Our results indicate that transient multiphase behavior is likely to be common in volcanic conduit flows, and that a key topic of future research is quantifying the types of time-dependent behaviors and their impacts on eruption column dynamics.