Hydrodynamic aspects of caldera‐forming eruptions: Numerical models

Abstract

Comparison of results from a two‐dimensional numerical eruption simulation (KACHINA) to calculations based upon a shock tube analog supports the conclusion that the hydrodynamics during the initial minutes of large caldera‐forming ash flow eruptions may be dominated by blast wave phenomena. Field evidence for this phenomenology is pyroclastic surge deposits commonly occurring both directly below caldera‐related ash flow sheets, on top of a preceding Plinian fall deposit (ground surge), and separating individual ash flow units. We model the eruption of the Tshirege member of the Bandelier Tuff (1.1 Ma B.P.) from the Valles caldera, New Mexico. In the model a magma chamber at 100 MPa (1 kbar) and 800°C is volatile rich, with an average H2O abundance above saturation greater than 8.7 wt % increasing to nearly 100 wt % near the very top of the chamber. Using a shock tube analogy, decompression of the chamber through a wide‐open dikelike vent 0.1 km wide and 1 to 5 km long forms a shock wave of 3 MPa (≃3O atm) with a velocity greater than 1.0 km s−1. Steady flow of material erupted from the vent begins after 20 to 100 s based upon a 7‐km depth from the ground surface to a reflective (density) boundary in the chamber and a rarefaction wave velocity of 100 to 600 m s−1. The velocity of the ash front behind the shock wave is 300 to 500 m s−1. The shock tube model serves as a basis to evaluate the consistency of the KACHINA code results which are similar to a one‐dimensional problem along the symmetry axis. The results of the KACHINA simulation show in some detail the effect of multiple reservoir rarefaction reflections and possibly Prandtl‐Meyer expansion in generating compressive wave fronts following the initial shock. The rarefaction resonance not only prolongs unsteady flow in the vent but tends to promote surging flow of ash behind the leading shock. Furthermore, these results are consistent with a blast wave characterized as a shock front followed by one or more pulses of entrained ash. The blast wave shocks ambient air to higher pressures and temperatures, the magnitudes of which depend strongly on the initial chamber overpressure, distance, and direction from the vent. In consideration of volcanic hazards our numerical model shows that a shock wave compressed the atmosphere to pressures of ≃0.2 to 0.7 MPa (2–7 atm) and temperatures of ≃200° to 300°C for distances to 10 km from the Bandelier vent(s).