Magmatic model for the Mount St. Helens blast of May 18, 1980

摘要: Analytical and numerical solutions to the hydrodynamic equations of motion, constrained by physical properties juvenile ejecta in Mount St. Helens blast deposit, were used investigate magmatic conditions required produce initial devastating phase eruption May 18, 1980. Evidence that was includes equivalence volume preemption inflation cone, substantial vesicularity this ejecta, continued vesiculation large clasts after eruption. Observed or inferred velocities 100 250 m/s are shown require 0.2 0.7 wt % water vapor preexisting magma unloaded a landslide 200 900 m thick. These imply total contents 1.7 %, respectively. Such low content suggests volcanic blasts may be regarded as normal consequence intrusion into an unstable edifice.