A MODEL FOR THE MECHANISM OF THE MAY 18, 1980 MOUNT ST-HELENS BLAST

A model is suggested for the mechanism of the blast that accompanied the May 18, 1980 Mount St. Helens eruption. The model explains the large duration of the blast process and accounts for several explosive episodes recognized during the blast. The mechanism is based on the idea of destruction of the near-solidus porous cryptodome by a fragmentation wave, termed the F-wave. Originating from cryptodome decompression induced by a landslide, the F-wave propagates from a free surface to the interior of the cryptodome at velocity N. The products of cryptodome fragmentation (a gas-pyroclast mixture) are ejected into the atmosphere at velocity u. Calculations show that for pressure of gas in the cryptodome pores of 20 MPa, and porosity 0.4-0.7, the F-wave travels at 5 m/s, and velocity of pyroclast ejection is 100-250 m/s. For a cryptodome with diameter of 700 m, the duration of blast fragmentation is estimated at 140 s. Energy released during the blast is estimated as 4.8-7.2 x 10(15) J, blast power is 3.4-5.1 x 10(13) Wt, gas-pyroclastic mixture mass flux is 5.0-7.5 x 10(3) kg/m2 . s, the maximum discharge is 1.9-2.9 x 10(9) kg/s and the weight fraction of water vapor in the cryptodome is 0.006-0.04. Several explosive episodes during the blast are accounted for by fragmentation wave stoppage, due to blockage of the exploding cryptodome by landslide material.