Geometrical constraints of the 1980 Mount St. Helens intrusion from analogue models

The 1980 Mount St. Helens cryptodome intrusion has been simulated by scaled experiments. Silicone, used as a magma analogue, was intruded into a cohes ive cone that models the edifice, Measuring surface deformation features as sociated with variations of injection rate and depth of cryptodome initiati on revealed linear relationships with the intrusion characteristics. Scaled to Mount St. Helens dimensions, the 1980 cryptodome is found to have had a maximum E-W width of 850 mi an initial depth of 680 to 835 m. It had an as cending velocity of 5.7-7.3 m/day that corresponds to a vertical displaceme nt of 330-380 m toward the surface within the two months of deformation. Th is brought the intrusion to less than 250 m below the crater floor at the p oint when the north flank failed on May 18, 1980.