The 1980 Mount St. Helens cryptodome intrusion has been simulated by scaled experiments. Silicone, used as a magma analogue, was intruded into a cohesive cone that models the edifice. Measuring surface deformation features associated with variations of injection rate and depth of cryptodome initiation 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 m, an initial depth of 680 to 835 m. It had an ascending velocity of 5.7--7.3 m/day that corresponds to a vertical displacement of 330--380 m toward the surface within the two months of deformation. This brought the intrusion to less than 250 m below the crater floor at the point when the north flank failed on May 18, 1980. ¿ 2001 American Geophysical Union