Experiments on the indentation process during cryptodome intrusions: new insights into Mount St. Helens deformation

Scaled experiments were carried out to study the deformation of a volcanic edifice by forcible intrusion of a cryptodome. As the magma analogue is injected vertically into a sand cone, asymmetric deformation is caused by the formation of a curved major shear fault, which dips inward from one side of the cone to the opposite edge of the intrusion. The path of the ascending silicone deviates to follow the trajectory of the fault, and a lateral bulge grows slowly from the footwall of the fault. The oblique push makes the bulge migrate outward, causing extension upslope to form an asymmetric graben in the hanging wall of the major shear fault. We suggest that the pattern of internal deformation within Mount St. Helens prior to the May 18, 1980, eruption was similar to that observed in our scaled models.