What type of lava/erupted material comes out of Mount St. Helens?
Mid- to low- silica felsic rock (granitic and viscous). Specifically, dacite rock was formed. But for more information about the (primary) eruption itself… The eruption began during a relatively quite period in which no steam explosions had occurred for four days. On May 18, at 8:32 a.m., a 5.0-M earthquake triggered a very rapid series of events. The entire northern slope above the bulge failed and the north flank of the volcano began to slide downward from almost the exact site of the east-west fracture at the summit. This gigantic landslide released a tremendous mass from above the hydrothermal system that had driven the precursor steam eruptions. The abrupt loss of confining pressure above the heated groundwater caused a massive flashing to steam, which initiated a hydrothermal blast that was directed laterally through the landslide scarp. The lateral hydrothermal blast rapidly overtook the avalanche and devastated a fan-shaped area to the north, which was nearly 30 km wide over
