The landslide exposed the dacite magma in St. Helens' neck to much lower pressure, causing the gas-charged, partially molten rock and high-pressure steam above it to explode a few seconds after the landslide started. Explosions burst through the trailing part of the landslide, blasting rock debris northward. The resulting blast directed the pyroclastic flow laterally. It consisted of very hot volcanic gases, ash and pumice formed from new lava, as well as pulverized old rock, which hugged the ground. Initially moving at approximately , the blast quickly accelerated to around , and it may have briefly passed the speed of sound.