Why did mt st Helens blow up?

Mount St. Helens erupted on May 18, 1980, due to a combination of tectonic activity and the buildup of pressure from molten rock, or magma, beneath the surface. The eruption was triggered by an earthquake that caused a massive landslide, allowing the magma to escape rapidly. This explosive event was fueled by the release of gas and steam, resulting in a powerful volcanic blast and ash plume that devastated the surrounding area. The eruption was a result of the complex interactions between the subduction of the Juan de Fuca Plate beneath the North American Plate.