Why did Mount St. Helens erupt?

What caused Mount St. Helens to erupt?

• Answer:

  The 1980 eruption resulted when a huge section of the mountaintop was forced outward and fell away from the mountain. The pressure of magma from within the mountain had been increasing over the years and the rock eventually gave way. With the blocking rock removed, huge amounts of hot rock, ash, and gas exploded from the opening with incredible force. As part of the Cascade Range of mountains, Mount St. Helens was formed by eruptive flows of magma as the Juan de Fuca tectonic plate is forced under the North American Plate. Beginning about 37,600 years ago, periodic eruptions and magma flows occurred in the area between Mount St. Helens and Mount Rainer. Huge eruptions occurred in the years 1480, 1482, and 1800, and smaller releases of steam between 1831 and 1857. The large magma chamber beneath Mount St. Helens will likely continue to force lava upward and cause future eruptions. However, as history indicates, they could be several hundred years apart.