What is a lateral blast?

A lateral blast, also known as a lateral eruption, is a volcanic eruption that occurs out of the side of a volcano instead of at its summit. An example of a lateral blast is the 1980 eruption of Mount St. Helens.

When the sides, or flanks, of a volcano are comprised of sufficiently weak material, the upswelling of magma can cause the top to collapse, decompressing the magma chamber and causing it to vent, or explode, out the flanks. During the eruption of Mount St. Helens, the force of the lateral blast knocked forests of trees over in a 19-by-23-mile swath.