According to the Oregon State University Department of Geosciences, a volcanic eruption may become violent if pressure builds up inside the volcano for any reason. An explosive eruption is much more dangerous than a steady flow of magma and can spread ash and pyroclastic material over a wide area. The eruption of Mount Saint Helens in 1980 was a textbook example of a violent, explosive volcanic eruption.
Under normal circumstances, the magma vent inside a volcano allows a steady stream of magma to escape to the surface, preventing any buildup of pressure beneath the volcano. However, if this vent becomes blocked, it can turn the magma reservoir below into a pressure cooker, allowing it to pressurize to explosive levels. In some cases, the vent may be structurally unstable and collapse in on itself, creating a plug that allows pressure to build up. In other cases, the composition of the magma may change, becoming thicker and more viscous. When the vent is particularly narrow, viscous magma can stop it up long enough for a pressure spike to occur.
When pressure builds up inside the volcano, it eventually takes the escape route of least resistance. In most cases, this means blowing out whatever material is blocking the vent, but in others the pressurized magma may create an entirely new route to the surface. In 1980, an explosive eruption blew out the side of Mount Saint Helens, sending out magma and pyroclastic material over a large area.