Located in Washington State's southwestern corner, Mount Saint Helens was responsible for one of the biggest volcanic eruptions in the history of the continent on the morning of May 18, 1980. The volcano shot millions of tons of ash and rock 65,000 feet into the sky, levelling the top of the mountain in the process. There are many potential causes of a cataclysmic eruption like this, but evidence recorded before and after Saint Helens blew point to a few factors in particular.
Magma Volume and Density
On the day before Mount Saint Helens' eruption, observers noted a key change in the mountain: It's northeast face had bulged outward nearly 450 feet. This sudden transformation was a strong indication that magma (molten rock) had risen to a high level inside the volcano. This means one of the key factors in causing the mountain to erupt was a deficit in the density between surface-level magma and its surrounding and overhead rocks. This happens because as the earth's crust melts, it creates lighter, buoyant magma that rises to the surface and erupts.
Another likely contributor to the eruption of Mount Saint Helens was the issue of dissolved volatiles in the magma, such as carbon dioxide, sulphur dioxide and water. While the amount of dissolved gas in magma at normal atmospheric pressures is negligible, it increases significantly with a rise in pressure inside the magma chamber of a volcano column. On May 18, there were enough pressurised volatiles under Mount Saint Helens to send an eruption column of ash and gas over 15 miles into the earth's atmosphere.
The eruption of a volcano can also be kick-started deep under the earth's crust as new magma is injected into an already full chamber. Whether or not the two magma types are similar or disparate, this injection forces the magma to move up in the conduit of the volcano, erupting at the surface. Mount Saint Helens' magma bulge prior to its eruption indicated a significant increase in magma volume, which an injection may have contributed.
According to the U.S. Geological Society, while volcanic eruptions are sometimes preceded and followed by minor to major seismic activity (in the form of earthquakes), the historical record of both types of activity does not indicate any easy, causative relationship. And while there are a few standalone historical examples of eruptions triggered by massive earthquakes, Mount Saint Helens in 1980 is not one of them. Therefore, the series of earthquakes that happened before and after that eruption should be viewed more accurately as indicators and aftershocks, respectively.