The August 7, 1980 Eruption of Mount St. Helens

How do you measure a high speed volcanic flow? An observer uses a camera, a timer, and the laws of physics to calculate the speed from an eruption at Mount St. Helens.

Volcanic eruptions have played a part in forming the Earth's atmosphere as well as many of its landforms both on land and under water. The geologic record contains evidence of eruptions much larger and explosive than any witnessed and even when it is possible to study an erupting volcano, direct observations are, needless to say, dangerous, and rare.

The August 7, 1980, eruption of Mount St. Helens was a unique opportunity to measure the speed of a hot ash flow. Rick Hoblitt, a Geologist for the US Geological Survey, was close to the mountain and recorded a series of photographs of a small ash cloud that rapidly descended the mountain.

Using the time stamp on his camera, Hoblitt was later able to locate the geographic postion of the flow front at discrete time intervals. By using the simple velocity equation:

he was able to determine the speed of the volcanic flow between subsequent photographs. This was one of the first times that the velocity of an ash-flow was recorded, and showed how it varied with changes in slope and channel width as it descended the volcano.

see related image

Hoblitt, Richard P (1986), Observations of the Eruptions of July 22 and August 7, 1980, at Mount St. Helens, Washington. U.S. Geological Survey Professional Paper 1335.