Can We Gain Evidence About Volcanic Pyroclastic Flows from Those Who Survive Them?
Pyroclastic flows have been recognized as one of the most devastating hazards of volcanic eruptions ever since one killed nearly 28,000 people in the town of St Pierre in 1902 AD. After more than 100 years, however, there are aspects about them that scientists still do not understand, including where they will go, how far they will travel, how their velocity varies laterally and vertically, and how volcanic ash is transported within them to be deposited on the ground. Scientists rely on indirect evidence from their ash deposits or from computer models to hypothesize about such unknowns. The fundamental problem is that no one has made in situ measurements within the flows. Despite their lethal nature, however, some people have lived through them. While being enveloped by searing hot ash must be unimaginably traumatic, it is possible that survivors remember details that could help inform scientists, especially if a collective memory can be developed from multiple accounts. This study uses recollections from eyewitnesses and survivors of the 18 May 1980 pyroclastic flow from Mount St Helens to develop a collective memory that shows the leading edge of the flow is relatively ash poor, cold and even carrying ice. It is shown that this collective memory is likely sound by comparing it to the physical record of the flow from its impact on inanimate objects. What is gained is that a cold flow front would not have otherwise been imagined. It is hoped that if future encounters occur, scientists seek out eyewitnesses and survivors to gain new and potentially unexpected evidence about pyroclastic flows.