The leaching legacy of Mount Saint Helens

This year marks the thirty fifth anniversary of the eruption of Mount Saint Helens, a volcano in the North-Western United States. Since its cataclysmic eruption on May 18th, 1980, Mount Saint Helens has become a ‘household name’ in volcanology; its legacy recorded in hundreds of scientific papers that advance our understanding of a volcano´s inner workings. Coinciding with the anniversary, researchers in the Department for Earth and Environmental Sciences at LMU show that there are still lessons to be learned in a new study published in Bulletin of Volcanology.

The summit of Mount Saint Helens, 35 years on

The Mount Saint Helens eruption blanketed much of the Pacific Northwest in ash coated with tiny salt crystals. These crystals form by the interaction of ash with volcanic gases during the eruption. Although difficult to directly analyse, they are easily dissolved when ash is mixed, or leached, with water. Scientists are able to determine the abundance and chemistry of the crystals by analysing these leaching solutions and use the data to gain insight into the eruption itself and understand the impact of an eruption on ash-affected areas.

In their new study, Dr Paul Ayris, Dr David Damby and Prof Dr Donald Dingwell, collaborating with researchers from Belgian and Norwegian universities, retrospectively analyse the wealth of existing leaching data from the Mount Saint Helens eruption. “Mount Saint Helens is the most well-studied volcano in history,“ says Dr Paul Ayris, “the studies of the 1980 eruption laid the foundation for our current understanding of eruption plume chemistry, but after thirty five years, we wanted to look back at that eruption with a fresh perspective“.

“The Mount Saint Helens eruption is the most well-leached and well-studied in history. The studies of that eruption are part of a golden age for ash leaching, which laid the foundation of our current understanding of eruption plume chemistry.”

Dr Paul Ayris, lead author

By compiling more than 300 leaching data from every study conducted and assessing the quality of each according to modern understanding, Dr Ayris and co-workers were able to map the spatial variation of salt concentrations across the ash deposit. This unveiled far more complex distributions in the abundance of these salts than had been previously realised.

Crucially, these new observations coincide with known chemical and physical features of the ash deposit, and “are in excellent agreement with our current theoretical and experimental understanding of the volcanic plume,“ explains Dr Ayris; “our study is an important step towards being able to predict the chemical effects of volcanic ashfall on our environment, whether that is a forest, farm or even garden.“

But the retrospective study offers more than just the potential validation of current models according to Dr Damby: “Looking back at the pioneering work of the 1980s, the extent of both ash sampling and analysis is remarkable. Our findings reiterate the depth of insight that such endeavours can offer,“ adding that such an “extensive campaign lays the groundwork for volcanologists preparing to investigate the impacts of the next big volcanic eruption.“

That too is part of Mount Saint Helen´s legacy.

“Spatial analysis of Mount St Helens tephra leachate compositions: implications for future sampling strategies“ has been published as an open-access paper in the Bulletin of Volcanology

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