On a clear early summer morning (see note 1) 11.81 million years ago, the ground exploded in region that is now part of the Snake River Plain in southern Idaho. A massive ash plume surged skyward, reaching well over 100,000 feet in height (see note 2), well above the influence of the jet stream. Reaching heights where winds are weaker and the air much thinner, this ash plume readily spread out in all directions, like a giant, rapidly expanding umbrella over what is now the northwestern United States.
Later that same day, as temperatures warmed along the crest of the Cascade Mountains, clouds gradually built up into towering thunderheads. These scattered thunderstorms began drifting eastward off of the crest of the Cascades and out onto the rolling terrain beyond. The storms may have looked like those in the image loop below from June 2018.
Farther east, the sky gradually darkened as the thick ash cloud billowed northwestward into eastern areas of current-day Oregon and Washington. While the ash cloud remained incredibly thick and dense, most of the larger particles in the cloud rained out farther east, leaving only the finest of particles to continue journeying northwestward. Lighter in weight, air currents were able to keep these smaller moats of ash aloft longer, but they too began to drift down upon reaching central areas of modern-day Washington.
One of the thunderheads drifting eastward off the Cascades began passing over what is now Mattawa, Washington just as the cloud of ash moving in from the east reached the same area. As the thick cloud of ash drifted over and into the thunderstorm cloud, temperatures around the region began to cool further, weakening the updraft in the storm. No longer held aloft by the rising air, hail within the storm cloud began to fall to the ground, through the fine ash slowly drifting downward. Initially, the hail plowed through the fine cloud of ash, but as the hail fell farther it reached warmer air closer to the ground and began to melt. With a thin layer of water coating the hailstone, ash particles no longer flowed around the falling hailstones, but began sticking to them. In time, the hailstones became covered in a thin layer of wet ash. Sulfur dioxide in the ash cloud also interacted with the layer of water on the hailstones, creating a weak sulfuric acid that mixed in with the wet coating of ash. Eventually, the hailstones smacked into the surface, already coated with a layer of ash. The hailstones continued to melt, but at the same time, the layer of acidic ash surrounding them began to cement together, forming a strong enough shell to hold its shape as the last of the ice within melted away. In time, the last of the hail reached the surface as the storm faded away, snuffed out by the thickening ash cloud. Ash continued to pile up on the ground for many more hours though, gradually burying the shells of the former hail stones deep enough to keep them well protected.
Millions of years later, as part of public works projects taking place all across the western United States, the deep layer of volcanic ash near Mattawa, Washington was mined in order to blend it into concrete being used on area dams. When the mining was finished, the entire 30-foot deep layer of volcanic tuff (the solidified layer of ash) was exposed and visible. Years later, a geologist studying the layer of volcanic tuff noticed strange shell-like balls near the bottom of the ash layer and took the following picture. Here, 11.81 million years after they first fell from a weakening thunderstorm, through a dense cloud of ash and onto the ground, are essentially the petrified remains of those ancient hailstones.
These are known geologically as accretionary lapilli, but typically have a stony interior instead of a hollow one, as formation on water coated rocks is more common than on hailstones. As a meteorologist, these are extraordinarily fascinating to me, and something I had never knew existed until a couple of weeks prior to writing this blog.
While these fossilized hailstones are interesting by themselves, the story of their formation runs a little deeper than discussed above. The volcanic eruption in southern Idaho was no ordinary volcanic eruption, but an eruption from a supervolcano. And not just any supervolcano, but one driven by the Yellowstone hotspot, the same one that has become famous for powering the awe-inspiring thermal features in the national park of the same name.
If you are wondering how a volcano in southern Idaho is related to Yellowstone, the answer is that the Yellowstone supervolcano hasn't always been located in northwestern Wyoming like it is today. The North American continent has been drifting southwestward over time, while the hotspot has remained stationary. The net result has been a parade of supervolcanic eruptions tracking across the North American continent. While the most recent eruptions from the hotspot have been in northwestern Wyoming if the trend continues, the hotspot will one day be erupting in the middle of Montana, well northeast of Yellowstone National Park.
These supervolcano eruptions produce quantities of ash that dwarf those of any modern-day volcanic eruption. The image below shows the relative size of several supervolcano eruptions compared to more well-known and recent eruptions such as Mount St. Helens and Mount Pinatubo.
So, even though the location of the accretionary lapilli in central Washington was far from the eruption location in southern Idaho, a deep layer of ash was still able to spread that far northwestward because of the massive size of the eruption. We know that the ash came from southern Idaho because the chemical signatures of the volcanic ash match those of the southern Idaho volcano and not those of other area volcanoes.
If you are interested in learning more about accretionary lapilli, super volcanoes, or other related topics, here are a few resources:
Supervolcanoes in the Pacific Northwest: https://www.youtube.com/watch?v=NcreTTI9Rew
This is a great video by Professor Nick Zentner and where I first learned about accretionary lapilli.
Pyroclastic Rocks: https://amzn.to/2VdYQ9r
USGS page on Yellowstone volcano: https://www.usgs.gov/faqs/when-will-yellowstone-erupt-again?qt-news_science_products=0#qt-news_science_products
https://www.volcanodiscovery.com/photoglossary/accretionary_lapilli.html
https://epod.usra.edu/blog/2013/04/accretionary-lapilli.html
https://en.wikipedia.org/wiki/Lapilli
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Additional Notes:
1. The eruption probably occurred during the warm season, but there is no way to know exactly what the weather was like or the day of the year.
2. Given the extent and volume of the resulting ash plume, a plume height over 100,000 feet was very likely.