And before this, the eruption history gets murkier, with the latest research suggesting at least four other super-eruptions occurred over the past 12 million years.
Finding evidence for specific eruptions is difficult, with significant volcanic deposits tending to overlap.
The latest study saw researchers attempt the most in-depth analysis of North America’s ancient volcanic rock tracts ever conducted.
The team correlated widely-separated volcanic deposits in Idaho and Nevada with seven characteristics, including the rock’s colour, age, its chemical composition and the polarity of magnetic minerals.
The research revealed a smattering of volcanic deposits previously attributed to a series of small eruptions resulted from two gargantuan ones.
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The oldest — called the McMullen Creek super-eruption — occurred approximately nine million years ago over a 4,600-square-mile (12,000 square km) stretch of what is now southern Idaho, the researchers found.