Since about mid-July, the earth beneath the volcano has been shifting in a sign that magma could be rushing into the caldera’s main chamber.
Since then, there have been roughly 2,500 small-scale earthquakes recorded near the volcano, the largest stretch on record. Previous estimates had assumed that the process that led to the eruption took millenniums to occur.
The same estimates that USGS based their warning on.
As the New York Times explains, the Yellowstone caldera is a behemoth far more powerful than your average volcano. It has the ability to expel more than 1,000 cubic kilometers of rock and ash at once, 2,500 times more material than erupted from Mount St. Helens in 1980, which killed 57 people. That could blanket most of the United States in a thick layer of ash and even plunge the Earth into a volcanic winter.
As the Times points out, scientists expect a supervolcano eruption to scar the planet once every 100,000 years.
To reach their conclusion, the team of scientists spent weeks at Yellowstone’s Lava Creek Tuff – a fossilized ash deposit from the volcano’s last supereruption, where they gathered samples and analyzed the volcanic leftovers. The analysis allowed the scientists to pin down changes in the lava flow before the last eruption. The crystalline structures of the rocks recorded changes in temperature, pressure and water content beneath the volcano just like tree rings do. (source)
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