Almost 17 million years ago, the deep plume of partly molten rock known as the Yellowstone hot spot first breached the surface in an eruption near what is now the Oregon-Idaho-Nevada border.
As North America drifted slowly southwest over the hot spot, there were more than 140 gargantuan caldera eruptions — the largest kind of eruption on Earth — along a northeast-trending path that is now Idaho’s Snake River Plain.
The hot spot finally reached Yellowstone about 2 million years ago, yielding three huge caldera eruptions about 2 million, 1.3 million and 642,000 years ago.
Two of the eruptions blanketed half of North America with volcanic ash, producing 2,500 times and 1,000 times more ash than the 1980 eruption of Mount St. Helens in Washington state. Smaller eruptions occurred at Yellowstone in between the big blasts and as recently as 70,000 years ago.