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Yellowstone National Park, Part 1 - Page 2

© Geoff Habiger

If you look carefully at a map showing the physical features of the area around Idaho and Wyoming, you will see a ring of mountains circling the area of the Snake River Plain. This ring has the appearance of a wake produced by a boat, and in a way, that’s exactly what they are. Around 16.5 million years ago (Ma) what we call the Yellowstone hot spot first erupted under the present location of the Oregon-Nevada border. Since then, the hot spot has remained in place, but the North American plate has moved southwest, inch-by-inch. Every few million years, as the plate moved, a new eruption would occur northeast of the previous eruption. Each eruption was a large, cataclysmic eruption of a caldera, producing craters tens of miles in size. As the continent moved on, less viscous basaltic magmas would erupt in the wake of the calderas, creating the smooth terrain of the Snake River Plain. This pattern of eruptions and the movement of the North American Plate have created the Yellowstone we see today.

Around 2 million years ago, the hot spot “reached” it’s present location in Yellowstone. At that time, it produced one of the largest eruptions geologists have studied. The eruption was 250 times larger than the eruption of Mt. Pinatubo in 1991, releasing 600 cubic miles of volcanic ash and debris. (Mt. Pinatubo only released 2.4 cubic miles of ash and debris.) This was the first caldera eruption in Yellowstone, but wouldn’t be the last. 1.3 Ma another eruption occurred, forming the Island Park caldera to the west of Yellowstone, just inside Idaho. This eruption was only 28 times as large as Mt. Pinatubo, releasing 67 cubic miles of volcanic debris. The last caldera eruption at Yellowstone occurred 630,000 years ago. It overlapped the first caldera eruption, creating a 45-by30-mile-wide crater and throwing 240 cubic miles of ash and debris into the air. This last eruption has nearly destroyed all evidence of the first two Yellowstone calderas making size estimates of their calderas nearly impossible. We do not know how destructive each of these caldera eruptions were, and do not have anything to reference them to, since no human has ever witnessed a large caldera eruption. The closest example we have is the eruption of Tambora, in Indonesia in 1815. Its eruption threw 36 cubic miles of ash and debris high into the atmosphere. So much as ash was present in the atmosphere that global temperatures fell causing it to snow in June in New England and causing rivers to freeze, making 1816 the Year Without a Summer. We can estimate how extensive the eruptions were, by locating the extent of the ash fall from the eruptions. Ash several yards deep fell on most of the western United States, and fell as far east as Minnesota, Iowa, and Missouri, and as far south as Mexico. In comparison, the ash from Mt. St. Helens, which erupted in May of 1980, traveled only about 700 miles.

Porcelain Basin

Lower Falls of the Yellowstone River

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