The simmering volcano has produced major eruptionsâ€”each a thousand times more powerful than Mount St. Helens’s 1980 eruptionâ€”three times in the past 2.1 million years. Yellowstone’s caldera, which covers a 25- by 37-mile (40- by 60-kilometer) swath of Wyoming, is an ancient crater formed after the last big blast, some 640,000 years ago.
(See “When Yellowstone Explodes” in National Geographic magazine.)
Since then, about 30 smaller eruptionsâ€”including one as recent as 70,000 years agoâ€”have filled the caldera with lava and ash, producing the relatively flat landscape we see today.
But beginning in 2004, scientists saw the ground above the caldera rise upward at rates as high as 2.8 inches (7 centimeters) a year. (Related: “Yellowstone Is Rising on Swollen ‘Supervolcano.'”)
The rate slowed between 2007 and 2010 to a centimeter a year or less. Still, since the start of the swelling, ground levels over the volcano have been raised by as much as 10 inches (25 centimeters) in places.
“It’s an extraordinary uplift, because it covers such a large area and the rates are so high,” said the University of Utah’s Bob Smith, a longtime expert in Yellowstone’s volcanism.
Video: Yellowstoneâ€”World’s First National Park.
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