The Yellowstone Caldera
Unbeknownst to most, Yellowstone National Park sits on a subterranean chamber of molten rock and gasses so vast that it is arguably one of the largest active volcanoes in the world. A magma chamber not far below the surface fuels all the volcanic attractions that Yellowstone is famous for. The last major eruption at Yellowstone, some 640,000 years ago, ejected 8,000 times the ash and lava of Mount St. Helens. It is alive and well today, and is the scientific basis for the hilarious volcanic explosion seen in the movie 2012 that blew up Woody Harrelson and, somehow, NOT John Cusack.
The Long Valley Caldera
Second only to Yellowstone in North America is the Long Valley caldera, in east-central California. The 200-square-mile caldera is just south of Mono Lake, near the Nevada state line. The biggest eruption from Long Valley was 760,000 years ago, which unleashed 2,000 to 3,000 times as much lava and ash as Mount St. Helens, after which the caldera floor dropped about a MILE, according to the U.S. Geological Survey. Some of the ash reached as far east as Nebraska. What worries geologists today was a swarm of strong earthquakes in 1980 and the 10-inch rise of about 100 square miles of caldera floor. Then, in the early 1990s, large amounts of carbon dioxide gas from magma below began seeping up through the ground and killing trees in the Mammoth Mountain part of the caldera. When these sorts of signs are present, it could mean trouble is centuries, decades, or even YEARS away, say volcanologists.
The 175-square-mile Valles caldera forms a large pock in the middle of northern New Mexico, west of Santa Fe. It last exploded 1.2 million and 1.6 million years ago, piling up 150 cubic miles of rock and blasting ash as far away as Iowa. As with other calderas, there are still signs of heat below: hot springs are still active around Valles. Geologists suspect the cause of Valles caldera has something to do with how the western United States' portion of the North American tectonic plate is being pulled apart.
The 1,080-square-mile Toba caldera in North Sumatra, Indonesia is the only supervolcano in existence that can be described as Yellowstone's "big" sister. About 74,000 years ago, Toba erupted and ejected several thousand times more material than erupted from Mount St. Helens in 1980. Some researchers think that Toba's ancient super eruption and the global cold spell it triggered might explain a mystery in the human genome. Our genes suggest we all come from a few thousand people just tens of thousands of years ago, instead of from a much older, bigger lineage — as the fossil evidence testifies. Both could be true if only a few small groups of humans survived the cold years following the Toba eruption.
New Zealand's Taupo caldera has been filled by water, creating what many describe as one of the world's most beautiful landscapes, but the lake itself was created by a massive eruption 26,500 years ago. The caldera — the collapsed and subsided basin left after the huge eruption — became today's lake. But Taupo is not dead. The 485-square-mile caldera let loose again in the year A.D. 181, with estimates of ash and magma reaching as high as 22 cubic miles. Today, there are plenty of signs of current volcanic activity in the form of hot springs and venting.
One of the most recently troubling calderas in the world is the 150-square-mile Aira caldera in southern Japan, on the edge of which sits the city of Kagoshima. 22,000 years ago 14 cubic miles of material burped out of the ground and formed the Aira caldera, which is now largely Kagoshima Bay. That is equal to about 50 Mount St. Helens eruptions. The Sakura-jima volcano, which forms part of the Aira caldera, has been active on and off for the past century and still causes earthquakes today, indicating that the caldera itself is far from sleeping.