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Hot spot: scientists prove beneath Yellowstone and find a seething volcanic basin that could one day blow its top
Science World, Feb 25, 2002 by Rachel Rivera
TOWERS OF scalding water gush up from the ground hundreds of meters into the sky. Bubbling mud ponds belch steaming gases from below. A scene from Middle Earth in the movie Lord of the Rings? No, it's Yellowstone National Park, home to more than 10,000 hot springs and geysers (superheated fountains) that draw hordes of gaping tourists from all over the world.
What many tourists don't realize is that cradling this awesome landscape is a caldera, the craterlike basin of a volcano, that is still active--and could one day explode again. Feeding the volcano is the "Yellowstone hot spot," a sizzling column of partly melted rock that extends about 20 kilometers (12.5 miles) across and 2,900 km (1,802 mi) down, nearly halfway to the center of Earth. This hot spot created the caldera and provides the heat source that fuels the park's famed geysers and hot springs.
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Scientists are taking a closer look at the hot spot (one of about 40 in the world) to learn how Earth's heated interior molds the planet's surface. One surprising discovery to date: The hot spot actually causes the volcanic basin to "breathe," or rise and sink in cycles, like a poorly baked cake in an oven. Even the bottom of tranquil Yellowstone Lake, where tourists lazily fish trout, bulges and deflates. In other words, the rise and fall triggered by the Yellowstone hot spot could one day lead to a monster explosion of steam from many geysers and a tidal wave in the lake. "You don't want to be around when something happens," says Ken Pierce of the U.S. Geological Survey (USGS) in Denver.
RESTLESS GIANT
Three major volcanic explosions gave birth to Yellowstone as it exists today. The first one, more than 2 million years ago, was one of the largest eruptions in the planet's history, blanketing the entire Western U.S. with ash nearly one meter (3.3 feet) thick. A second eruption took place about 1.3 million years ago, followed by a third, 640,000 years ago, which formed the present-day caldera. Extending about 48 km (30 mi) across, the caldera now occupies center stage at Yellowstone (see map, p. 18).
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"The caldera was like a blister underneath Earth's crust [outermost layer]," explains park ranger Mark Williams. A mass of molten rock, or magma, heated deep within Earth, thrust up the crust and formed a bulge. When the pressure got too great, the magma exploded and the dome collapsed in on itself to create a caldera. But the Yellowstone hot spot continues to push up against the crust, causing the caldera to heave and sink (see diagrams, p. 19).
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HEAVY BREATHER
With the lung capacity of a whale five times the size of New York City, an "inhaling" (rising) caldera can lift 320 sq km (124 sq mi) of land as much as 30.5 m (100 ft).
In the last 9,000 years the caldera has "inhaled" and "exhaled" about five times, says Pierce. By measuring the ancient shorelines of Yellowstone Lake, he determined that the caldera rose and fell each time by an average of 7.6 m (25 ft). Pierce says it's currently exhaling, deflating at a rate of about 1 cm (0.4 in.) a year. (Yet other evidence is contradictory. Newly installed sensors suggest the caldera may continuously rise and fall. The sensors reveal that in 1987 the caldera started to fall by 2.5 cm, or 1 in., a year. But in 1995 the caldera began to bulge again.) Pierce is waiting to see if the current release of pressure could result in a violent explosion at the surface. An exhale sometimes triggers a blast of boiling hot water and steam that might scald an area about 1.6 to 3.2 km (1 to 2 mi) across.
Meanwhile, scientists watch for other signs of volcanic eruption. Geologists have scanned 40 percent of the Yellowstone Lake bottom with sound waves, remote-control devices, and scuba gear. Their discovery: The lake bed's rise and fall could also unleash steam explosions and wild waves. And a fault line (crack in Earth's crust) has been found to run right into the lake bed. "We could see earthquakes, and the ground would swell tens of meters," says USGS geologist Robert Christiansen. "There's no reason for tourists not to go to Yellowstone right now," he quickly adds. "But another eruption will occur sometime within the next 100,000 years."
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Cross-Curricular Connection
History: Research and report on some of the most disastrous volcanic eruptions in history.
Did You Know?
* Each year Yellowstone's caldera emits as much carbon dioxide as 10 to 20 fossil fuel plants, killing many animals and small patches of forest.
* Most hot spots lie beneath the ocean floor. Island chains, like the Hawaiian Islands, are often formed when magma erupts from underwater hot spots.
* In May 2001, Yellowstone was made one of the nation's official volcano observatories. Together with four other U.S. Geological Survey observatories in Alaska, California, the Pacific Northwest, and Hawaii, it monitors 43 of approximately 70 potentially hazardous volcanoes in the U.S.
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