Scientists have known for decades that hidden under those impressive vistas at sites such as [Death Valley] and [Yellowstone National Park]are magma pools that under the right conditions can trigger explosive eruptions.
Now, new research is changing scientists' understanding of the timing of those eruptions, and prompting them to call for greater monitoring of sites to help save lives when the next big volcano explodes.
Two recent papers highlight the shift. One looked at a Death Valley volcano thought to be 10,000 years old and found it last erupted just 800 years ago, and is still an eruption danger. The other found that large caldera volcanoes, such as the one under Crater Lake in Oregon, can recharge in a matter of decades, rather than the thousands of years previously thought.
"The understanding of the timing of eruptions and the timing of the building up to eruptions is changing," says Margaret Mangan, the scientist in charge of volcano monitoring in California for the [U.S. Geological Survey]. "These two papers are very nice examples of good scientific work."
One thing that's coming to light is that eruptions are often clustered, with "long stretches of inactivity punctuated by periods of activity that can go on for years," Mangan says.
The first paper looked at the Ubehebe Crater (you-bee-HE-bee) at the northern end of California's Death Valley. It's about half a mile wide and 700 feet deep. It was long believed to have been caused by a volcanic eruption sometime in the past 10,000 years or so.
However, researchers recently looked at beryllium in the rocks and were able to date the last series of eruptions to just 800 years ago. They say the ingredients necessary for another eruption are all still there.
Ubehebe Crater is the result of what's known as a phreatomagmatic (free-at-oh-mag-MAT-ick) eruption. That means that it has a huge pocket of molten rock, or magma, deep below it. When it begins to push to the surface and comes into contact with water, superhot steam is created, building up pressure until it explodes.
It had been thought that the eruptions would occur only during wet climate periods, and as Death Valley is famously dry now, there was little concern. But using U.S. Geological Survey data, the scientists show that the current water table may be just 500 feet below the surface of the crater. The paper was published this month in the journal [Geophysical Research Letters].
The relatively recent eruption means that the magma underground, which can take thousands of years to cool, is likely still hot. When water and hot magma come into contact, it can produce something "like a very large bomb going off," says Brent Goehring, a geochemist at the department of Earth and Atmospheric Sciences at Purdue University in West Lafayette, Ind., and one of the paper's authors.
Another worry is that caldera volcanoes, such as Mount Pinatubo in the Philippines and Krakatoa in Indonesia, may be able to blow much more quickly than previously believed.
Caldera volcanoes consist of large underground lakes of magma. As more magma builds up, the pressure builds and the magma starts getting pushed upward through cracks in the Earth's surface. When the pressure gets too great, it explodes.
Caldera volcanoes typically have a long quiet period prior to eruptions. Writing in Wednesday's edition of the journal Nature, researchers looked at the eruption of the Santorini volcano in Greece around 1,600 B.C., which released as much as 12 cubic miles of magma.
By analyzing feldspar crystals formed within the magma and then ejected during the eruption, the researchers found that the volcano's magma chamber grew by as much as 10% in the final few decades before it blew, says Tim Druitt, a volcanologist at Blaise Pascal University, Clermont-Ferrand, France, and senior author on the paper.
For this reason, scientists are calling for greater monitoring, including satellite surveillance, to detect ground swelling.
There are several large and still active calderas in the United States, including the one under Yellowstone National Park. All are closely monitored. What worries the researchers are other unmonitored calderas around the world with the potential to send huge clouds of ash into the atmosphere, causing massive ecological and climate damage.