Then, on June 15, the volcano blew its top in the second largest volcanic eruption of this century. The eruption caused high-speed avalanches of hot ash and gas (pyroclastic flows), giant mudflows (lahars), and a cloud of volcanic ash that spanned hundreds of miles across.
The largest volcanic eruption in recent history, the blast of Mount Pinatubo in the Philippines, affected climate around the world, causing temperatures to drop and Asian rain patterns to shift temporarily.
That eruption occurred 20 years ago this month. And unfortunately, volcanic eruptions like it will be difficult to predict, although larger events with much greater impacts on climate will likely come with more notice.
If Pinatubo sticks to its record — its prior eruption occurred about 500 years ago — we won't have much to worry about for a while, according to Richard Hoblitt, a geologist at the United States Geological Survey's Cascades Volcano Observatory. [In Photos: The Colossal Eruption of Mount Pinatubo]
"It's most likely that it's going to stay in repose again for hundreds of years," Hoblitt said, "but there's always a possibility that it can deviate from that pattern. These volcanoes are not metronomes; they tend to vary on a theme. Though we don't expect to see one again in our lifetime, it's not impossible."
The Pinatubo eruption pushed an umbrella-like cloud of rock, ash and gas more than 22 miles (35 kilometers) into the sky above the Philippines, and planet-cooling aerosols left by the gas lingered in the air around the globe for as long as three years.
Scientists agree that similar eruptions around the world are inevitable. Mont Pelée, Katmai, Mount St. Helens, El Chichón — the 20th century was peppered by significant eruptions. Much larger giants may awaken one day, potentially altering the climate in dramatic ways. The Yellowstone Caldera produced a super-eruption about 640,000 years ago, with enough force to blanket much of the North American continent in a layer of ash and chill the planet for years. And massive volcanic activity about 250 million years ago, unlike any humans have known, may have warmed the planet and prompted the largest mass extinction in the history of life.
In the future
Scientists knew little about Pinatubo's potential to erupt when small earthquakes and steam explosions began in spring of 1991, but they quickly realized it could produce large eruptions. [History's Biggest Volcanic Eruptions]
Nearly a million Filipinos and two U.S. military bases shared the island of Luzon with the volcano, making the decision to evacuate a must.
"Evacuation recommendations can never be made lightly, and here the pressure to get it right, 'just in time,' was intense," Chris Newhall, who was the USGS scientist leading the response team, wrote in an email.
Even so, hundreds of people died in the eruption.
Like Pinatubo, the most dangerous future eruptions would come from volcanoes near large populations, according to Philipp Ruprecht, postdoctoral researcher at Columbia University's Lamont-Doherty Earth Observatory.
These include Vesuvius, which devastated the ancient city of Pompeii and now has 550,000 neighbors living in the "red zone," and Washington's Mount Rainier, where even a small eruption could melt glaciers on the mountain and create mud flows, according to Ruprecht.
Although scientists can recover past records of volcanic activity, predicting the future is difficult.
"I wouldn't be surprised if one happened tomorrow, but I wouldn't be surprised if another didn't occur for another 20 years," said Alan Robock, a climatologist at Rutgers University. "Nobody can predict how often they occur, and nobody can predict, even after the volcano starts to rumble, if it's even going to erupt with a big eruption or not. All we can do is look at past data and see how often they have occurred."
Big, but not too big
Volcanoes merit their own ranking system, called the Volcanic Explosivity Index (VEI), which runs from 0 to 8, with each score indicating an increase of about a factor of 10. The Pinatubo blast scored a 6. The VEI describes the magnitude of explosive eruptions based on a number of factors, including the volume of magma and the height of the ash cloud the volcano produces. VEI does not factor in impact on climate.
In the hierarchy of volcanoes, Pinatubo falls behind the 1815 eruption of Tambora in Indonesia, which scored 7, and the most recent super-eruption of the now-slumbering Yellowstone volcanic basin, which topped out the scale at 8.
Another VEI-8 eruption at Yellowstone or elsewhere would certainly create havoc, according to Jacob Lowenstern, the scientist in charge of the Yellowstone Volcano Observatory for the United States Geological Survey.
"More ash would be deposited close to Yellowstone, but even far away there could be millimeters to centimeters of ash. Most estimates predict several degrees of temperature drop for several years, though even for super-eruptions, the effects aren't expected to last for more than a decade," Lowenstern wrote in an email to LiveScience.
You don't need to stay up at night worrying over a super-eruption at Yellowstone; the odds are tiny and, because the volcano has been quiet for a long time, there would be earthquakes warning of an impending eruption, Lowenstern said.
Pinatubo's global reach
During the eruption of Pinatubo on June 15, 1991, a cloud 684 miles wide (1,100 kilometers) and 22 miles high (35 kilometers) formed over the volcano, carrying about 17 megatons of sulfur dioxide into the stratosphere, according to researchers led by Stephen Self of the University of Hawaii at Manoa writing in the USGS publication "Fire and Mud."
While the larger particles of ash fell out of the sky fairly quickly, the sulfur dioxide became fine droplets, or aerosols, of sulfuric acid. These prevented inbound solar energy from reaching the planet's surface, which caused global cooling. The cloud of aerosols created by Pinatubo created spread around the globe in about three weeks and ultimately caused a dramatic decrease in the amount of solar energy reaching the planet, according to the researchers.
As a result, from 1992 to 1993, large parts of the planet cooled as much as 0.7 degrees Fahrenheit (0.4 degrees Celsius), they wrote.
These tiny droplets remained suspended for one to three years, but the effects they produced in that time were complex, according to David Pyle, a professor of earth sciences at the University of Oxford.
Parts of the Northern Hemisphere experienced relatively cool summers for a couple of years, while in other places winter temperatures were slightly warmer. "When you cool the atmosphere, you change the pattern of weather systems," Pyle said.
This has implications for rainfall. A study of tree rings showed that after big eruptions, including those of Pinatubo and Tambora, large parts of Mongolia and southern China consistently received less rainfall while the mainland of Southeast Asia received more.
"Pinatubo is a fantastic case study, and there are still developing hypotheses based on observations of Pinatubo," Pyle said.
In addition to the scale and the contents of the eruption plume are other factors determining the amount of global cooling caused by a volcano. The location of the eruption matters, because the height of the stratosphere — the layer of atmosphere that the aerosols must enter to have any global impact — varies with latitude, as do air circulation patterns and the amount of light reflected by the Earth's surface.
Climate patterns matter, too. After Mexico's El Chichón erupted, its potential cooling effect was counteracted by an active El Niño, according to Robock.
An agent of change
Volcanoes also have the potential to warm the planet's surface by the carbon dioxide they emit. The amount of that greenhouse gas from a single eruption would cause only a trivial amount of warming, but over long time scales, the carbon dioxide of multiple eruptions could build up, Robock said.
Some scientists have controversially linked volcanic emissions with mass extinctions, including the largest extinction event in Earth's history, the Permian-Triassic extinction. Dubbed the Great Dying, it wiped out 90 percent of all marine species about 250 million years ago. At about the same time, massive volcanic eruptions occurred over a swath of Siberia, caused by a rising plume of abnormally hot rock.
The carbon dioxide these eruptions released would have caused the Earth's surface to warm and triggered a cascade of ultimately deadly effects, including the stagnation of the oceans, according to Paul Wignall, a University of Leeds professor of paleoenvironments.
It is difficult, however, to compare the volcanic eruptions of recorded history with the cataclysmic eruptions that occur irregularly every 20 million to 50 million years or so. Those eruptions would have been preceded by hundreds of thousands of years of warning as hot magma welled up beneath the continent, Wignall said.
LiveScience writer Stephanie Pappas contributed to this story.