If you'd been on the Philippines island of Luzon on June 15, 1991, you'd have been forgiven for thinking that the world was ending.
Typhoon Yunya was screaming across the island, but the angry skies were nothing compared with the angry Earth. Mount Pinatubo, an unassuming volcano that no one had even realized was active until a few years prior, was blowing its top. A column of ash spewed 21 miles (34 kilometers) into the atmosphere, opening like an umbrella to form a cloud 249 miles (400 km) across.
On the way back down, Pinatubo's ash mixed with Yunya's rain, creating a concretelike mud that collapsed roofs more than 9 miles (15 km) away. The streets were choked with evacuees, including the scientists in charge of monitoring the volcano, who were reduced to rinsing their ash-streaked windshield with a six-pack of cherry soda.
The eruption of Mount Pinatubo was the second-largest volcanic cataclysm of the 20th century, second only to a 1912 eruption on the sparsely populated Alaskan Peninsula. Mount Pinatubo, on the other hand, threatened tens of thousands who lived on or near its slopes. [In Photos: The Colossal Eruption of Mount Pinatubo]
Nonetheless, only a few hundred people died in the initial eruption, thanks to round-the-clock volcano monitoring, proactive evacuation plans, and a little bit of luck. It was a success that helped cement the importance of close volcano-watching — though scientists are still searching for the unique clues that might warn them that an eruption will be as large as Pinatubo's.
A giant awakens
The first sign that Pinatubo might be restless came in mid-March 1991. Frequent earthquakes shook the area, alarming residents. On April 2, an explosion opened a line of steaming vents along the mountain.
The Philippine Institute of Volcanology and Seismology (PHIVOLCS) rushed to the scene, bearing five seismic monitors. They also called in the United States Geologic Survey's (USGS) Volcano Disaster Assistance Program, a group formed five years earlier to help with volcanic emergencies around the globe.
Pinatubo had not emitted so much as a volcanic burp in living memory, but a look at the area's geology revealed that the mountain could produce very large eruptions, said USGS team leader Chris Newhall, now the volcano group leader at the Earth Observatory of Singapore.
"But we didn't know whether it WOULD erupt, or whether an eruption would be LARGE," Newhall wrote in an email to LiveScience. "So you can't recommend an evacuation right away."
Instead, the team gathered every bit of information they could about the mountain. They deployed additional seismometers, walked around looking at ancient volcanic deposits and flew helicopter missions close to the volcano to see what was going on.
"We're kind of coming into a volcano that no one knew much about," said John Ewert, now the scientist-in-charge at the USGS Cascades Volcano Observatory. "There had been no monitoring, no baseline has been collected. No geologic map, no hazards map, nothing. … There was an element of building the boat and rowing it at the same time."
Buildup to a blowout
On top of the scientific challenge, there was also a massive amount of personal pressure. About 500,000 people lived near Pinatubo, and convincing them that this unassuming, jungle-covered lump was a threat was no small feat. In addition, Clark Airbase, the U.S. facility that became the scientists' command center and home away from home, sat about 9 miles (15 km) from Pinatubo, and a base evacuation was not to be taken lightly.
In other words, if the scientists were going to urge an evacuation, the volcano had darn well better blow. [10 Biggest Volcanic Eruptions in History]
As May turned into June, the volcano continued rumbling and emitting streams of ash. By the morning of June 8, a lava dome appeared on the peak. This was a sign that hot magma was pushing up against the surface, stretching and bulging the land above. By June 9, officials had ordered 25,000 people out of the area. On June 10, American officials made the call to evacuate more than 14,000 service members and their families from Clark Airbase.
The suddenly empty airbase was just one more source of pressure for the scientists, who took the opportunity to move their base of operations to the side of the base farthest from the steaming volcano. USGS volcanologist Richard Hoblitt, now at Cascades Volcano Observatory, described the anxiety wryly.
"If the volcano had not performed, it wouldn't have been very good for our careers, let's put it that way," Hoblitt told LiveScience.
They wouldn't have to wait long. At 8:51 a.m. local time on June 12, the seismometers on the mountain's slopes went haywire as the volcano sent a column of ash and steam 11 miles (19 km) into the air at the speed of an F-15 jet.
As Hoblitt and Ewert watched the volcano blow, they felt relief. Their admonitions — and the evacuation of more than 60,000 people — had not been in vain. But the relief was followed by concern, as after the initial 40-minute eruption, Pinatubo started rumbling again. The volcano was just getting warmed up.
Over the next three days, the volcano spit out three more vertical eruptions and 13 smaller eruptions that produced pyroclastic flows(molten mixtures of ash, gas and rock that can sweep over the landscape at more than 60 miles (100 km) per hour).
And then, on June 15, Pinatubo really let loose. Sometime around 1:42 p.m., the stop-and-go eruptions turned into one long, roaring explosion. Chunks of pumice the size of golf balls fell at Clark Airbase. Ash spewed higher than it ever had before, and lahars, or mudflows, rushed down the slopes in a clatter of banging boulders.
With all of their seismometers swallowed by the volcano's pyroclastic flows, Hoblitt, Ewert and their colleagues evacuated Clark Airbase as Typhoon Yunya dumped rain overhead. Rain-soaked ash streaked their truck's windshield, Hoblitt said, so the scientists wiped it clean with cherry soda, the only beverage left in the airbase commissary that no one wanted to drink. They rode out the night 24 miles (38 km) away at an agricultural college, feeling the earth quake as Pinatubo's summit collapsed into a 1.5-mile (2.5 km) caldera.
By the time Pinatubo's climactic eruption ended, a snowlike blanket of ash at least a half-inch (1 centimeter) thick coated 4,660 square miles (7,500 square km) of the island of Luzon. Over the next year, the ash spewed into the atmosphere by Pinatubo would lower global temperatures by an average of 0.9 degrees Fahrenheit (0.5 degrees Celsius). The volcano would continue smoldering and smoking for months. And for years afterward, monsoon season would wash ash down the mountain's slopes in devastating lahars, raising the initial death toll of the eruption from 200 or 300 to more than 700.
But for as large as Pinatubo's eruption was, the death toll was remarkably small. At least 20,000 lives were saved. Aggressive monitoring and public information campaigns did their job, PHIVOLCS director Raymundo S. Punongbayan wrote in "Fire and Mud," a USGS collection of papers about Pinatubo's eruption.
But a big part of the Pinatubo evacuation's success is owed to the volcano's timing, Hoblitt said.
"The fact is that the pace of the activity was just about right for people to react to it," he said. "There was a steady escalation and that escalated up and culminated in an eruption."
Many eruptions are much more strung out, he said, which makes people impatient and unlikely to listen to warnings that the big one is coming.
Pinatubo is not expected to erupt again for hundreds of years, assuming it keeps to its usual schedule. But if the mountain did blow today, Ewert said, monitoring it would be slightly easier, thanks to 20 years of technological advances. Today, the Internet makes it easier to consult with far-off scientists, he said, and GPS and radar satellites make it possible to measure changes in the ground caused by moving magma.
But despite the new tools, Hoblitt said, it's still difficult to predict exactly when a volcano will blow and how large its eruption will be.
"This is still not a 'turn the crank and solve the problem' skill," Hoblitt said. "There's a lot of experience and art that goes into dealing with restless volcanoes."
You can follow LiveScience senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.
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.
"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 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.
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.
Science news from NBCNews.com
- Cosmic rays may spark Earth's lightning
All lightning on Earth may have its roots in space, new research suggests.
- How our brains can track a 100 mph pitch
- Moth found to have ultrasonic hearing
- Quantum network could secure Internet
- Cosmic rays may spark Earth's lightning
"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 and 22 miles high 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 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, 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.
© 2012 LiveScience.com. All rights reserved.