The world’s largest volcanic eruption to happen in the past 100 years was the June 15, 1991, eruption of Mount Pinatubo in the Philippines.
Bursts of gas-charged magma exploded into umbrella ash clouds, hot flows of gas and ash descended the volcano’s flanks and lahars swept down valleys. The collaborative work of scientists from the U.S. Geological Survey (USGS), and the Philippine Institute of Volcanology and Seismology (PHIVOLCS) saved more than 5,000 lives and $250 million in property by forecasting Pinatubo's 1991 climactic eruption in time to evacuate local residents and the U.S. Clark Air Force Base that happened to be situated only 9 miles from the volcano.
U.S. and Filipino scientists worked with U.S. military commanders and Filipino public officials to put evacuation plans in place and carry them out 48 hours before the catastrophic eruption. As in 1991 at Pinatubo, today the USGS is supported by The US Agency for International Development’s (USAID) Office of U.S. Foreign Disaster Assistance to provide scientific assistance to countries around the world though VDAP, the Volcano Disaster Assistance Program. The program and its partners respond to volcanic unrest, build monitoring infrastructure, assess hazards and vulnerability, and improve understanding of eruptive processes and forecasting to prevent natural hazards, such as volcanic eruptions, from becoming human tragedies.
Seismographs at Mount Pinatubo Monitoring Observatory site at Clark Air Base, Philippines. Public domain
Monitoring: 10 weeks before the eruption
At Pinatubo, the volcanic unrest began April 2, 1991, with a series of small steam explosions. In Manila, Dr. Raymundo Punongbayan, Director of PHIVOLCS, dispatched a team to investigate a fissure that opened on the north side of the volcano and was emitting steam and sulfur fumes. PHIVOLCS set up a seismograph and began monitoring earthquakes. Dr. Punongbayan also called his friend, Dr. Chris Newhall, at the USGS. The two scientists began working on how to get the USGS-USAID Volcano Disaster Assistance Program team to the Philippines to help monitor Pinatubo.
Three weeks later, Newhall, along with VDAP volcanologists Andy Lockhart, John Power, John Ewert, Rick Hoblitt and Dave Harlow, began unpacking 35 trunks of gear at temporary quarters on Clark Air Base. The seismic drum room was a maze of wires and cables; the daily drum roll of seismicity posted on the walls. Instrumentation was drawn principally from a permanent supply of specialized equipment kept ready for volcano crises under the auspices of the USGS Volcano Hazards Program and the joint USGS-USAID VDAP. They nicknamed the place PVO—the Pinatubo Volcano Observatory.
U.S. Air Force helicopter dropping off USGS and PHIVOLCS scientists and gear to be installed on the flanks of Mount Pinatubo.Public domain
With air assistance from the U.S. military, the PHIVOLCS-VDAP team installed seven telemetered seismic sites, two telemetered tiltmeters to measure ground deformation, and used a COSPEC (correlation spectrometry) instrument to measure sulfur dioxide gases that would presage arrival of new magma deep in the volcano’s plumbing. All efforts were focused on answering the questions — will Pinatubo erupt catastrophically, and when?
Volcanologists are first to admit that forecasting what a volcano will do next is a challenge. In late May, the number of seismic events under the volcano fluctuated from day-to-day. Trends in rate and character of seismicity, earthquake hypocenter locations, or other measured parameters were not conclusive in forecasting an eruption. A software program called RSAM (real-time seismic amplitude measurement), developed in 1985 to keep an eye on Mount St. Helens, helped scientists analyze seismic data to estimate the pent-up energy within Pinatubo that might indicate an imminent eruption.
There was no existing volcanic hazards map of the Pinatubo volcano, so one was quickly compiled by the PHIVOLCS-VDAP team to show areas most susceptible to ashflows, mudflows and ashfall. The map was based on the maximum known extent of each type of deposit from past eruptions and was intended to be a worst-case scenario. The map proved to forecast closely the areas that would be devastated on June 15.
USGS volcano seismologist David Harlow conducts analysis of Mount Pinatubo seismicity. Public domain
USGS scientists set up instrumentation to monitor Mount Pinatubo. Public domain
USGS and PHIVOLCS scientists loading volcano monitoring gear into an Air Force helicopter. Public domain
Evacuation: 48 hours before the first ash eruption
The Clark Air Base sprawled over nearly 10,000 acres with its western end nestled in the lush, gently rolling foothills of the Zambales Mountains–only 9 miles (14 km) east of Mount Pinatubo. Military housing was located on the “Hill” closest to the volcano, with nearly 2,000 homes, elementary schools, a middle school, a new high school, a convenience store and restaurant. At the time, the population of Clark and nearby cities of Angeles, Sapangbato, Dau and Mabalacat numbered about 250,000. The PHIVOLCS-VDAP team developed an alert system and distributed it to civil defense and local officials as a simple means to communicate changing volcanic risk.
Senior base officials listened to daily briefings and put together plans to evacuate. Everyone agreed that if there were an evacuation, people must be moved to an area where they would be safe—not statistically safe, but perfectly safe. The location chosen was 25 miles (40 km) away at Naval Station Subic Bay and Naval Air Station Cubi Point.
Beginning June 6, a swarm of progressively shallower volcano-tectonic earthquakes accompanied by inflationary tilt (the “puffing up” of the volcano) on the upper east flank of the mountain, culminated in the extrusion of a small lava dome, and continuous low-level ash emission. Early June 10, in the face of a growing dome, increasing ash emission and worrisome seismicity, 15,000 nonessential personnel and dependents were evacuated by road from Clark to Subic Bay. By then, almost all aircraft had been removed from Clark and local residents had evacuated. The USGS and PHIVOLCS scientists did their own “bugout,” moving the monitoring observatory to an alternate command post located just inside the base perimeter near the Dau gate, an additional five miles (8 km) away from the volcano.
The June 12 eruption of Mount Pinatubo, three days before the much larger eruption on June 15, 1991. Public domain
On June 12 (Philippine Independence Day), the volcano’s first spectacular eruption sent an ash column 12 miles (19 km) into the air. Additional explosions occurred overnight and the morning of June 13. Seismic activity during this period became intense. The visual display of umbrella-shaped ash clouds convinced everyone that evacuations were the right thing to do.
Eruption: June 15, 1991
When even more highly gas-charged magma reached Pinatubo's surface June 15, the volcano exploded. The ash cloud rose 28 miles (40 km) into the air. Volcanic ash and pumice blanketed the countryside. Huge avalanches of searing hot ash, gas and pumice fragments, called pyroclastic flows, roared down the flanks of Pinatubo, filling once-deep valleys with fresh volcanic deposits as much as 660 feet (200 meters) thick. The eruption removed so much magma and rock from beneath the volcano that the summit collapsed to form a small caldera 1.6 miles (2.5 km) across.
If the huge volcanic eruption were not enough, Typhoon Yunya moved ashore at the same time with rain and high winds. The effect was to bring ashfall to not only those areas that expected it, but also many areas (including Manila and Subic Bay) that did not. Fine ash fell as far away as the Indian Ocean, and satellites tracked the ash cloud as it traveled several times around the globe. At least 17 commercial jets inadvertently flew through the drifting ash cloud, sustaining about $100 million in damage.
With the ashfall came darkness and the sounds of lahars rumbling down the rivers. Several smaller lahars washed through Clark, flowing across the base in enormously powerful sheets, slamming into buildings and scattering cars as if they were toys. Nearly every bridge within 18 miles (30 km) of Mount Pinatubo was destroyed. Several lowland towns were flooded or partially buried in mud.
The volcanologists at the Dau command post watched monitoring stations on Pinatubo fail, destroyed by the eruption. They watched telemetry go down but then come back up – a sign that a pyroclastic flow was headed down valley and temporarily interfering with the radio links. They moved to the back of a cinderblock structure to maybe provide a little more protection from hot gas and ash; there was nowhere else for them to go. Fortunately, the flow stopped before it reached the building.
World Airways DC10 airplane sitting on its tail because of the weight of wet volcanic ash. Public domain
NASA's TOMS satellite image showing how Pinatubo's ash had circled the globe by June 30, 1991. Public domain.
The eruption of Mount Pinatubo sent lahars and pyroclastic flows down the mountain, wiping out bridges and other infrastructure downstream.Public domain
Aftermath: Adapting and learning
The post-eruption landscape at Pinatubo was disorienting; familiar but at the same time, totally different. Acacia trees lay in gray heaps, trees and shrubs were covered in ash. Roofs collapsed from the tremendous stresses of wet ash and continuing earthquakes. No matter which way one turned, everything looked the same shade of gray.
Damage from volcanic ash fall at Clark Air Force Base from the June 15, 1991 eruption of Mount Pinatubo. Public domain
Damage to Clark Air Force Base airplane hangers collapsed under the weight of wet volcanic ash from the eruption of Mount Pinatubo in 1991. Public domain
Most of the deaths (more than 840 people) and injuries from the eruption were from the collapse of roofs under wet heavy ash. Many of these roof failures would not have occurred if there had been no typhoon. Rain continued to create hazards over the next several years, as the volcanic deposits were remobilized into secondary mudflows. Damage to bridges, irrigation-canal systems, roads, cropland and urban areas occurred in the wake of each significant rainfall. Many more people were affected for much longer by rain-induced lahars than by the eruption itself.
By the end of 1991, and into 1992, more than 23 USGS geologists, seismologists, hydrologists, and electronics and computer specialists had each spent between three and eight weeks at Pinatubo and helped PHIVOLCS advise community and national leaders and those at-risk and studying the volcano to better understand what causes giant eruptions and how to forecast them, whether in the U.S. or abroad.
Much weaker but still spectacular eruptions of ash occurred occasionally through early September 1991. From July to October 1992, a lava dome grew in the new caldera as fresh magma rose from deep beneath Pinatubo. For now, the volcano is quiet, and the U.S. transferred Clark Air Force Base to the Philippine government in November 1991. The base has been repurposed as a trade and commercial center with large airport.
What would be different if the situation occurred today? Consider that in 1991 there was no easy access to the internet, no connections to other data sets or scientists other than by telephone. The first popular web browser was a couple of years off, CD writers cost around $10,000, and scientific data and analysis were shared mainly by fax. The Pinatubo Volcano Observatory in 1991 was a self-contained unit; data from the monitoring network were radioed to it and the analysis was done by scientists on-site. Today, data received at PVO would be forwarded to colleagues in the U.S. and elsewhere for more sophisticated analysis with the results quickly transmitted back to PVO. Satellite data measuring ground temperatures, gas emissions, and inflation or deflation of the volcano would be sent to PVO where it would be integrated with other data sources to develop forecasts and inform hazard mitigation efforts. Tools and expertise would no longer be confined to what was physically at the observatory, but instead a global support group would be available to aid the response. Monitoring instruments have also improved greatly in performance while at the same time dropping in price and power consumption. There is no doubt that with the communication and monitoring tools available to us today, we would learn much more about the buildup to the eruptions and have more and better data to guide our decision-making.
For successful natural hazard mitigation, it all comes down to the right combination of monitoring data and scientific skill, and then just as important, scientists and public officials who are effective at communicating with each other and with the public who may be in harm's way. At Pinatubo, the quick deployment of monitoring instruments and preparation of a volcanic hazards map by the PHIVOLCS and VDAP team helped to better understand the precursors of volcanic activity and provided the basis for accurate warnings of impending eruptions. The willingness of base commanders, public officials and citizens to take the necessary precautions lessened the risk from this catastrophic eruption.
Pinatubo 1991 Case Study, Volcanic Ash Impact & Mitigation
The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines, USGS Fact Sheet 113-97
Benefits of Volcano Monitoring Far Outweigh Costs–The Case of Mount Pinatubo USGS Fact Sheet 115-97
FIRE and MUD: Eruptions and Lahars of Mount Pinatubo, Philippines, edited by Christopher G. Newhall and Raymundo S. Punongbayan, 1996
NOVA: In the Path of a Killer Volcano, TV program
The Ash Warriors, by C.R. Anderegg
The International Association of Volcanology and Chemistry of the Earth's Interior’s (IAVCEI) video for crisis education
USGS-USAID Volcano Disaster Assistance Program
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.
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"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.
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