Release Date: JUNE 13, 2016 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

Release Date: JUNE 13, 2016

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 (drum reorders) in on a table, with seismograms on the wall
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. Navy helicopter dropping off scientists and gear in open grassy field
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.

Photo of scientist sitting at a computer with seismograms on the wall behind him.
USGS volcano seismologist David Harlow conducts analysis of Mount Pinatubo seismicity. Public domain
Scienitists installing antenna at Pinatubo volcano
USGS scientists set up instrumentation to monitor Mount Pinatubo. Public domain
scientists loading gear into helicopter
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.

Distant view of erupting volcanic ash cloud rising over cars in parking lot in forground
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.

 

 

Damaged jumbo jet airplane covered in volcanic ash, with collapsed rear end
World Airways DC10 airplane sitting on its tail because of the weight of wet volcanic ash. Public domain
TOMS satellite image showing how ash had circled the globe by 30 Jun 91
NASA’s TOMS satellite image showing how Pinatubo’s ash had circled the globe by June 30, 1991. Public domain.

 

 

 

 

 

 


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