Before a cataclysmic 1991 eruption, Pinatubo was an unassuming mountain peak in the Philippines.
Mount Pinatubo shows early signs of activity in April 1991. The volcano, located on the Philippine island of Luzon, had been quiet for centuries, and scientists knew little about its eruptive history.
Monitoring Ground Swell
Electronic tiltmeters are being installed along the rim of Pinatubo's caldera to monitor the ground's inflation (a sign that magma is reaching the surface), on June 1, 1991.
Looking for clues
Field assessments of the volcano's activity were undertaken just before the colossal eruption.
A large eruption occurred at Pinatubo on June 12, sending up a cloudy plume that reached an altitude of nearly 12 miles (19 km). This was the first in a series of powerful eruptions that would culminate on June 15 with the largest land eruption living history has seen.
The Big Day
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 eruption plume from the June 15 eruption (shown here from Clark Air Force Base) shot some 25 miles (40 kilometers) into the air, creating a giant mushroom cloud in the middle to lower stratosphere and injecting twice as much sulfur dioxide into the air as the 1982 eruption of El Chichón, Mexico. The resulting sulfate aerosols spread rapidly around the Earth in about three weeks, reaching global coverage by about one year after the eruption, according to the USGS.
The June 15 eruption created a 1.5-mile-wide (2.5 km) collapse caldera (shown here on June 22, 1991) and filled valleys around Pinatubo with pyroclastic-flow deposits. The new summit elevation of Mount Pinatubo dropped from its pre-eruption elevation of 5,725 feet (1,745 meters) above sea level to 4,872 feet (1,485 m).
This view to the west from Clark Air Base shows Pinatubo's lateral blast cloud of eruption at 5:55 a.m. on June 15.
Ash-covered cars at Clark Air Force Base, on June 16, 1991.
Cars and people traverse the flooded river after mudflows wiped out the bridges after the Pinatubo eruption in June 1991.