MT. PINATUBO ERUPTION: JUNE 15, 1991

In early 1991, Mt. Pinatubo, a volcano north of Manila on the Philippine island of Luzon, had been dormant for almost 500 years. Few geologists would have guessed that it would produce one of the world’s most explosive eruptions in the twentieth century. Indications of unrest started a few months before the June 1991 eruption, but the size and impact of the eruption were completely unexpected. During the June 12-15 eruptive climax, the top of the mountain was blown off, lowering the elevation by roughly 150 m. About 8 to 10 km2 of material (Scott, et al., 1996) spewed out of the volcano onto the surrounding slopes. The eruption forced evacuation of more than 50,000 people, and effectively shut down two major US military bases (Clark Air Force Base and Subic Bay Naval Base); it was ultimately responsible for taking several hundred human lives.

Aerial Photo of Mt. Pinatubo eruption in 1991 (Photo credit unknown

Satellite Images of Mt. Pinatubo

This photograph was taken by Space Shuttle astronauts in March 1982, 9 years before the eruption. At the time of this photograph, the elevation of Mt. Pinatubo was 1745 m. Manila, a city of 10 million people, lies 100 km southeast of Pinatubo. Photo courtesy of earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center, NASA image STS003-10-567.

This view was taken by Shuttle astronauts looking westward over Mt. Pinatubo about one year after the volcano erupted; it shows much of the regional damage. The summit of Pinatubo was blasted away, the forests were blown down, and hundreds of meters of loose sand and gravel rained down on the mountain’s upper slopes. The light-colored mud-filled rivers radiating away from the summit were the scene of floods of hot volcanic slurries streaming down the mountain slopes. Photo courtesy of earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center, NASA image STS050-52-26.

Astronauts observed Pinatubo just a few days after heavy monsoon rainfalls triggered lahars, or volcanic debris flows down the western slope of the mountain. This is the first detailed view of Pinatubo after the eruption. Fresh lahars on the Bucao River (top left) created extensive floods and swept away whole villages. NASA image STS046-75-79A.

The areas covered by lahars in early August 1992 (shown in red, and mapped from STS046-75-79A) are overlaid on a March 14,1995 image of Pinatubo (STS067-721a-31). By 1995, more than 400 km2 of land was buried by lahars. Photos courtesy of earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center.

Before the eruption

Before the eruption, Mt. Pinatubo was a forested, deeply dissected and unimposing mountain on Luzon’s Bataan Peninsula. Although the upper slopes were steep and not well suited for agriculture, the lower slopes were heavily populated and supported extensive rice fields.

During the eruption, the upper slopes of the mountain suffered immediate destruction. The climactic explosions of June 14–16, 1991, blasted away the summit of Pinatubo, blew down surrounding forests, and rained hundreds of cubic meters of loose sand and gravel down on the mountain’s upper slopes. Floods of hot volcanic slurries were responsible for long-lasting damage downslope.

Astronauts did not observe the June 1991 eruption of the volcano—but they have routinely monitored subsequent changes around Mt. Pinatubo. The eruption had two major environmental effects which are readily documented from low-Earth orbit: the distribution of vast quantities of sulfur dioxide aerosols into the stratosphere; and the post-eruption mudflows, or lahars, which are recognized as the major natural hazard from the eruption.

Aerosols, Floods, and Mudslides

Vast quantities of volcanic gases [mostly sulfur dioxide (SO2)] were blown into the atmosphere during the eruption. The globally distributed aerosols were responsible for several phenomena, from spectacular sunsets to global cooling.

The immediate consequences of the eruption—ashfall, mudflows, crop devastation, and destruction of villages and roads—affected tens of thousands of people. However the worst impact on the people of the region surrounding Pinatubo resulted from the lahars (volcanic mudflows). When the top of the mountain was blasted off, ash rained down on the upper slopes of Mt. Pinatubo, accumulating as loosely consolidated gravels and muds up to a few hundred meters thick. So much material was heaped on the mountain that all the natural river drainages were clogged. The main eruption occurred during a typhoon, and heavy rainfall flushed the volcanic muds and gravels downslope as lahars. Soon, all the downstream reaches of streams flowing down the mountain were also clogged—new drainage networks had to be reestablished. Periodically, loosely dammed drainages broke through catastrophically and swept downslope as more destructive mudflows. Described as floods with the consistency of wet cement, the lahars jumped river banks, and swept away or buried everything (towns, roads, bridges) in their paths (Rodolfo, 1995).

In subsequent months and years, heavy rains (frequent during the monsoon months of June through November) mobilized more lahars. Villages in river floodplains on the lower slopes of the mountain were repeatedly threatened. In the first five years after the eruption, at least 400 km2 of land was covered by mudflows. More than 53,000 people were permanently displaced, including 20,000 indigenous Ayta people (Bautista, 1996; Pinatubo Observatory Team, 1991). These people remain removed from their cultural and spiritual home. Further, the extensive loss of agricultural land and irrigation sources, the destruction of roads and bridges, repeated floods and disease have affected more than 1,000,000 Filipino people.

Municipalities have attempted to mitigate future lahar damage by constructing earthen levees along the edges of rivers, with hopes of channeling mudflows. Unfortunately, several levees have failed, such as along the Santo Tomas River on the southwest side of the mountain, and repeatedly along the Pasig-Potrero River on the southeast side of Pinatubo (Rodolfo, 1995; Newhall and Punongbayan, 1996; Umbal, 1997). Levees concentrated the lahar flows, occasionally resulting in catastrophic breakthroughs and floods. Although less frequent today, Pinatubo lahars still occur after exceptionally heavy rains.

Astronauts continue to photograph the changing landscape around Mt. Pinatubo. Several types of changes are being monitored: The lahars (mudflows) are easily identified on photographs from space: the light, even-colored river drainages radiate outward from the center of the mountain. An excellent time series of images taken before and after monsoon seasons from 1992 through 1996 captures downstream changes on the major river systems draining Mt. Pinatubo. In a sense, Mt. Pinatubo serves as a laboratory for studying how drainage networks become established over a very short time. Coastal changes continue. As large quantities of materials moved downstream and to the coast of the South China Sea, shorelines around river mouths changed. Delta formation and longshore drift of sediment around Santo Tomas and Bucao River systems create rapidly changing shorelines. (Siringan and Ringor, 1995)

Civil engineering efforts continue—in later years, efforts to construct more permanent levees and dams to retain the muds can be seen. The region around the Pasig Potrero has been walled off by levees and sediment retention structures. Astronaut photographs document the ponding of mudflows behind these structures. Forest and agriculture regrowth is underway. Ten years after the eruption, the mountain slopes are becoming green with vegetation. It will be decades before the tropical jungle is fully reestablished on the upper slopes of the mountain, but the vantage point from space allows for excellent documentation of the changes in vegetation. As lahar activity diminishes, the fertile mudflow deposits are being cultivated for crops such as rice.

Today, astronauts and cosmonauts aboard the International Space Station Alpha and the Space Shuttles are on the constant lookout for volcanic eruptions in other parts of the world. The crews use their long-term observations of Mt. Pinatubo as an example of the types and magnitudes of changes that can be monitored over other volcanic regions.

By Cindy Evans, Johnson Space Center