Topographic Volcano Maps by Remote Sensing

The flight of NASA’s Shuttle Radar Topography Mission (SRTM) in February, which collected topographic data over most of the Earth’s surface, is the latest contribution to a growing data set that volcanologists use to study lahars and other volcanic hazards in the Philippines. Mapping of lahars builds on previous remote-sensing studies in which scientists trace probable routes of lava flows and measure how much lava an eruption produced.

Scientists at the University of Hawaii and the Philippine Institute of Volcanology and Seismology are using Mt. Pinatubo as a testbed to demonstrate the value of remote sensing for lahar monitoring. Using observations of the volcano from spaceborne and airborne instruments along with measurements made on the ground, they are honing their mapping technique and extending it to other lahar-prone volcanoes.

Peter Mouginis-Mark of the University of Hawaii has been studying volcanoes up close and from a great distance for more than 25 years. Since many of the volcanoes he studies have never been explored on foot (they are on Mars), he is familiar with the unique capabilities of remote-sensing observations and believes they are well-suited to studying lahar hazards.

"Lahars are one of the few natural hazards that evolve over time," says Mouginis-Mark. "The changes they produce in the landscape occur over a wide geographic area. Remote-sensing observations let you watch the changes evolve over a very large area, something you can’t do from the ground."
 

Lahar deposits 300 meters wide fill a section of the Sacobia-Bamban River valley, seen here winding from Mt. Pinatubo toward the northeast. Most of these deposits are just a few years old, since a major change in the river system in 1993. full size (130K) (Photo by Peter Mouginis-Mark)

Filipino colleague Ronnie Torres of the Philippine Institute of Volcanology and Seismology in Quezon City agrees. "It is already clear that field campaigns alone cannot cope with the magnitude and dynamic situation of post-eruption hazards at Mt. Pinatubo," says Torres. As part of his hazard-monitoring work, Torres is keeping an eye on the rising level of the lake at the summit of the volcano. A breakout of that lake could create another major lahar.

Mouginis-Mark has used data from several radar mapping instruments to study Italy’s Mount Vesuvius and volcanoes in Hawaii, the Galapagos Islands, and Africa. Mt. Pinatubo’s lahar deposits are the first he has studied with remote-sensing data. In addition to his research interest in volcanoes, Mouginis-Mark is chief scientist of the Pacific Disaster Center, a U.S. Department of Defense initiative dedicated to using remote sensing to monitor natural hazards in the Pacific Rim and Indian Ocean.

For lahars to be considered a major hazard after an eruption, two conditions must be met: the eruption threw out a great deal of ash and rock, and the volcano is in a part of the world that receives heavy rainfall. Mt. Pinatubo and several other volcanoes in tropical regions such as Central America and Indonesia meet these requirements.

A lahar is defined as a rock-laden flood made up of 40 percent or more by weight volcanic debris. A lahar flows like wet concrete and is very fast, outstripping a normal water-only flow. Lahars have been clocked at 65 kilometers per hour (40 mph).

Walking on a lahar deposit is like "walking on a pebbly beach that is constantly wet," says Mouginis-Mark, who hiked over miles of Pinatubo lahar with Torres and a team of scientists in November 1999. When subsequent floods erode these loose, gravelly deposits, they cut deep, steep ravines that can easily crumble if you stand too close to the edge.

The lahar hazard usually disappears from around a volcano in just a few years if the eruption produced little debris. Once the rubble and ash are carried downslope or stabilized by the regrowth of vegetation, lahars are no longer a danger. The immense size of the Mt. Pinatubo eruption means that it will be lahar-prone for some time to come. Mouginis-Mark says that the volcano may take a total of at least 20 years to rid itself of lahars.

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Lahars can quickly reshape the landscape downslope of a volcano long after an eruption ends. Three years after Mt. Pinatubo exploded, a major lahar inundated the Pasig-Potrero river valley (dark area, lower right). These false-color radar images taken in 1994 from NASA’s space shuttle show the region east of the volcano at the beginning (April 14) and end (Oct. 5) of the annual rainy season. (NASA image, P-44729)