When Rivers of Rock Flow
 
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On the night of September 22, 1994, a river of rock returned with a vengeance to the Filipino village of San Antonio. The villagers had seen their first "lahar" just three years before when Mount Pinatubo erupted for the first time in centuries. These raging slurries of volcanic rock and ash and water, powered by torrential summer rains and typhoons, sliced roads, buried villages, and covered farmland beneath thick layers of volcanic rubble.

The lahars seemed to diminish a year after the 1991 eruption, and people returned to the village. But in the summer of 1994, high up the river valley that runs down the side of Mt. Pinatubo toward San Antonio, a lahar cut off a segment of the Pasig-Potrero River behind a dam of loose volcanic debris. A lake began to form behind this dam as the rains fell on the east side of the volcano. On September 22, after a moderate rain, the swollen lake finally burst the dam and tore down the valley toward San Antonio.

When the villagers heard the rumbling of tons of rock and water, they rushed to higher ground or climbed onto their rooftops. In the dark they could only watch helplessly as the lahar spread through the village. Many homes were buried up to their rooftops. Two dozen people in the path of the lahar never made it to safety.

The explosion of Mt. Pinatubo on June 15, 1991, just 60 miles northwest of Manila, was the world’s largest volcanic eruption in nearly 80 years. Due to active monitoring of the volcano by scientists and warnings of an impending eruption, only a few hundred people died during the main eruption. Most casualties were caused by the collapse of buildings under the weight of rain-soaked ash and the lahars spawned by the eruption.
 

 

Map
Mt. Pinatubo is part of a chain of major volcanoes in the Philippines. Its eruption in June 1991 was the world’s largest in nearly 80 years. The most recent Philippine volcano to erupt was Mayon, in February 2000.

House buried by a lahar

But the hazards posed by Mt. Pinatubo continue to this day, even though the volcano has remained quiet since 1991. Volcanologists estimate that damaging lahar flows may continue for another 10 years. More than 5 cubic kilometers (1 cubic mile) of volcanic ash and debris were deposited on the slopes around Mt. Pinatubo. Entire river valleys were filled. Each year part of that new volcanic layer is sent churning downslope by the heavy monsoon rains and typhoons that hit central Luzon between June and October. Scientists estimate that as much as 60 percent of the rocky debris from the eruption will eventually erode into the lowlands around the volcano. That’s enough sediment to cover the state of Rhode Island in a layer 1 meter deep.

Monitoring the continuing dangers from lahars is difficult. Whereas the onset of a volcanic eruption can be monitored by observing a relatively small area, lahars can cover tens of square kilometers and are a moving hazard that changes location and shape with each heavy rainfall. Volcanologists are now turning to remote-sensing observations from spacecraft and airplanes to track this dynamic hazard.

next Topographic Volcano Maps by Remote Sensing

 

Volcanic rubble and ash mixed with torrential rain produces devastating lahars that can bury villages, roads, and bridges. This house in the Pasig-Potrero river valley east of Mt. Pinatubo was completely buried in mudflows, which are now being eroded away. (Photo by Peter Mouginis-Mark, University of Hawaii at Manoa)

 

Topographic Volcano Maps by Remote Sensing

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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 from the Air
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)

Radar Comparison

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.

next A Decade of High-Flying Radar
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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)

 

aerial photo of lahar

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A Decade of High-Flying Radar

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Remote sensing of volcano topography took off in the early 1990s with the launch of several satellites carrying radar-mapping instruments. Radar sends rapid pulses of electromagnetic waves to the Earth’s surface and carefully times the return of the faint signals that bounce off the surface. Each returned signal yields a precise measure of the distance from the satellite to a point on Earth. These measured distances are combined into digital elevation maps that show the topography of the land surface. The wavelengths that the radars use pierce clouds and fog and can take measurements night and day.
 

   
TOPSAR Overview of Mt. Pinatubo

With an advanced technique called radar interferometry, two radar images are taken of the same area from slightly different angles. When combined they show subtle differences in elevation. This technique is capable of mapping surface features as small as 10-25 meters across and 2-5 meters deep.

The first civilian spaceborne imaging radar was launched in 1978 on NASA’s short-lived Seasat spacecraft, which was lost after less than a year. The launch of the European Remote Sensing (ERS-1) satellite in 1991 returned radar to space. It was followed by a twin spacecraft (ERS-2), the Japanese Earth Resource Satellite (JERS-1), and the Canadian Radarsat.

NASA flew an Earth-mapping radar on two space shuttle flights in 1994. The Spaceborne Imaging Radar C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) mapped Mt. Pinatubo in April and October, providing topographic maps of lahar deposits at the beginning and end of that year’s wet season. The SRTM mission was an upgrade of this radar system.

Radar maps of volcanoes have also been made by aircraft over small areas. The Topographic Synthetic Aperture Radar (TOPSAR) instrument flew aboard a NASA research aircraft in November 1996 to map Pacific Rim volcanoes including parts of Mt. Pinatubo. TOPSAR produced very detailed topographic maps with surface features just 10 meters across and as shallow as 2 meters deep.
 

 

The most detailed digital elevation maps of Mt. Pinatubo produced to date were made by the Topographic Synthetic Aperture Radar (TOPSAR) instrument flown in November 1996 on a NASA aircraft. The area inside the white box was the site of damaging lahar flows. The image below shows this area in more detail. (Image courtesy Peter Mouginis-Mark and Harold Garbeil, University of Hawaii at Manoa)

TOPSAR detail

With a single digital topographic map of a volcano, scientists can identify potential hazards from lava flows and lahar flows, information that can be used to advise local authorities on specific hazardous areas. With two radar maps of the same area taken some time apart, researchers can measure how much the topography has changed and calculate how much volcanic material has moved downstream.

After the same TOPSAR radar flies over Mt. Pinatubo again this spring, Mouginis-Mark hopes to produce a "topographic difference" map by subtracting one image from another to see how the lahar deposits have changed in four years. "By measuring this ‘sediment budget’ we hope to be able to say when the upper slopes are stable and the risk to people downstream is decreased," says Mouginis-Mark.

Torres is using data from the two 1994 space shuttle flights to study the location of lahar hazards along the Pasig-Potrero river system on the eastern flank of Mt. Pinatubo. Although these radar maps can only pick out features about 25 meters across and vertical features are nearly imperceptible, preliminary results from this work show good agreement between the landscape changes seen from space and those surveyed on the ground. Torres plans to use the more detailed maps from this spring’s TOPSAR airborne radar flights to locate flood-prone areas adjacent to river beds that have been raised by lahar sediments.

next The Essential Role of Ground Surveys
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This high-resolution TOPSAR map, taken in 1996, shows the spread of lahar sediments upstream of a manmade dike built on the Pasig-Potrero River. (Image courtesy Peter Mouginis-Mark and Harold Garbeil, University of Hawaii at Manoa)

 

The Essential Role of Ground Surveys

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A critical part of demonstrating that a mapping instrument in orbit or in the air has accurately seen what’s on the surface is to directly compare the remote data with measurements made on the ground. In November 1999 Mouginis-Mark led a group of University of Hawaii scientists on an expedition to Mt. Pinatubo to map some of the steep valleys cut into lahar deposits and the broad sediment outflows spread across the landscape.

   
Scientists explore an eroded lahar

On their way by helicopter from Clark Air Base to begin their expedition on the west flank of the volcano, the scientists flew over the Pasig-Potrero River and the summit of Mt. Pinatubo. Mouginis-Mark remembers seeing "complex networks of deep, deep canyons cut into the lahar. This type of very steep erosion is unique to lahars. This must have been what the mythical Greek labyrinth looked like."

Later as the team hiked over the lahar-covered landscape, they visited a village partially buried by lahar deposits. Mouginis-Mark recalls stepping directly into the second story of a centuries-old church half buried by lahar deposits that the villagers were excavating.

The team surveyed the dimensions of several "erosional valleys" using a laser range finder. The valleys are a few kilometers long, from 50-80 meters wide, and 30-50 meters deep. These precise measurements will be compared with SRTM digital maps to see how well SRTM can see into these valleys, which have nearly vertical walls. Mouginis-Mark expects to begin this work this summer when he receives his first batch of SRTM Mt. Pinatubo data.

"We don’t have a complete digital topographic data set for the volcano," says Mouginis-Mark. "SRTM is our mission of choice for a data set that will give us a quantum step forward in our knowledge of the erosion of the volcano." He will also use extensive field measurements and TOPSAR data of Hawaiian volcanoes to validate the accuracy of the SRTM maps.

Then it will be on to studying other tropical volcanoes with SRTM maps, in particular volcanoes in Java and other parts of Indonesia. "Many of these volcanoes won’t be studied on the ground due to personal safety concerns related to the local political situation. Our Mt. Pinatubo work will give us a good calibration point that we can use to evaluate how effective the SRTM data are in studying volcanoes in tropical environments." The other advantage of remote-sensing observations of volcanoes, says Mouginis-Mark, is that it allows volcanologists to study whole volcanic regions, not just one volcano at a time, which has been the traditional focus of earthbound scientists.
 

 

Scientists from the Philippines and the University of Hawaii hike across an "erosional valley" toward the nearly vertical edge of a massive 1991 lahar deposit over 60 meters high. The team measured the dimensions of these valleys to find out how well these features can be seen by instruments in space. (Photo by Peter Mouginis-Mark)

Karisimbi Volcano

One limitation of radar mapping systems flown to date is that they do not penetrate thick forest cover very well. The radar data from the airborne TOPSAR, for example, only slightly pierces the forest canopy. Most of the signal bounces off of the dense tree tops before reaching the ground. Consequently, the maps made over heavily forested areas "see" the surface somewhere between the top of the trees and the surface.

The upcoming launch of another NASA mission will help fix this limitation. The Vegetation Canopy Lidar (VCL) spacecraft, a joint project with the University of Maryland, will simultaneously measure where the tops of trees are and where the ground is using lidar, a radar-like instrument that uses a laser beam. Mouginis-Mark plans to use VCL data to subtract out the forests from the SRTM data to generate a "bald Earth" topographic map of volcanoes that pose significant local hazards.

  • References
  • Massonnet, D. and Feigl, K. L., 1998: Changes in the Earth’s Surface, Reviews of Geophysics, 36, 4, pp. 441-500.
  • Newhall, C. G. and Punongbayan, R. S., editors, 1997: Fire and Mud: Eruptions and Lahars of Mt. Pinatubo, Philippines, Philippine Institute of Volcanology and Seismology (Quezon City) and University of Washington (Seattle). http://pubs.usgs.gov/pinatubo/
  • Newhall, C., Stauffer, P.H., and Hendley, J. W., 1997: Lahars of Mount Pinatubo, Philippines; U. S. Geological Survey Fact Sheet 114-97, U. S. Geological Survey Cascades Volcano Observatory, Vancouver, Wash.
  • Rowland, S. K., MacKay, M. E., Garbeil, H., and Mouginis-Mark, P. J., 1999: Topographic Analyses of Kilauea Volcano, Hawaii, From Interferometric Airborne Radar, Bulletin Volcanology, 61, pp. 1-14.
  • Self, S. and Mouginis-Mark, P. J., 1995: Volcanic Eruptions, Prediction, Hazard Assessment, Remote Sensing, and Societal Implications, Reviews of Geophysics, 33 supplement (U.S. National Report to the IUGG, 1991-1994)
  • Related Web Site
  • Dr. Peter Mouginis-Mark's photos of Mt. Pinatubo.

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By comparing digital topographic maps made before and after an eruption, scientists can precisely measure the amount of material produced by a volcano. This technique, used here to calculate the thickness of lava flows on Karisimbi volcano in Zaire, will be used on Mt. Pinatubo to calculate how much volcanic debris has been carried off the volcano by lahars. (Image courtesy Mary MacKay and Harold Garbeil, University of Hawaii at Manoa)