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Mount Pinatubo Revisited; A Study of Lahar Erosion

browse graphicGravelly lahar terraces from debris flows. The original level of 1991 pyroclastic flow deposits is on top of the hill showing patches of unvegetated deposit.
At dawn on June 15, 1991, a cataclysmic eruption began with a tremendous explosion that destroyed ten deserted villages. This eruption deposited approximately 5 to 7 km3 of volcanic fragments in pyroclastic flows on the slopes of the volcano and over neighboring towns and agricultural areas. It is this material that continues to threaten structures and lives in the area, in the form of lahars (debris flows) during heavy rainstorms. The lahars from the Mount Pinatubo volcano have been particularly damaging to the surrounding area. This set of slides shows how the disaster that began at Pinatubo in 1991 continues to threaten the population in the area. Mount Pinatubo is located on the Island of Luzon in the Philippines, about 100 km northwest of Manila. The volcano, with K-Ar datings of approximately 1.1 million years, and with the youngest carbon-14 dating of + 400 years B.P. (before present), is the youngest volcano in the western Luzon volcanic arc. On April 2, 1991, Pinatubo, which had been rumbling for months, stirred to life. Over the next six weeks earth tremors and minor explosions occurred. These natural warnings led to the evacuation of personnel at Clark Air Base and of 55,000 people in nearby towns and villages. At dawn on June 15, 1991, a cataclysmic eruption began with a tremendous explosion that destroyed ten deserted villages. The eruption deposited approximately 5 to 7 km3 of volcanic fragments in pyroclastic flows on the slopes of the volcano and over neighboring towns and agricultural areas. It is this material that continues to threaten structures and lives in the area, inthe form of lahars (debris flows) during heavy rainstorms. The Pinatubo deposits are subdivided into two general groups based on the lithology and age of emplacement: the "ancestral" and the "modern." The ancestral Pinatubo (+/- 1 million to +/-35,000 years B.P.) is an andesite-dacite stratovolcano of mostly laval flows and breccia deposits oflaharic origin. On its slopes are numerous elongated to sub-rounded hills made up of breccia, created mostly by the ancestral lahars. The modern Pinatubo (+/- 35,000 years to present) shows signs of repeated, very explosive eruptions which have produced large volumes of pumiceouspyroclastic flows. Pyroclastic flows, also known as nuee ardentes, or glowing avalanches, are extremely hot (+/-1,000 degrees Celsius), often incandescent, highly fluid, gravity-driven density currents of gas and volcanic fragments that sweep down slope and travel at hurricane speed (+/-100 km per hour). Pyroclastic flows are generated when the density of the rising column of volcanic fragments and gas exceeds that of the surrounding atmosphere. Gravity causes a portion or all of the column to collapse and flow down the flanks of the volcano. Most of the 1991 pyroclastic flow deposits were emplaced during the June 15 eruption. The pre-eruption magma temperature of Pinatubo was about 800 degrees Celsius and the temperature of the emplaced pyroclastic flow was on the order of 600 degrees Celsius. The deposits are non-welded, dry, and very loose. The accumulated thickness of the pyroclastic flows varies, depending on the proximity to the crater and the pre-eruption morphology. It reaches more than 200 meters along deep pre-eruption valleys. The pyroclastic flow deposits of 1991 affected eight major watersheds around the slopes of the volcano and radically altered the hydrological regimes, leading to unprecedented amounts of erosion and sediment delivery in the form of destructive lahars. Lahars predominantly occur during the rainy season in the monsoon period, which lasts from June until November. Long-duration and high-intensity rainfall, associated with the occurrence of strong typhoons, are responsible for the production of large-magnitude lahars. Other factors contributing to the rapid erosion of lahars are: failure of lahar dams, secondary explosions produced by rapid vertical and lateral erosion of the pyroclastic flow deposits, and stream and river capture bringing deposits into new drainage systems. The rapid erosion or removal of the pyroclastic flow deposits has been one of the major social and scientific concerns after the 1991 eruption ofthe Pinatubo Volcano. The lahars from Pinatubo have resulted in loss of lives and damage to properties in areas surrounding the volcano. About 500,000 persons were left homeless and indirect impacts such as flooding and isolation have affected more than 1.3 million people in 39 different towns and four large cities. About 1,000 km2 of prime agricultural lands are atrisk. Lahars will continue to be an important hazard on the lower slopes of Pinatubo for a number of years.

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    Distribution Formats
    • TIFF
    Distributor DOC/NOAA/NESDIS/NGDC > National Geophysical Data Center, NESDIS, NOAA, U.S. Department of Commerce
    Point of Contact Heather McCullough
    DOC/NOAA/NESDIS/NGDC > National Geophysical Data Center, NESDIS, NOAA, U.S. Department of Commerce
    (303) 497-3707
    Heather.McCullough@noaa.gov
    Documentation links not available.
    Originator
    • DOC/NOAA/NESDIS/NGDC > National Geophysical Data Center, NESDIS, NOAA, U.S. Department of Commerce (comp)
    Publisher
    • National Geophysical Data Center
    Date(s)
    • publication: 1994
    Edition: First
    Dataset Progress Status Complete
    Data Update Frequency: Not planned
    Purpose: Make available Damage Photos for research and education
    Time Period: 1991-04-02  to  1993-10-06
    Spatial Bounding Box Coordinates:
    N: 15.08
    S: 15.08
    E: 120.21
    W: 120.21
    Spatial Coverage Map:
    Themes
    • EARTH SCIENCE > SOLID EARTH > Volcanoes > Eruption Dynamics
    • EARTH SCIENCE > SOLID EARTH > Volcanoes > Lava
    • EARTH SCIENCE > SOLID EARTH > Volcanoes > Magma
    • EARTH SCIENCE > SOLID EARTH > Volcanoes > Pyroclastics
    • EARTH SCIENCE > SOLID EARTH > Volcanoes > Volcanic Ash/Dust
    • INFOTERRA > Lithosphere > Seismic activity > Seismic activity
    • INFOTERRA > Lithosphere > Volcanoes > Volcanoes
    • WDC/MGG, BOULDER > World Data Center for Marine Geology and Geophysics, Boulder
    • DOC/NOAA/NESDIS/NGDC > National Geophysical Data Center, NESDIS, NOAA, U.S. Department of Commerce
    Places
    • Asia > Philippines > Luzon Island
    • Asia > Philippines > Zamables > Pinatubo, Mount
    Use Constraints
    • Access Constraints: None Use Constraints: None Distribution Liability: While every effort has been made to ensure that these data are accurate and reliable within the limits of the current state of the art, NOAA cannot assume liability for any damages caused by any errors or omissions in the data, nor as a result of the failure of the data to function on a particular system. NOAA makes no warranty, expressed or implied, nor does the fact of distribution constitute such a warranty.
    Access Constraints
    • Access Constraints: None Use Constraints: None Distribution Liability: While every effort has been made to ensure that these data are accurate and reliable within the limits of the current state of the art, NOAA cannot assume liability for any damages caused by any errors or omissions in the data, nor as a result of the failure of the data to function on a particular system. NOAA makes no warranty, expressed or implied, nor does the fact of distribution constitute such a warranty.
    Fees
    • $25.00 plus handling and shipping outside the USA None $25.00 plus handling and shipping outside the USA
    Lineage Statement Lineage statement not available.

    Metadata Last Modified: 2011-04-06

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