Mount Pinatubo had likely been dormant for hundreds of years. There had been no historical records of volcanic eruptions. Local residents in this part of the Philippines hardly believed Pinatubo was a volcano, so it was difficult to convince them to evacuate once it began showing signs of an eruption throughout the spring of 1991. When it did erupt explosively on June 15, 1991, it was one of the largest volcanic eruptions of the 20th century.
The ash cloud rose 40 kilometers high, and volcanic ash blanketed the region. Making the disaster worse was the arrival of Typhoon Yunya, which hit on the same day. The heavy rains from the storm sent flows of mud and volcanic debris rushing down the mountainside in all directions. Rice and sugarcane fields were smothered. Rooftops collapsed from the weight of ash saturated with rain. Nearly all bridges within 29 kilometers of the mountain were destroyed.
Today, the mountain is relatively quiet, and about 300 meters shorter than it was before the eruption.
Landsat’s historical record reveals the changes and regrowth as it happens, something that can’t be witnessed from the ground. Furthermore, images from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor on NASA’s Terra satellite add more information about the changes to the mountain and surrounding region. Data from these satellites can help us analyze a larger area in a much shorter amount of time than ground surveys, providing valuable information for local decision making.
After the eruption, many areas near the volcano were stripped of vegetation. The series of images reveals the regrowth of vegetation, but it progressed at different rates. The eastern side recovered the fastest, where the area was mainly affected by falling ash. Wind and rain removed the ash, and most areas on this eastern side recovered to almost pre-eruption levels in about 7 years.
The mountain’s western side experienced the worst of the pyroclastic flows, causing regrowth to be slower and more variable. In fact, some areas are still bare.
A remarkable feature of the series of images, even in the most recent images, is that mudflows can still be seen trailing away from the mountain’s summit more than 25 years after the eruption. Runoff from monsoon rains and typhoons continued to erode and redistribute the pyroclastic deposits years after the 1991 eruption.
Another change visible at this scale is the summit itself. The eruption caused the summit to collapse into a caldera about 2.5 kilometers wide. Water has collected in the crater to form a small lake.
Landsat’s infrared bands set up a clear contrast between vegetation and lahar deposits. The images make it clear that the lahar hazards continued, and many occurred in inaccessible areas. Vegetation shows up as bright green, while the sediment and bare ground from lahars is pink. It also shows the progression of the revegetated areas on the mountain’s slopes.
This progression is aided by additional scenes from Terra’s ASTER sensor. ASTER uses similar infrared bands to Landsat, and its 15-meter resolution reveals slightly more detail than Landsat’s 30-meter resolution.
The Pasig-Potrero River drains the east side of Mount Pinatubo, just south of Clark Air Force Base. Following the 1991 eruption, pyroclastic flows buried about one-third of the Pasig-Potrero watershed.
A series of lahars that occurred in the years after the eruption made the Pasig-Potrero channel wider. After the eruption image from July 1991 (really just a brown and gray mess under a few puffy clouds), a bright purple-pink lahar appears downstream on the Pasig-Potrero River. The sediments are redistributed over time, until the October 1994 image shows a larger change to the river. A lahar alongside a previous one more than doubled the area covered by sediment. The 1996 image shows more clearly even further lahar action with bright pink hues. Recent images show the reestablishment of farm fields, though the land and river course are altered from what they were before the 1991 eruption.
The 1996 image indicates the beginning of dike construction. These dikes, along with a reinforced dam built along the southern slopes, create a sediment retention area. The dikes stand 5 to 15 meters tall.
These images also hint at the lingering hazard in this region. A growing population is indicated by the urban growth of the city of Angeles in the upper center of the images. This city, with a 2016 population of about 900,000, is near Clark Air Force Base; the long straight lines are the runways of the air base. Clark Air Force Base closed soon after the Pinatubo eruption. However, it reopened to U.S. forces in June 2012, when it was called Clark Air Base. A new highway was also completed by 2007, seen in the image winding past the air base and to the lower left.
In early 2017, clouds covered the Pasig-Potrero River area in all available Landsat images. The image ASTER captured on February 23, however, was clear. With many cloudy days in this part of the world, these sensors complement each other, filling in when the other sensor detects mostly clouds.
Even more than two and a half decades after the eruption, lahar hazards may continue. Landsat and ASTER help monitor changes caused by these hazards and how the changes to the land affect the population.
The lahars produced by Pinatubo’s eruption blocked the flow of the Mapanuepe River and created a new lake, named Mapanuepe Lake. The formation of the lake flooded the small towns of Aglao, Buhawen, and Pili. Breakout lahars continued to be a threat in the years after the lake formed, but artificial channels help to stabilize the lake’s water level.
In the series of images that accompany this section, Pinatubo’s summit crater is in the upper right corner. In the 1989 image, the maroon colors indicated moisture in the soils southwest of Pinatubo’s summit. After the eruption, the bright gray or pink colors streaming from the summit are sediment-laden lahars in the Marella River, which flows into the Santo Tomas River Valley. The lahars effectively widened this river plain, and the sediment persists, indicated by the shades of pink. The brighter mottled white and gray in the upper left of the image after the eruption gradually returned to forested green tones.
Mapanuepe Lake is the dark shape in the lower right. Southeast of the lake is the copper and gold Dizon Mines. Cleared land for the mining activity appears in pink shades in the midst of the green forested areas. The small dark shape is water at the bottom of the mine’s main open pit.
Again, a few ASTER images in this series help fill in some gaps where Landsat imagery was cloudy. The complementary imagery of the changes over time to these valleys help scientists understand more fully the trajectories of the changes.
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