Mount Pinatubo had likely been dormant for hundreds of years. Local residents on the northern island of Luzon within the Philippines hardly believed Pinatubo was a volcano, making it 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.

Today the mountain is relatively quiet, and about 300 meters shorter than it was before the eruption. This Landsat image pair shows the mountain several months after the eruption and more recently in 2017. The additional green near the summit indicates forest regrowth, but scars from fast-moving floods of volcanic ash and water are visible as pink stretches streaming away from the mountain. Called lahars, these floods affected more people than the eruption itself.

Twenty-six years after the eruption, lahar hazards continue. Landsat helps scientists monitor changes caused by these hazards, and how the changes to the land affect the population. Landsat can show these changes in a way that can’t be witnessed from the ground. Landsat data also allow scientists to analyze a larger area in a much shorter amount of time than ground surveys, providing valuable information for local decision making.