Within hours, heavy rains began to wash this material down into the surrounding lowlands in giant, fast-moving mudflows called lahars. In the next four rainy seasons, lahars carried about half of the deposits off the volcano, causing even more destruction in the lowlands than the eruption itself. Background Before April 1991, when steam explosions blasted out three small craters on the north flank of Mount Pinatubo on the island of Luzon in the Philippines, the volcano had been dormant for 500 years, allowing lush tropical vegetation to cover its slopes. They also concluded that lahars would again rush down Pinatubo's slopes in its next major eruption, threatening the communities that were built while it slumbered.
Inset photo shows the house before it was buried.
Explosive eruptions can deposit huge amounts ash and other volcanic debris on a volcano's slopes. Lahars form when water from intense rainfall, melting snow and ice, or the sudden failure of a natural dam, mixes with this loose volcanic material, creating mudflows that can be particularly dangerous and destructive. Since that eruption, lahars have destroyed the homes of more than 100,000 people in the area surrounding Pinatubo. Ash and other deposits from Mount Pinatubo's 1991 eruption destroyed or buried much of the lush vegetation that had covered the volcano. Fortunately, vegetation is beginning to grow on the 1991 deposits, and new stream channels are becoming established, reducing the runoff and erosion that create lahars.