When Mt.Pinatubo erupted it was very devastating. It happened in June of 1991, in the Philippines. What caused it was frequnt earthquaks that shook the area. The volcano erupted because there were earthquacks vents open up a line of steaming vents along the mountian. When the erution happened, the ash mixed with the rain and turned into mud. Since it turned into mud, it became very hard like concrete. Since it was very hard, it collapsed roof more than nine miles away from the volcano. Luckily, only a few hunderd people died. What is very great is maybe 20,000 people were saved because they listened to the authorities and avacuated.