Mount Pinatubo is part of a chain of composite volcanoes along the Luzon arc on the west coast of the island (area map). The arc of volcanoes is due to the subduction of the Manila trench to the west. The volcano experienced major eruptions approximately 500, 3000, and 5500 years ago.
The events of the 1991 Mount Pinatubo eruption began in July 1990, when a magnitude 7.8 earthquake occurred 100 kilometers (62 miles) northeast of the Pinatubo region, determined to be a result of the reawakening of Mount Pinatubo.
In mid-March 1991, villagers around Mount Pinatubo began feeling earthquakes and volcanologists began to study the mountain. (Approximately 30,000 people lived on the flanks of the volcano prior to the disaster.) On April 2, small explosions from vents dusted local villages with ash. The first evacuations of 5,000 people were ordered later that month.