Mount St. Helens blew its top 36 years ago, but it remains alive today. See how Earth's 500-some active volcanoes were formed. | National Geographic

MOUNT PINATUBO ERUPTION IN 1991 Mount Pinatubo is an active stratovolcano in the Zambales Mountains, located on the tripoint boundary of the Philippine provinces of Zambales, Tarlac and Pampanga, all in Central Luzon on the northern island of Luzon. Its eruptive history was unknown to most before the pre-eruption volcanic activities of 1991, just before June. Pinatubo was heavily eroded, inconspicuous and obscured from view. It was covered with dense forests which supported a population of sever