Although the mountains are volcanic in origin, Mount Pinatubo is the only active volcano in the mountain range. Its eruption on June 15, 1991 was the second most powerful volcanic eruption of the 20th century after the 1912 eruption of Novarupta in Alaska. The volcanic eruption, which was complicated by the arrival of typhoon Yunya, covered the region with thick volcanic ash and lahar including the U.S. military base at Clark Field near Angeles, Pampanga.