<P> There, the relatively low pressure allows water and other volatiles (mainly CO, SO, Cl, and H O) dissolved in the magma to escape from solution, as occurs when a bottle of carbonated water is opened, releasing CO . Once a critical volume of magma and gas accumulates, the plug (solidified blockage) of the volcanic vent is broken, leading to a sudden explosive eruption . </P> <P> In recorded history, explosive eruptions at subduction zone (convergent - boundary) volcanoes have posed the greatest hazard to civilizations . Subduction - zone stratovolcanoes, such as Mount St. Helens, Mount Etna and Mount Pinatubo, typically erupt with explosive force: the magma is too stiff to allow easy escape of volcanic gases . As a consequence, the tremendous internal pressures of the trapped volcanic gases remain and intermingle in the pasty magma . Following the breaching of the vent and the opening of the crater, the magma degasses explosively . The magma and gases blast out with high speed and full force . </P> <P> Since 1600 CE, nearly 300,000 people have been killed by volcanic eruptions . Most deaths were caused by pyroclastic flows and lahars, deadly hazards that often accompany explosive eruptions of subduction - zone stratovolcanoes . Pyroclastic flows are swift, avalanche - like, ground - sweeping, incandescent mixtures of hot volcanic debris, fine ash, fragmented lava and superheated gases that can travel at speeds in excess of 160 km / h (100 mph). Around 30,000 people were killed by pyroclastic flows during the 1902 eruption of Mont Pelée on the island of Martinique in the Caribbean . In March to April 1982, three explosive eruptions of El Chichón Volcano in the State of Chiapas in southeastern Mexico, caused the worst volcanic disaster in that country's history . Villages within 8 km (5 mi) of the volcano were destroyed by pyroclastic flows, killing more than 2,000 people . </P> <P> Two Decade Volcanoes that erupted in 1991 provide examples of stratovolcano hazards . On June 15, Mount Pinatubo spewed an ash cloud 40 km (25 mi) into the air and produced huge pyroclastic surges and lahar floods that devastated a large area around the volcano . Pinatubo, located in Central Luzon just 90 km (56 mi) west - northwest from Manila, had been dormant for 6 centuries before the 1991 eruption, which ranks as one of the largest eruptions in the 20th century . Also in 1991, Japan's Unzen Volcano, located on the island of Kyushu about 40 km (25 mi) east of Nagasaki, awakened from its 200 - year slumber to produce a new lava dome at its summit . Beginning in June, repeated collapse of this erupting dome generated ash flows that swept down the mountain's slopes at speeds as high as 200 km / h (120 mph). Unzen is one of more than 75 active volcanoes in Japan; an eruption in 1792 killed more than 15,000 people--the worst volcanic disaster in the nation's history . </P>

When was the last time a composite volcano erupted