Stratovolcano - Hazards

Hazards

In recorded history, explosive eruptions at subduction zone (convergent-boundary) volcanoes have posed the greatest hazard to civilizations. Subduction-zone stratovolcanoes, like Mount St. Helens 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 in the pasty magma. Following the breaching of the magma chamber, the magma degasses explosively. Such an explosive process can be likened to shaking a bottle of carbonated water vigorously, and then quickly removing the cap. The shaking action nucleates the dissolution of CO₂ from the liquid as bubbles, increasing the internal volume. The gases and water gush out with speed and force.

Two Decade Volcanoes that erupted in 1991 provide examples of stratovolcano hazards. On June 15, Mount Pinatubo spewed ash 40 kilometres (25 mi) into the air and produced huge pyroclastic flows and mudflows that devastated a large area around the volcano. Pinatubo, located 90 km (56 mi) from Manila, had been dormant for 600 years 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 country's history.

The 79 CE Plinian eruption of Mount Vesuvius, a stratovolcano looming adjacent to Naples, completely covered the cities of Pompeii and Herculaneum with pyroclastic surge deposits. The death toll ranged between 10,000 and 25,000. Mount Vesuvius is recognized as one of the most dangerous volcanoes, jointly because of its potential for powerful explosive eruptions and the high population density of the area (around 3 million people) around its perimeter.