Both Mount Etna and Mount St. Helens are active stratovolcanoes, also known as composite volcanoes. In addition, both have experienced major activity in the twentieth and twenty-first centuries.
Stratovolcanoes are steep volcanoes with conical shapes composed of layers built up by previous eruptions. They are the most destructive volcanoes in the world, and, in the past four centuries of recorded history, they have caused the deaths of about 300,000 people. They are characterized by such hazards as lava flow, mud flow, clouds of ash and rock projectiles of various sizes.
Mount Etna has a history of activity which includes numerous ongoing eruptions. In 1928, an eruption destroyed an entire village, a train line and a train station. Other eruptions occurred in 1949, 1971, 1981, 1983 and 1991-93. In 1995-2001, its four craters were almost continuously active, and this culminated in the 2001 eruption through seven fissures on the south slope. In 2002-2003, an eruption caused an ash cloud so large it could be seen from space. In subsequent years, further outbursts of lava and ash have followed, some severe enough to shut down the airport in Catalina, Sicily. The major modern Mount St. Helens event was the 1980 eruption that blasted away part of the mountainside. The eruption killed 57 people and destroyed 250 homes. In 2004-2008, the mountain became active again, with lava flows, ash clouds and seismic activity.