Stratovolcano is a tall volcanic mountain that typically has steep sides. Stratovolcanoes are formed by a combination of explosive and nonexplosive eruptions. Explosive eruptions can deposit layers of ejecta (ash and rocky debris). Nonexplosive eruptions produce flows of lava that cool and harden. Over time, layers of ejecta and hardened lava flows build up to form a cone around the vent. The vent is the hole through which eruptions occur. A stratovolcano may also be called a composite volcano. This name refers to the mixture of ejecta deposits and lava flows that make up the cone’s structure. Most eruptions at a stratovolcano occur at the summit. Occasionally, however, vents open on a stratovolcano’s sides.
Stratovolcanoes may erupt explosively. The explosions produce ash plumes and pyroclastic flows. A pyroclastic flow is a cloud of hot gas and rock that moves quickly along the ground. Stratovolcano magma (molten rock) tends to be mostly composed of rock called andesite, but other types of rock may also be present. This magma is less fluid than the basalt magma that erupts at wider, flatter shield volcanoes . The less fluid magma traps gas prior to eruption. Pressure from this trapped gas is what drives the explosions. However, stratovolcanoes sometimes undergo effusive (nonexplosive) eruptions. These eruptions produce lava flows or lava domes that often move smoothly down the sides of the volcano cone.
Well-known stratovolcanoes include Mount Rainier in the United States, Mount Fuji in Japan, Mount Vesuvius in Italy, and Cotopaxi in Ecuador. Stratovolcanoes near large cities present a significant hazard when they become active. Examples include Mount Rainier, near Seattle, and Mount Vesuvius, near Naples.
