What sets volcanoes apart from other mountains is the manner in which they form. Volcanic eruptions are fueled by heat released from magma, a mixture of molten rock, suspended minerals, and dissolved gases. Most rocks melt at temperatures between 800°C and 1,200°C (1472°F and 2192°F). This occurs in the layers of Earth's mantle known as the lithosphere and asthenosphere, at depths of 50 km to 250 km (31 mi to 155 mi) beneath the surface. Because magma is molten, it is less dense than the surrounding rocks. This causes magma to move upward and eventually either come into contact with and intrude into portions of Earth's crust or be released through volcanic vents.

Volcanic eruptions vary greatly in their intensity. In general, the most explosive eruptions come from magmas that, due to their high silica content, maintain high gas levels as they reach the surface. Silica-rich magmas, including andesite and dacite, are also thicker, or more viscous. The more viscous a substance is, the more it resists flow. Because these magmas do not flow quickly, pressure builds behind them as they move through vents in the volcano. In contrast, magmas that are low in silica, such as basalt, tend to be relatively fluid. They allow gas to easily escape and produce streaming lava flows and typically less volatile eruptions.

Scientists classify volcanoes according to how explosively they erupt, as well as their height and shape. The four types of volcanoes they recognize are cinder cones, lava domes, shield volcanoes, and composite volcanoes. Cinder cones are relatively small, symmetrical, steep-sided volcanoes that form on or near other volcanoes of the same or different types. Their shape and vertical structure result from volcanic material being launched nearly straight up into the air during eruptions and then falling back down and settling near the volcano's peak. Lava domes are also relatively small volcanoes that form on or near other larger volcanoes. What distinguishes lava domes from cinder cones are their rounded, lumpy shapes that result from the very thick, or viscous, magma that feeds them. Shield volcanoes are typically very large, symmetrical volcanoes created by non-explosive eruptions of low viscosity lava flowing long distances to create the volcano's gentle slopes.

In contrast, composite volcanoes (also called stratovolcanoes) such as Nyiragongo, located in the Democratic Republic of Congo, are typically asymmetrical and can erupt explosively. Although most composite volcanoes produce lava of high viscosity, Nyiragongo and others combine explosiveness with low-viscosity, fast-flowing lava. This combination has been deadly in the past and is the reason experts consider this volcano to be one of the most dangerous in the world.

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