Volcanoes

Magma, lava, and the vent

A volcano is the vent through which molten rock and other materials escape from Earth's interior, together with the landform that accumulates around that vent. The melt that feeds a volcano is called magma while it is still underground. Once magma reaches the surface, the same material is called lava. The name change is not a formality: the behavior of the melt below the ground controls what an eruption looks like at the surface.

Erupting material is more than pure melt. Magma carries dissolved gases, mostly water vapor and carbon dioxide, and it can pick up fragments of older rock as it rises. As pressure drops during ascent, dissolved gas comes out of solution, the way bubbles form when a bottle is opened. The expansion of that gas, together with the temperature and composition of the melt, decides whether an eruption pours fluid lava, fountains glowing spray, or hurls rock fragments into the air.

Three common volcano types and their eruption styles

Volcanoes are often grouped by the shape of the landform, which reflects the composition and gas content of the magma that built it. Shield volcanoes have broad, gently sloping profiles created by many fluid lava flows that spread far from the vent. Mauna Loa in Hawai'i, the largest active volcano on Earth, is a shield volcano, and nearby Kīlauea erupted almost continuously from 1983 for decades. Composite volcanoes, also called stratovolcanoes, are tall, steep-sided mountains built from alternating layers of lava and fragmental material; their viscous, gas-rich magmas tend to erupt explosively. Mount Rainier and Mount St. Helens are composite volcanoes. Cinder cones are the smallest of the three common types: steep-sided hills of loose fragments ejected by fountains of gas-rich magma, usually from a single vent.

Magma composition drives these differences. Basaltic magma is hot and fluid, so it produces broad shields and extensive lava flows. Magma richer in silica, such as the dacite that built Mount St. Helens, is more viscous and traps gas, favoring explosive eruptions that generate ash and pyroclastic debris. A single volcano can change its eruption style over time as the magma feeding it changes.

Where volcanoes occur

Most volcanoes follow the pattern set by plate tectonics. Many rise above subduction zones at convergent boundaries, where a descending plate releases water that helps generate magma in the overlying mantle; the Ring of Fire around the Pacific Ocean is the classic example. Others form along divergent boundaries, where plates pull apart and mantle material rises and melts, as in Iceland. A third setting is the hotspot: mantle material rising within a plate builds volcanoes far from boundaries, such as the Hawaiian Islands and Yellowstone. Plate tectonics supplies the setting, while this lesson focuses on what happens when magma actually reaches the surface.

Hazards and monitoring

Volcanic hazards take several forms. Lava flows are streams of molten rock that usually move slowly: basaltic flows typically advance at less than 1 km/h, though steep slopes can speed them to about 10 km/h and channels can carry them faster. Pyroclastic flows are hot avalanches of gas, ash, and rock that typically travel faster than 80 km/h at temperatures of 200-700 °C. Lahars are flows of volcanic debris mixed with water that can race down valleys faster than 200 km/h and grow to more than ten times their original size as they pick up material. Ashfall spreads particles smaller than 2 mm across wide areas, where they can collapse roofs, close airports, and clog engines. Volcanic gases, dominated by water vapor, also escape; volcanoes worldwide release an estimated 180 to 440 million tonnes of carbon dioxide per year.

Because many eruptions are preceded by change, scientists watch for warning signs. Rising seismicity reflects magma moving through rock. Ground deformation, measured with GPS and tiltmeters, can reveal magma accumulating underground. Rising gas output, changes in hot-spring chemistry, and satellite observations of heat and ground movement add further clues. The 1980 eruption of Mount St. Helens shows the stakes: on May 18, a magnitude 5.1 earthquake at 8:32 a.m. triggered a lateral blast that moved at least 480 km/h, devastated about 600 km² of forest, and sent roughly 540 million tons of ash into the atmosphere, lowering the summit by about 400 m to its present elevation of about 2,549 m. Monitoring cannot predict eruptions hour by hour, but it gives communities the best available warning.