What a volcano is
A volcano is an opening in the crust where molten rock, ash and gas escape to the surface. The molten rock is called magma while it is underground and lava once it reaches the surface. Over many eruptions the material piles up around the vent, building the cone or mound people usually picture, although some volcanoes are broad plateaus, fissures or collapsed craters called calderas.
Why magma rises
Magma forms where part of the rock in the crust or upper mantle melts. This happens in a few typical settings, which tie closely to plate tectonics:
- Subduction zones, where one plate sinks beneath another. Water released from the sinking plate lowers the melting point of the rock above it.
- Spreading ridges and rifts, where plates pull apart and hot mantle rock rises and partly melts as the pressure on it drops.
- Hot spots, where plumes of unusually hot material from deep in the mantle feed volcanoes away from plate edges, as in Hawaii.
Magma is generally less dense than the solid rock around it, so it tends to rise and collect in reservoirs called magma chambers. Many eruptions begin when new magma arrives, or when the pressure in a chamber builds until the surrounding rock breaks.
The role of dissolved gas
Magma contains dissolved gases, mostly water vapor and carbon dioxide, along with sulfur compounds. Deep underground, the high pressure keeps them dissolved, much as pressure keeps carbon dioxide dissolved in a sealed bottle of soda. As magma rises, the pressure falls and the gas comes out of solution as bubbles, a shift between states of matter that makes the bubbles expand.
What happens next depends on how easily the magma flows, which is called its viscosity. Magma low in silica, such as basalt, is runny, so gas bubbles can escape fairly easily. Magma rich in silica, such as that forming rhyolite, is sticky, so bubbles are trapped, pressure builds, and the magma can fragment violently into ash and pumice. Magma temperature and the amount of crystals and gas it holds also matter. In general, runny magma tends toward gentle eruptions, and thick, gas-rich magma tends toward explosive ones, although there are exceptions.
Main eruption styles
Volcanologists classify eruptions into styles, often named after example volcanoes. A single volcano can show several styles over its history.
- Effusive (Hawaiian) eruptions release fluid lava that flows out in streams or sprays up in fountains. They can be long-lasting but are usually not explosive.
- Strombolian eruptions are moderate, with bursts of gas throwing glowing lava fragments into the air at intervals.
- Vulcanian eruptions are short, sharp explosions that clear a plug of cooled lava from the vent, sending ash and blocks upward.
- Plinian eruptions are very powerful, with sustained columns of gas and ash that can rise many kilometers into the atmosphere. The eruption of Vesuvius in 79 CE, which buried Pompeii in the Roman Empire, is the type example and is named after the Roman writer Pliny the Younger, who described it.
Hazards beyond lava
Lava flows usually move slowly enough that people can get out of the way, though they destroy what is in their path. Other products are often more dangerous:
- Pyroclastic flows are fast, hot avalanches of gas and volcanic fragments that sweep down slopes.
- Ash fall can collapse roofs, disrupt aircraft and harm crops and water supplies.
- Lahars are mudflows of volcanic debris mixed with water.
- Volcanic gases such as carbon dioxide and sulfur dioxide can be hazardous in low-lying areas close to a vent.
Monitoring and warning
Scientists watch for changes that often precede eruptions, including swarms of small earthquakes, ground swelling measured by satellites and GPS, and shifts in the amount and type of gas released. These signs can raise the probability of an eruption, but they rarely tell exactly when or how large it will be. Official alerts and evacuation guidance from local authorities should be followed near active volcanoes.
Eruptions and the wider Earth system
Large explosive eruptions can inject sulfur compounds into the upper atmosphere, where they form fine particles that reflect sunlight. The 1991 eruption of Mount Pinatubo is generally associated with a temporary global cooling of a fraction of a degree. Volcanoes also release carbon dioxide, but by most estimates the amount is small compared with human emissions, a point that comes up in discussions of the greenhouse effect.
On much longer timescales, some of the largest outpourings of lava in Earth’s history have been linked by researchers to mass die-offs, although the causes of any given case of species extinction are debated. Volcanic activity is also part of how new land forms, and volcanic rock is one of the starting points in the cycle that creates other types of rock.