A volcano is an opening in Earth’s crust where magma—a mixture of red-hot molten rock, mineral crystals, rock fragments, and dissolved gases—from inside the planet erupts onto the surface. Magmatic volcanoes like this are by far the best known type, but there is a second, less well known type that erupts mud rather than magma.
CREATION OF A VOLCANO
Magma is produced by the melting of rock in the Earth’s upper mantle and lower crust. This occurs only at certain places, notably at convergent and divergent plate boundaries and at hotspots or mantle plumes.
Magma is usually less dense than the surrounding rock because it is hotter (heating matter causes it to expand), so it rises up, traveling through weaknesses or fractures in the crust all the while incorporating small to large amounts of the surrounding bedrock called lithic fragments.
Eventually it collects in large cavities called magma chambers, several miles below Earth’s surface. From there, the magma rises through channels called conduits or pipes until it reaches the surface or, in the case of a submarine volcano, the ocean floor. There, the magma escapes either through an opening, called a vent, or a crack, called a fissure.
The escape of the magma is known as an eruption, and it can vary from a quiet outpouring—in the form of a fountain or stream of lava—to a highly explosive event in which the magma and contained gases are blown violently into the air and can travel down the slopes of the volcano at great speeds in the form of a pyroclastic flow, a mixture of hot rock, gas, and ash.
GROWTH OF A VOLCANO
Volcanoes grow mainly from the accumulation of their own eruptive products—solidified lava, cinders, and ash. Lava is the name given to magma when it flows out of a volcano. This molten material eventually cools and solidifies to form solid rock.
Cinders and ash are magma that has been blown into the air, cooled then deposited as solid fragments. Different types of volcanoes build up either by the accumulation of one main material, such as cinders of lava flows, or from a combination of several products.
Volcanoes can also grow partly by intrusion—when magma moves up within the volcano and solidifies internally, pushing overlying rock upward to form a bulge. As they grow, many volcanoes develop a classic volcano shape, a steep-sided cone. However, not all volcanoes are cones: some are broad, gently sloping shield-shaped structures while others consist of enormous shallow craters or water-filled depressions in the ground.
Volcanoes vary considerably in their activity, so their growth is very intermittent. Some can continue growing for millions of years before the supply of magma runs out and they become extinct.
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