Underground lava is called magma. It forms deep in the Earth, but not very deep. The Earth's mantle is about 2900 kilometers thick, but most magma comes from the top 100 kilometers or so. Two things make rock melt: the addition of water and the release of pressure. Where volcanoes occur, which is mostly in subduction zones and continental rifts, both of these mechanisms are at work, but water is the dominant one. (Volcanism in a Nutshell has more on this subject.)
Magma has a strong urge to rise, because it is less dense than solid rock. As it rises it moves into harder, colder rock of the lithosphere, where it slows down and collects in larger bodies. These magma chambers are not large lakes of lava but rather mazes of interconnected channels. Eventually magma nears the surface, and there its dissolved gases begin to come out of solution. The combination of buoyancy and degassing is what powers volcanic eruptions.
Magma changes in composition as it rises and cools, generally starting from basalt. Mineral crystals form and settle out of the magma, starting with olivine and the pyroxene minerals. The magma thus evolves toward a higher content of silica. Also, the magma may melt the crustal rocks around it and gain silica, especially in the siliceous continental crust. Thus volcanoes can erupt lavas with varying compositions, depending on what part of their magma chambers they're tapping.
The more silica magma has, the thicker and stickier it is, and the more explosive the eruptions it causes can be. Volcano shapes reflect the composition of their magma and the amount of magma they have feeding them. There are several types of volcanoesthe five most important are discussed below, but these and more are shown in the Gallery of Volcano Types.
The smallest volcanoes are made by short episodes of eruption (a few years to a few thousand years) that empty small magma chambers in isolated vents. High-silica lava shooting into the air and coming down in small piecesvolcanic ashbuilds steep-sided cones. Being loose piles, cinder cones erode away relatively quickly.
If lavas are very sticky, they pile up instead of flowing away. Mount St. Helens has a good example of a lava dome inside its crater. These too are short-lived structures because they either blow up or are covered by later ash eruptions.
Composite or stratovolcanoes
Larger volcanoes contain many layers of ash and lava flows. Flows of lava harden into strong rock beds that resist erosion. Active stratovolcanoes are often tall and symmetricalJapan's Fujiyama is a classic example. They are usually part of a field of lesser cones and flows. But even large stratovolcanoes are fragile structures compared to ordinary mountains. They erode easily, the magma moving inside them disturbs them, and volcanic gases inside them cook their rocks into rotten, loose crud. Stratovolcanoes tend to have mudslides called lahars or, even worse, have great chunks slip off their sides that expose the inner magma and trigger explosive eruptions. That happened to Mount St. Helens in 1980. Stratovolcanoes are nearly all younger than a million years.
If a magma chamber produces fluid basaltic lava and little ash, the resulting volcano is broad rather than tall. Lava flows in all directions from the central crater. Shield volcanoes make up the islands of Hawaii and Iceland, but many occur on the continents too. In the extreme case of flood volcanism, no volcano is built at all, just a flat expanse of basalt that pours from fissures in the ground. Idaho's Snake River Plain is an example.
Where magma chambers are at shallow depths, the overlying crust tends to collapse into them forming round sunken plugs with relatively flat floors. These are called calderas after the Spanish word for cauldron. The vent of the shield volcano Mauna Loa, for instance, is a caldera a few kilometers across.
The world's largest volcanic structures are calderas. Many have only been found by geologic mapping or by taking pictures from space. They overlie enormous magma chambers that erupt only a few times in a million years. Geologists don't use the term, but these are truly "supervolcanoes." Once these high-silica magma chambers launch into eruption, they spew thousands of cubic kilometers of material far and wide and into the upper atmosphere. They rank magnitude 8, the maximum, on the Volcanic Explosivity Index or VEI scale.
Eruptions of this size can disturb the global climate and devastate regions the size of continents. Civilization has never faced one. Twelve VEI 8 eruptions have been documented since the end of the Eocene Epoch, 34 million years ago. The last one occurred 74,000 years ago in Sumatra, at the 100-by-30-kilometer Toba caldera. Genetic evidence suggests that the world's human population was drastically reduced at that time. That would happen again the next time any of the world's largest calderas erupts.