The majority of volcanoes occur along the boundaries of tectonic plates. At these violent places, slabs of solid rock 5 to 100 kilometers (3 to 62 miles) thick are created and destroyed. Subduction zones, where oceanic crust is forced back into Earth's molten mantle, give rise to 80 percent of the world's volcanoes. The circum-Pacific belt, or Ring of Fire, which stretches up the western coasts of South and North America, across the Aleutian Islands, and down the eastern coasts of Asia, is one such zone.

Areas where two plates move away from each other, called rift zones, also give rise to volcanic activity. The East African Rift is a wide crack in Earth's crust that runs for more than 1,930 kilometers (1,200 miles) from Malawi through Tanzania, Kenya, and Ethiopia to the Gulf of Aden. Along this rift is a string of active volcanoes that includes Nyiragongo, a so-called stratovolcano located just north of the city of Goma in the Democratic Republic of Congo. Nyiragongo is one of the world's most active and dangerous volcanoes.

In an effort to forecast volcanic eruptions, vulcanologists monitor three types of activity within and around volcanoes: seismic activity, ground deformation, and gas emission. To measure seismic activity within a volcano, scientists use a seismograph -- the same apparatus used to measure earthquakes -- to record vibrations of Earth's crust. When rocks crack or slip past each other or when magma applies pressure to the inside of a volcano's magma channel, the surrounding rock vibrates much the way a tuning fork vibrates when it is struck. Seismographs record both the intensity and the frequency of the vibration, providing information that helps scientists determine the location, amount, and type of activity within a volcano.

Deformation of the ground on a volcano's slopes and gases emitted from a volcano's vents provide external signs of activity within the volcano. Pressure building inside a volcano's magma chamber or lava tubes may cause the ground surface to bulge outward. Satellite images and sensitive instruments positioned on a volcano's slopes allow scientists to measure elevation changes of a few centimeters or less. Analysis of the composition of gases emitted from a volcano also helps scientists determine the composition of magma inside, an indication of the volcano's explosiveness.

Unfortunately, no two volcanoes are alike. A combination that causes one volcano to erupt violently may cause no such result with another volcano. Having a limited number of cases to study, scientists are only beginning to understand the reasons for these variations.

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