Estimates suggest that approximately fifty to sixty volcanoes erupt every month around the world. Although some volcanoes only erupt once every few decades, others—like Kilauea on the island of Hawaii—are almost continually active. A single eruption may last between several hours and several weeks, but the volcano is considered to be active for around ten years afterwards. Because the superheated lava and poisonous gases released during an eruption can cause catastrophic damage, people living near active volcanoes often have to be ready to evacuate their homes without much notice. Unfortunately, impending eruptions cannot always be accurately forecasted.
Predicting an eruption is a complex process as forewarning behaviours differ greatly from one volcano to the next. In some, an imminent eruption is revealed by a surge in the frequency and intensity of earthquakes occurring in the local area. With others, an observed increase in the volcano's height, caused by magma expanding beneath the surface, signals pending activity. In yet other cases, indicators include explosions of ash at the summit and a rise in the number of vents ejecting hot gas. Forecasting a volcanic eruption, therefore, requires specific knowledge of the volcano in question. Scientists often have only the historical behaviour of a volcano to guide their predictions, and this data is often scarce, especially relating to volcanoes that erupt only occasionally. Little is known about Agung volcano in Indonesia, for example, because the last time it erupted was in 1963, when seismometers and satellite monitoring were not widely used. Therefore, even when geological changes are observed at Agung, scientists cannot accurately predict how much time will pass before an eruption occurs.