As volcanoes modify the surface of the earth, they also enrich it, drawing people to live in volcanically active areas in order to cultivate the soil. The ash from volcanic eruptions provides vital nutrients to enrich and renew the soil, and soil formed from decomposed basalt (volcanic rock) is very fertile. Consequently, volcanic areas such as Hawaii, the Philippines the Andes of South America and eastern Africa all have large populations living at the bases of active volcanoes. Predicting and understanding the hazards that these people face is of great importance to earth scientists and emergency management personnel.
The first, and most obvious hazard of volcanoes is lava. However, lava is generally not life threatening because most lava flows move a the speed of a few kilometers per hour, so one can avoid advancing lava, even on foot. Lava temperatures range from 500ºC to 1400ºC, so combustible materials, such as homes and forests often burn. In addition, in regions that are covered by new lava flows, the cooled lava forms rock, rendering the land unusable for cultivation.
One of the most dangerous byproducts of a volcanic eruption, especially an eruption from a silica rich volcano, is a pyroclastic flow. A pyroclastic flow is a mixture of hot gas (about 1000ºC), rocks and ash that sweep down the sides of a volcano at speeds in excess of 1000 kilometers per hour. Pyroclastic flows are extremely dangerous because they are often produced with the first stage of a large eruption, and can catch residents unaware, and they move so quickly they are impossible to escape. The majority of the damage from the 1980 eruption of Mount St. Helens in Washington was the result of a pyroclastic flow.
Another danger from volcanic eruptions is lahars or mudslides. When Mount Pinatubo erupted in the Philippines in 1991, the torrential rains that followed the eruption turned all of the ash into a heavy cement-like sludge that raced down mountain valleys obliterating small villages in their path. Read about the lahars of Mount Pinatubo at this USGS web site.
Toxic gasses are another danger in volcanically active areas. Volcanoes release gasses such as carbon dioxide, water vapor, hydrochloric and hydrofluoric acids and various sulfuric gasses. In late 1986, in Cameroon, Africa, 1700 people and many animals died when accumulated gasses trapped in the sediment of Lake Nyos, the crater of a dormant volcano, were released. Carbon dioxide is odorless and more dense than regular air, so it accumulated in a layer near the ground, suffocating any living thing that passed through it.