Active volcanoes can be extremely dangerous to live near because of their tendency to eject debris, gas and harmful chemicals at irregular intervals. According to Geology.com, any kind of volcano can produce potentially lethal phenomena in unstable and hard-to-predict ways. Certain hazards, however, are relatively well understood.
The first hazard many people think of in relation to volcanoes is lava. Lava is molten rock; it regularly reaches temperatures between 1,800 and 3,600 degrees Fahrenheit, according to Geology.com. Lava flows can be thick and slow moving or thin and fast, and an active flow poses an immediate hazard to fixed objects such as roads and buildings.
Pyroclastic currents are another hazard of volcanoes. When molten rock interacts with water, the resulting energy release can cause a tremendous explosion that drives ash and debris downhill anywhere from short distances to hundreds of miles. The Soufrière Hills volcano in Monserrat last erupted in this way and ejected a pyroclastic flow of material heated up to 750 degrees Fahrenheit that moved at speeds up to 650 mph.
Volcanoes can also inundate their surroundings with catastrophic mudflows, known as lahars, and poisonous gasses. Geology.com reports that these gasses are typically rich in sulfur and can easily combine with water vapor to produce acid rain downwind of the areas immediately affected by the eruption.