Will scientists ever be able to accurately predict volcanic eruptions? Vulcanologists around the world study gas emissions, ground deformation, temperature changes and seismic and electro-magnetic data to try and predict when volcanoes will erupt. Their task is vitally important, as many of the 1,500 active volcanoes on the planet are considered utterly unpredictable. Spewing forth lava and ash, they kill an average of 800 people per year, and threaten 500 million more.