Mount Vesuvius erupted in 79 CE with little warning. The volcano’s pyroclastic flows—hot avalanches composed of air, ash, and rock—obliterated everything in their path. The Roman cities of Pompeii and Herculaneum and the remains of about 1500 people were later excavated from under several meters of ash. In contrast to the viscous, ambling lava flows of other volcanoes, such as Mauna Loa in Hawaii, pyroclastic flows cruise across land at speeds of about 10–30 meters per second for tens of kilometers without slowing down.
Pyroclastic flows have long perplexed volcanologists. Given volcanic particles’ high static friction, they should stay put on slopes of up to 35–45°. But scientists have observed pyroclastic flows traveling over land surfaces with average slopes of just 8° and sometimes even upslope for short distances.
Volcanologists can’t easily measure the physical properties of pyroclastic flows in the environment because of the danger to people and the...
