Pyroclastic flows are avalanches containing hot volcanic gases, ash and volcanic bombs. On steep volcanoes pyroclastic flows can reach speeds of over 100 miles per hour.
Possibly one of the most famous pyroclastic flows was the one caused by the eruption of Mount Vesuvius in 79 AD which covered the Roman city of Pompeii with ash and volcanic debris. 1.5 million tons of rock, pumice and ash we released every second.
The video below shows a more recent pyroclastic flow at Mount Ontake, Japan.