A volcano's deadly pyroclastic flow - the same thing that buried Pompeii. You can't outrun it: it travels at 700 km/h (450 mph) and you can't weather it in anything but highly specialized shelters: it's over 1,000 °C (1,830 °F). Once you see that in your rear view mirror, you're either already out of reach or you're not.
Pileus cloud above the Sarychev volcano as it erupts, Kuril Islands, Russia. Pileus clouds, also called scarf or cap clouds, are small clouds that form on top of a bigger cloud. In this photo a pileus cloud (centre) has formed above a cloud of volcanic ash from the Sarychev volcano. A large plume of smoke, steam and ash is erupting from the volcano while pyroclastic flow of denser ash descends the volcano sides. The picture was taken by astronauts aboard the International Space Station…
A pyroclastic flow (also known scientifically as a pyroclastic density current) is a fast-moving current of hot gas and rock (collectively known as tephra), which reaches speeds moving away from a volcano of up to 700 km/h (450 mph).
This area in New Mexico owes its remarkable geology to layers of volcanic rock and ash deposited by pyroclastic flow from a volcanic explosion. Over time weathering and erosion of these layers has created canyons and tent rocks. The tent rocks themselves are cones of soft pumice and tuff beneath harder caprocks, and vary in height from a few feet to 90 feet.