http://www.newsweek.com/pyroclastic-flow-volcanic-eruptions-surf-air-1388699

Scientists have discovered that the scorching material spewed from a volcano during eruptions generates a layer of air between it and the ground, allowing it to surf along at incredible speeds, destroying everything in its path.

Pyroclastic flows are made up of a mix of hot lava, pumice, ash and volcanic gasses. They can reach temperatures of up to 1,000 degrees Celsius and can—in extreme cases—move down the slopes of volcanoes at over 400 miles per hour. They are responsible for around 50 percent of all deaths from volcanic eruptions globally. Pyroclastic flows were what destroyed the ancient cities of Pompeii and Herculaneum when Mount Vesuvius erupted in 79 AD.