Pyroclastic surges are low density flows of pyroclastic material containing mostly gases which can can asphyxiate people. Pyroclastic surges are very fast and not constrained by topography. They are extremely hazardous and can bury, burn, and destroy things upon impact.
The currents can be as hot as 800' C incinerating trees and houses, and causing volcanic particles to fuse and form solid sheets of black glass, welded onto the landscape. Vulcanologists are still ignorant of how and why these events occur as they do. It is the continental volcanoes that are most likely to produce such an event as they produce viscous magmas in which gasses become trapped, reaching intense pressures until they are released in cataclysmic explosions, pulverising magma into ash. No instrument yet devised can withstand the heat and crushing impacts of such a surge, hindering the search for knowledge.
Many people have been killed by pyroclastic surges. Probably the greatest number of people killed by pyroclastic surges was in 1902 near Mount Pelee in the town of St. Pierre when 30,000 people lost their lives.
7.50 am Thursday 8th of May 1902, the town of Saint Pierre on the caribbean island of Martinique experienced what is still regarded as the worst volcanic disaster this century. Of the 28,000 people in the town preparing to celebrate the feast day only 2 survived. One of the biggest this century was at Mount Pinatubo in the Philippines, spewing out over nine cubic kilometers of ash and pumice in five hours. It buried an entire landscape beneath up to 200 meters of ash and incinerated hundreds of square kilometers of forest. Ancient events include the Campanian pyroclastic current which covered 32,000 square kilometers in and around the bay of Naples to a depth of up to 100 meters with a total volume of 500 cubic Kilometers. At yellowstone in the USA, 2 million years ago 2500 cubic kilometers were vented. Volcanic calderas in the San Juan mountains contain ignimbrites up to 3 kilometers thick! The extent of the current engulfed large parts of three states, and the airbourne dust covered most of what is now the USA in a grey ash blanket. If this occurred today the destruction would be immense with industry, transport and agriculture collapsing overnight. It is estimated that on average, eruptions of this scale occur every 100,000 years.