Pyroclastic density current

Fast-moving, lethal, hot clouds of ash, rocks and gas, caused by a volcanic eruption. They are controlled by gravity, moving laterally and usually down topographical lows at high speeds (usually between 40 to 100 km per hour). They can travel a few hundreds of metres to kilometres from the source. In large but rare caldera-forming eruptions they can travel tens of kilometres. They are sometimes referred to as ‘PDCs’, and types include pyroclastic flows, pyroclastic [base] surges, and block and ash flows.