This bugged me for a while:
Pyroclastic flows are fast (up to 700 km/h (430 mph) according to Wikipedia) and typically roll down the slopes of a volcano. E.g. here https://www.youtube.com/watch?v=Cvjwt9nnwXY.
Also according to Wikipedia, and other sources, they are hot, up to 1000 degrees Celsius, and consist of gasses.
This makes me wonder, why do those hot gasses not ascend up into the atmosphere? This is what hot air does, so there must be something very special about theses gasses.
Now, I know that C02 and CO are heavier than air, but at 1000 degrees?