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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?

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    $\begingroup$ Don't skip part of lines when reading Wikipedia: "[...]fast-moving current of hot gas and volcanic matter [...]" There is your simple answer. $\endgroup$ – Erik Feb 15 at 14:06
  • $\begingroup$ I actually read the "matter" part, but guessed that this was meant to be only the ashes which flow with the gasses. Gasses and matter would not chemically mix in the flows, I guess. $\endgroup$ – hitchhiker Feb 15 at 20:38
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Volcanic plumes and pyroclastic flows are two behaviours of basically the same thing. A volume of hot gasses and tephra (a.k.a. a mix of ash, pumice, and volcanic rocks of different sizes).

If the gas is hot enough and the tephra content is light enough the mix will be buoyant and it will rise as a volcanic plume. On the other hand if gas is too cold to support the plume, or the weight of the tephra is too heavy. It will collapse and rush downhill as a pyroclastic flow. That is referred to as a "Fountain collapse", and is the most common cause of pyroclastic flows.

That is why the are also referred as density or gravity currents.

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  • $\begingroup$ I can not understand how gas and matter would mix to create a flow. To me, the gas and matter would separate, with the gas part going up like steam, while the matter part would rain down to earth. Surely I must have misunderstood something basic, but why would gas and matter mix (or keep being mixed)? $\endgroup$ – hitchhiker Feb 15 at 20:43
  • $\begingroup$ Yes, that's exactly what happens, but if the gas is going up "like steam" fast enough, it will carry the tephra along with it. Like sand carried by the wind (but here the wind goes up), if the wind is not strong enough to keep pushing the tephra along, then the column collapses and falls to the ground to create a pyroclastic flow. Does that makes sense? $\endgroup$ – Camilo Rada Feb 15 at 20:54
  • $\begingroup$ I think the tephra/gas mix would be too turbulent for the gas to be able to separate out of the mixture. It's also my understanding that much of the gas gets trapped under the flow, which is how the pyroclastic flow can travel as fast as it does. $\endgroup$ – BillDOe Feb 15 at 21:27
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    $\begingroup$ @hitchhiker: Consider smoke. It is hot gas and ash particles mixed, and usually goes up. In the right conditions, though, it can go downward (say from your rooftop chimney) or along the ground, from a campfire. $\endgroup$ – jamesqf Feb 16 at 0:03

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