posted on Jul, 23 2007 @ 03:16 AM
No, it wasn't a pyroclastic flow. It was a gravity current, which describes the way a pyroclastic flow works. In this case, the dust cloud of the
WTC was denser than the surrounding air. This causes it to fall downwards, and as it does so it spreads outwards. This also causes it to speed up,
and due to it being full of dust and ash and whatever else, you can see it. Admittedly it looks a lot like a pyroclastic flow, and both are gravity
currents, but the similarities really end there.
The flow from a volcano occurs (generally) when the force in the eruptive column is insufficient to propel it upwards, and it flows downwards,
creating the flow. And due to the large amounts of energy involved, it is very fast. The WTC cloud was not that fast.
And sorry, but it's not just seen with volcanic eruptions in nature. Avalanches are an example too.