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.

I'm sorry, it IS only seen elsewhere in volcanic eruptions. An avalanche cannot supply the HEAT (H.E.A.T.) required for the flow to be PYROclastic (PYRO meaning Fire/hot) and neither can a simple building collapse - explosives however create the neccessary heat AND debris for a pyroclastic flow to occur, hence the phenomenom is seen in building demolitions and eruptions.

I provided a link to a video where an eye witness said the cloud was "Hot" and "like having gravel thrown at your back."

Pyroclastic = Pyro (latin for heat/fire) and clastic (meaning gravel/rocks) hence PYROCLASTC Flow. The eye witness said the cloud was HOT and GRAVELLY, you see - PYRO-CLASTIC. In a demolition the explosives provide the heat, in an eruption the lava.

[edit on 23-7-2007 by ad19t]