Both Towers exploded into vast dust clouds, which photographs show to be several times the volumes of the intact buildings by the time the destruction reached the ground. The dust clouds continued to expand rapidly thereafter, growing to easily five times the buildings' original volume by 30 seconds after the initiation of each collapse.
(The first 15 pictures are of the North Tower Collapse, the rest are of The South Tower Collapse)
The dust clouds rapidly invaded the surrounding city, filling the cavernous spaces between nearby skyscrapers in seconds. Eyewitness reports were consistent that it was impossible to outrun the dust clouds. Photographs can be used to calculate the speed at which the dust cloud from the North Tower grew. There is a photograph of the North Tower dust showing the spire and showing dust 700 feet in front of the nearest part of the building's footprint. That distance is calculated using buildings as reference points. Since it is known from real-time movies that the spire fell about 30 seconds after the initiation of the collapse, and that it took about 10 seconds for the bottom of the dust cloud to reach the ground, the average speed of advance on the ground in that direction was approximately 35 feet per second.
Another feature of the dust clouds was that they upwelled in immense columns, climbing to over the height of Building 7 (over 600 feet) in the seconds immediately after each collapse.
Such behavior clearly indicates the input of huge quantities of heat far in excess of what the friction of a gravity-driven collapse could produce.