Typical physical processes during an ideally deep underground nuclear explosion.
Nuclear explosion starts to heat the rock around its hypocenter.
Rock is vaporized. As a result of the disappearance of the vaporized rock, a “primary cavity” appears and is filled with the former rock which now exists in gaseous form. The extremely high pressures from the gases in the cavity now begin to expand the actual primary cavity at the expense of neighboring areas of the still solid rock.
The actual cavity reaches its final “secondary” size because of the extremely high pressure from the gases inside of it and as such expands from its original size (shown by the dotted line) to an even bigger size (shown by the firm line). Given that this expansion occurs at the expense of the neighboring areas, these neighboring areas of rock become tightly compressed.
Final picture. White: the underground cavity (the secondary size); blue: the “crushed zone” – totally pulverized rock (crushed into complete microscopic dust ~100 micron particle size); green: the “damaged zone” – partly crushed rock.
This pictorial rendition schematically outlines all the important physical processes of an ideally deep (meaning that it occurs sufficiently far from the Earth surface) underground nuclear explosion. So, now it should be clear that the extreme pressures from the vaporized rock inside the cavity takes on at least two important tasks: 1) it expands the actual cavity from its "primary" size to its "secondary" size; and 2) because it does this expansion at the expense of the neighboring areas of the rock, it produces two damaged zones around itself, each representing a different degree of damage.
The zone immediately adjacent to the cavity in nuclear jargon is called the "crushed zone". This zone can be as thick as the diameter of the cavity itself and is filled with a very interesting matter. It is filled with rock that is now completely pulverized. It is reduced into a fine microscopic dust, an approximate particle of which is about 100 microns in size. Moreover, the particular state of material within this "crushed zone" is in a very interesting state – nothing in the world can produce the following phenomenon other than an underground nuclear explosion:
If you were to pick up a stone from this zone, but do so very gently, it might still stick together and resemble a stone by its form and its color. However, it you just squeeze the stone a little bit with your fingers, this "stone" will immediately be crushed into complete microscopic dust which it actually now consists of. The second zone – just outside of and surrounding the "crushed zone" is called the "damaged zone" in professional nuclear jargon. This "damaged zone" is filled with rock crushed to various pieces - from very small (millimeters in size), to some relatively larger fragments. The closer to the border of the "crushed zone" you get, the smaller the debris becomes, and the further away from hypocenter you go - the larger the debris. Finally, outside the "damaged zone’s" border, there would be virtually no damage inflicted to the surrounding rock.
However, the physical processes we discussed above are true in an "ideally deep" underground nuclear blast. When a nuclear charge is not buried deep enough, the picture will be slightly different. The "damaged" and "crushed" zones will not appear as round as in the prior example. They will be rather elliptic – whereby the longer end is directed upward – like the shape of an egg. This happens because the pressure from the gases encounters less resistance in the direction of the Earth’s surface (given that it is so close), and both the "crushed zone" and "damaged zone" will expand upwards as well in the same fashion.
The drawing above is an illustration of the resistance of the surrounding rock when a cavity is located not very deep below the Earth’s surface. Evidently, the resistance of the rock towards the Earth’s surface will be much less than in any other direction. Given that everything goes in the way of least resistance, it is understandable to note that the cavity will expand more towards the Earth’s surface and won’t look so “round”. It will look more like an egg. In other words, it will be ellipsoidal in shape.
When the pressure propagates upwards, the upper boundaries of the "damaged zone" and the "crushed zone" eventually reach the underground foundations of the Tower they are about to demolish, the picture is even more different. This is because the actual materials the Tower is built of differ from the surrounding granite rock in the sense of their resistance. Besides, there is a lot of empty space inside the Tower, while the remaining granite rock in all other directions (to the sides and below the cavity) is solid. So, the expansion of the upper boundaries of the "damaged" and "crushed" zones by the Tower's structure will be the furthest. In the case of the WTC Twin Towers and the Sears Tower, the "damaged zone" could likely reach up to 350-370 meters, while the "crushed zone" which follows immediately, will likely reach up to 290-310 meters. However, in the case of the much shorter WTC-7, its entire length was well within the "crushed zone" - so it was pulverized completely from bottom to top. The ability of a nuclear demolition to pulverize steel and concrete alike is one of its unique features.
The picture above shows an example of the fine microscopic dust that covered all of Manhattan after the WTC demolition. Many people mistakenly believed that it was "concrete dust". No, it was not. It was dust – but mainly pulverized steel. Despite the common misconception, the WTC structures did not contain much concrete at all. Concrete was used only in some limited quantities to make very thin floor slabs at most. It was not used anywhere else. The majority of the WTC Twin Towers was steel, not concrete. So accordingly, the majority of this ultra-fine dust is represented by steel dust. However, it was not only "steel dust" alone - it was also "furniture dust", "wood dust", "paper dust", "carpet dust", "computer parts dust" and even "human dust", given that human beings were left to be pulverized in the Towers the same manner in which the steel, concrete and furniture were.
Some may wonder how WTC-7 collapsed so neatly into its own footprint, and in its entirety, while the Twin Towers came down not only scattering dust, but even larger debris and ejecting them to such far distances. This question is very easy to answer – you just have to look at the distribution of the "crushed" and "damaged" zones within the Twin Towers structures and the answer will become obvious.
The picture above represents the approximate distribution of the damaged zones in the scenario of a nuclear demolition of a skyscraper using a 150 kiloton thermo-nuclear charge positioned 50 meters deeper than the lowest underground foundations of the building. Don't forget that the demolition charges in this particular case were buried not "ideally deep", which is why the formations of the "crushed" and "damaged" zones were not "ideally round" either - they were elliptic, with their sharper ends facing upwards – like an egg – in the way of least resistance. It is easy to understand that the entire length of the WTC-7 fit well into the “crushed zone” alone so there were not any undamaged areas on top of it that might produce the effect of an undamaged top falling down like we saw during the collapse of the North and South Towers.
The particular distribution of damage within the skyscrapers’ structures inflicted by such a process could be better understood when you watch the videos that show the details of the collapses of the WTC Twin Towers and of WTC-7. These contemporary videos are widely available on YouTube.
The North Tower just began to collapse a moment ago.