Why Does a Bomb Blast Create a Mushroom Cloud and Then a Ring of Smoke?

[AdityaDev]

I attended the navy day celebrations and the marine commandos demonstrated a bomb blast by setting a time bomb in a small installation at the middle of the sea. The commandos dived from their boat and set the explosives from underwater.
After the explosion, what I saw first was a cloud of black gas which soon took the shape of a mushroom. Then after some time you could only see a dark ring of smoke above the location which remains for some time. How can you explain the two phenomena?
Why does it take the shape of mushroom? Why not a sphere or cube?
Why does it change into a large ring of gas which floats?

 

[DaveC426913]

I was going to write up a short explanation but I realized these pages do a better job:

http://en.wikipedia.org/wiki/Vortex_ring
http://en.wikipedia.org/wiki/Mushroom_cloud

Notice how, on the mushroom cloud page here:
http://en.wikipedia.org/wiki/Mushroom_cloud#Cloud_composition
the image at the left shows a cloud part way between a mushroom cloud and a smoke ring.

 

[A.T.]