bilby
Fair dinkum thinkum
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Not exactly. Generally, ammonium nitrate is stable.Some sources are saying it was 3000 tons of ammonia nitrate that blew. Ammonia nitrate is inert unless there is a fuel and a detonator. It is used in mining and fuel oil is added and a detonator.
The red smoke also supports the ammonia nitrate
That is, when not supplied with pressure, heat, and confinement...
But when you have giant tonne+ bags of it left in giant piles, it will cake and become like concrete, and when it's piled like that you get the pressure...
Really all it would take is a bit of heat.
Caking it isn't enough to let it be set off by heat. It takes a substantial shock--it won't even go off for a normal blasting cap. However, there were explosives going off nearby, maybe something landed in it. I don't know if it can be set off by a bit of it being caught under enough falling weight (say, a caked mass dislodged by the fire or the firefighting efforts.)
There are plenty of cases of people blowing themselves up by trying to break up caked AN fertiliser with a crowbar or pick. It needs a fair old whack, but it certainly can be done.
AN makes a fairly good explosive all on its own; 2,750 tonnes of AN yields about the same explosion as 1,000 tonnes of TNT.
In mining, they usually mix it with fuel oil (ANFO), as this significantly increases the explosive power, at very low cost. When AN goes up, it yields a big excess of oxygen, and adding hydrocarbons to scavenge that excess is a big boost to efficiency.
A one kiloton explosion is certainly enough to raze a small town, or a large fraction of a city.
Halifax, Nova Scotia, in 1917; and Port Texas in 1947 were both practically destroyed by a few thousand tonnes of AN. West, Texas was a very small town, and was devastated by an amount of AN exploding that was about ten percent of the size of Halifax, Port Texas, or Beirut.

