Actually, this "explosion" occurs because of the following:
Thermite reaches such high temperatures that when in contact with ice (which is water -> Hâ‚‚0) the Hâ‚‚0 molecule decomposes into Hydrogen and Oxygen gas. When these are mixed and combined with the immense temperature -> explosion
Lol. I did'nt notice maxovitsj not saying the reaction needs to be started . It would be completely unlogical if it would ignite and explode by itself.