Freezing a volcano is a really annoying but useful trick. Basically, the volcano caster will get mad because he took so long to charge that huge spell, and as he casted it, the damages of the volcano were only 0% to 20%.

When a volcano is casted, and if some landbridges are casted (usually 3 or 4) on top of it, it can freeze the volcano. Note that the right timing is needed, aswel as using the landbridges on the right place, but as long as the landbridges are casted on the volcano cone, it should be fine. 

When a volcano is going to get frozen, the cone will still create a hill, destructing the nearby buildings, and lava on top of the cone, burning units in a small area, but it won't expel more lava, nulling the big powerspike of this spell. At the end, the lava that was in the volcano's cone will remain there, but it's harmless - any unit can walk on it without being burnt.

 

In some advanced stages of levels, where many tribes own many buildings and units (obect data in the level is too high), the chances of a volcano freezing are better - and it is even possible to cast spells (not necessarly landbridges) far away from it, in order to freeze it (example, when a volcano is being cast, a shaman in the other side of the planet can cast some landbridges or some other spells, and the volcano have a good chance of freezing).

On the other hand, on levels with many objects (buildings, units), a volcano might freeze by itself, annoying the caster.

 

You can practice this trick in offline mode, by using mana cheats to get instant volcanos and landbridges.