The Movie: Spock is dropped off inside an erupting volcano because apparently that was the mission. He fixes the problem with his handy cold-fusion bomb, which freezes the eruption. Check and mate, volcano.
Real Life: Spock would either melt or boil in his suit. Sure, we're told he's in heat-protective gear, but no air conditioning system is that good. Volcanoes can reach temperatures of 1600-2000 degrees Celsius, well beyond the melting point for most metals. Even if we accept that it some kind of magical suit, what's with calling the freeze bomb a cold-fusion device? Cold fusion is a theoretical technique in which two atoms are fused, releasing buttloads of energy. It's not even cold; it would be done at room temperature, which is only cold compared to nuclear fission. So Spock actually set off a juiced-up atom bomb on top of a geological hot spot. Probably not the best call for saving a planet.