Bomb rock
Bomb rocks are explosive rocks in the Pikmin games. In the first game, they can be carried by Yellow Pikmin, where their main purpose is to destroy rock walls. They can also be used to defeat enemies, and are particularly useful for doing so with bosses. They are usually found inside tin cans, pipes, or large animal skulls.
In Pikmin 2, they are much larger and cannot be carried by Yellow Pikmin, but can be detonated by attacking them. In certain dungeons, you can trick nearby Emperor Bulblaxes into swallowing them, which will stun the beast and deal considerable damage. They are also produced by Careening Dirigibugs. Volatile Dweevils carry bomb rocks on their backs, presumably for protection, even though they themselves are killed in the blast. Never throw Purple Pikmin onto a bomb rock, as that can cause near-instant detonation. Bomb rocks can even be petrified, so do that if you are in a pinch, but keep in mind that they will not release nectar or spray like many beasts do if killed in that state. In the 2-player battle stage, Hostile Territory, bomb rocks can be picked up and thrown by your captain, which you can use to defeat enemies, or deal a serious blow to your opponent's Pikmin count.
Instant explosion
Bomb-rocks will explode with a shorter fuse when a Purple Pikmin is thrown directly onto them: be warned, though, that the Pikmin is more than likely to die. Also, if the "One Hit Kills Everything" Action Replay code is used, they will explode about as fast as if a Purple Pikmin fell onto them.