A blowout preventer with a block having a channel and one or more plates to close the channel. The block is placed over a pipe through which oil or other fluid is escaping, so that the pipe is in the channel. Alongside the channel are one or more circular plates, having diameters somewhat larger than the diameter of the channel. Explosive charges propel the plates into the channel to seal it off and stop the leak. The explosive charges may be ignited by radio, electric or sonic signals or other suitable means. There are passages for the plates that are orthogonal to the cylindrical channel, that allow the plates to move into place to block the channel, while preventing them from moving too far and overshooting the channel. Ratchets keep the plates from moving back after they have blocked the channel. Vents allow gas from the explosive charges to escape.