"Black Smokers" on the Sea Floor

"Black Smokers" are named for the soot-like appearance of the ejected material billowing out of the "chimneys". This substance is super-heated water with very high concentrations of dissolved minerals. As the super-heated water meets the very cold ocean-bottom water, the dissolved minerals precipitate out and settle onto the rock around them. This causes the chimneys to grow in height over time.