The "black smokers" of the deep sea floor have company near the Mid-Atlantic Ridge: the remarkable white carbonate chimneys of the Lost City hydrothermal field. Discovered in 2001, Lost City is an example of a whole new class of structure—and the product of a major geochemical process.
Lost City is quite different from a black smoker. Black smokers are very hot springs that lie in the central rift of deep-sea spreading zones. They form as superheated seawater (200 to 400 degrees C) reacts with hot rocks of the oceanic crust, specifically basalt and its coarse-grained version, gabbro. The water turns to an acidic fluid full of iron and sulfur minerals, which drop out of solution as a dark "smoke" and build up black chimneys on the rocky seafloor.
Lost City, though, is located away from the spreading rift. There seawater reacts not with basalt of the upper crust, but the mantle rocks beneath. These are peridotites, low-silica rocks containing olivine and pyroxenes, a mineral group high in magnesium as well as iron (that is, they are mafic). At relatively cool temperatures, these minerals oxidize to serpentinite plus brucite (Mg(OH)2), magnetite (Fe3O4), and leftover hydrogen ions. The resulting fluid is alkaline rather than acidic, and rich in calcium.
The warm, alkaline fluids bring up different minerals than the hot acid waters of black smokers. Brucite and aragonite (a form of calcium carbonate) build up the white stone towers of Lost City. Surely there are many other places like it, but we haven't been looking for them before.
Another chemical that forms at Lost City is methane, which is something that microorganisms can eat. In fact, the white stony towers of Lost City may form around the mineralized bodies of such microbes.
This chemical reaction—serpentinization—does two things: it produces some heat and it expands the rock by roughly one-third. Naturally the peridotite shatters under this expansion, which gives seawater even better access to fresh rock and creates large amounts of serpentinite mud. With a big enough body of peridotite, this can go on for millions of years—and in fact the whole upper part of the world's oceanic mantle becomes serpentinized as it cools.
Because of serpentinization, oceanic plates take up a great deal of water: a cubic meter of rock can gain as much as 300 kg of water. When oceanic plates undergo subduction, the heat and pressure reverse the serpentinization reaction and release the water into the deep lithosphere, where it gives rise to volcanoes. So the mechanism building Lost City is one that carries huge amounts of water down into the mantle, part of the vast and complex cycles that keep the Earth running.
See lots more about Lost City at the Lost City Expedition site.