BLACK SMOKER CHIMNEY ROCKS
Some of the most inaccessible rocks on Planet Earth are on the seafloor, at mid-ocean ridges (spreading centers, where two tectonic plates are separating). Seafloor hydrothermal vents at mid-ocean ridges were first discovered back in the 1970s, but they still demand much awe and wonder.
Near spreading centers, cold ocean water that has percolated downward into oceanic crust gets heated up and rises through fracture systems to emerge as seafloor hydrothermal springs. While in the subsurface, the superheated waters leach out metals and other materials from the rocks. As the waters emerge from the seafloor, the surrounding cold bottom waters induce precipitation of dissolved minerals. The result is small to large chimneys that appear to be smoking (see example; another example). ItŐs not really smoke. The chimneys are emitting clouds of finely-crystalline precipitates. The chimneys themselves are composed of several different minerals, and are gradually built up by consolidation of precipitates. The rocks making up these black smokers are dominated by sulfide minerals.
Here's a partial cross-section slice from two amalgamated seafloor chimneys. The golden-brown, metallic-lustered, sparkly material along the upper left margin is principally chalcopyrite (CuFeS2 - copper iron sulfide). The dark material in the rest of the specimen is mostly a mix of chalcopyrite and sphalerite (ZnS - zinc sulfide). The whitish material is principally anhydrite (CaSO4 - calcium sulfate).
Black smoker chimney rock (above & below; 8.8 cm across) - partial cross-section slice of two amalgamated chimneys.
Below: labels indicate chimney interiors and white line shows approximate boundary between the two fused chimneys.
Black smoker chimneys vary in size from quite small to well over 100' tall. They are moderately unstable on short geologic time scales. An abundance of odd life forms occupy the environs of these seafloor hydrothermal vents. Unlike ecosystems anywhere else, the base of the food chain for these vent biotas is not photosynthetic organisms, but chemosynthetic organisms. It's been hypothesized that life on Earth originated in seafloor hydrothermal vent areas.
Collected 19 November 1981 by the deep-sea submersible Alvin (Alvin dive # 1156).
Locality: seafloor hydrothermal vent at 2597 meters depth, northern East Pacific Rise, near-easternmost Pacific Basin. Coordinates: 20ˇ 51.00Ő North, 109ˇ 04.00Ő West.