TECTONICS/Hydrothermal Vents At Mid-Ocean Ridges 393 Figure Morphological and mineralogical evolution of chimneys (Left): A time series of seafloor photographs showing the morpho logical development of a chimney that grew on top of lava flows that erupted in 1991 on the crest of the East Pacific Rise near 9 50.30 N Within a few days to weeks after the eruption, anhydrite rich Stage I ‘protochimneys’ a few centimetres high had formed where hot fluids emerged from volcanic outcrops that were covered with white microbial mats (top left) Eleven months later, the chimney consisted of cylindrical Stage II anhydrite sulphide mineral spires approximately m in height, and as yet unpopulated by macrofauna (middle left) Three and a half years after the eruption, the cylindrical conduits had coalesced into a m high chimney structure that was covered with inhabited Alvinelline wormtubes (bottom left) (Right): Photomicrographs of chimney samples from the eruption area that show how the chimneys evolved from Stage I (anhydrite dominated; top right) to Stage II (metal sulphide dominated) mineral compositions (see text) As the fluids passing through the chimneys cooled below $330 C during Stage II, the CuFe sulphide minerals in the chimney walls (middle right) were replaced by Zn sulphide and Fe sulphide minerals (bottom right) Abbreviations: gr, grained; po, pyrrhotite; an, anhydrite; py, pyrite; cp, chalcopyrite Photographs reproduced from Haymon R, Fornari D, Von Damm K, et al (1993) Volcanic eruption of the mid ocean ridge along the East Pacific Rise at 9 45 520 N: direct submersible observation of seafloor phenomena associated with an eruption event in April, 1991 Earth and Planetary Science Letters 119: 85 101