USGS1985
Necker Ridge, Horizon Guyot and S.P. Lee Guyot in the Central Pacific were sampled, seismically surveyed, and photographed by bottom cameras in order to better understand the distribution, origin, and evolution of ferromanganese crusts. Necker Ridge is over 600 km long with a rugged crest, pods of sediment to 146 m thick, slopes that average 12° to 20°, and debris aprons that cover some of the lower flanks. Substrate lithologies are mostly hyaloclastite, volcaniclastic breccia, and minor alkalic basalt. Horizon Guyot, 300 km long and 75 km wide, is capped by at least 160 m of sediment, which buries stepped terraces. Substrate lithologies are similar to those on Necker Ridge, although previous workers sampled much tholeiitic basalt on Horizon. S.P. Lee Guyot, 125 km long and 80 km wide, is capped by at least 300 m of sediment, and contains talus aprons along its lower flanks. Ferromanganese-encrusted rocks were recovered in every dredge and are thickest on Necker Ridge. Crust thicknesses average about 2.5, 1.5, and 0.8 cm for Necker, Horizon, and S.P. Lee, respectively. Crusts range from smooth or porous surfaces to knobby and botryoidal. The entire crust is laminated, however, two distinct layers commonly exist, separated by a paper-thin layer of phosphorite. The dominant mineral of all crusts is vernadite (δ-MnO 2 ), while quartz, feldspar, apatite, and, in three rocks todorokite, are minor phases. Quartz and feldspar decrease with decreasing latitude of occurrence, and is suggested to be related to eolian input. On the average, apatite also increases within the crusts with decreasing latitude of occurrence, which may be related to high biological productivity in the zone of equatorial upwelling. Phosphorite substrates are more abundant on Necker Ridge and S.P. Lee Guyot than they are on Horizon Guyot. Seamount ferromanganese nodules are distinct from abyssal nodules in their chemistry and internal structure.