The topography and morphology of the Japan Trench and its vicinity are compiled in Figures 1 and 2, based on the results of submarine topographic and geological surveys, including multinarrow beam soundings. A typical feature of the Japan Trench and its vicinity from land to sea, consists of the continental shelf, the continental slope, the trench itself (trench landward slope, trench axis basin and trench seaward slope), and the marginal swell between the trench and the typical ocean floor. On the continental slope, generally on its outer margin, elevations of basement are often found. The structural basin on the back side of elevations is usually filled up with younger sediments and forms the deep-sea terrace. Benches are recognized on the trench landward slope. They repesent probably a hinge zone between whole uplifting of the Tohoku Arc and sinking of the lower part of the trench landward slope originated from the subduction around the trench axis since the late Miocene. Topographies of drainages, hollows and mounds caused by submarine landslides are developed along the foot of the trench slope, and in some places the trench-slope apron stretches from the foot of the trench landward slope to the trench axis. The trench axis consists of a series of the elongated basins arranged weakly in echelon. From the trench seaward slope to the marginal swell are found many narrow but long depressions, showing the horst and graben structure. These depressions develop in parallel with formation of the marginal swell under the tensile stress. The subducting angle of the oceanic plate beneath the Japan Trench is 14° at 110km west of the trench axis, based on the analysis of geomagnetic data.

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The chemical composition of marine ferromanganese nodules and crusts are reviewed in relation to their genesis. 1) Crusts on seamounts are usually of hydrogenous origin and have high contents of Co and low contents of Cu. Their mineralogy is vernadite (δ-MnO₂). The high content of Co is owing to the highly oxidizing environment and the low growth rate of hydrogenous crusts. The low content of Cu relative to Ni is attributed to the presence of organically bound copper in seawater, in spite of the fact that the concentration of Cu in deep seawater is similar to that of Ni. 2) Nodules almost buried in pelagic siliceous oozes are commonly of oxic diagenetic origin and have high contents of Ni and Cu. Their mineralogy is todorokite. The high contents of Ni and Cu are owing to their high concentrations in interstitial water relative to deep seawater. Elements associated with settling particles are released into interstitial water during early diagenesis and taken up by manganese nodules. 3) Suboxic diagenetic nodules in hemipelagic environments have low contents of cationic transition elements, especially of Co. The low content of Co is attributed to less oxidizing environments because of a large supply of organic matter by settling particles. The low contents of Ni and Cu are attributed to the high growth rate of suboxic diagenetic nodules. An alternative explanation is the hypothesis of non-steady-state growth. 4) Hydrothermal crusts of which mineralogy is todorokite or birnessite have extremely low contents of cationic transition elements, which are mainly due to the rapid growth of hydrothermal crusts, although the contents of oxyanionic elements in hydrothermal crusts are almost equivalent to those in nodules of other origins. The chemical composition of deep-sea sediments can be simulated by admixtures of aluminosilicates with the average shale composition and Fe-Mn oxides with manganese nodule compositions of four different origins. Therefore, a further understanding of the factors controlling the chemical composition of manganese nodules is necessary to clarify the geochemical cycle of elements in the marine environment.

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