Description and composition iron-manganese concretions from the Pacific

One the most interesting features of ocean sedimentation is the manganese formations on the surface of the ocean floor in some areas. These are especially widespread in the Pacific Ocean as concretions, grains, and crusts on rock fragments and bedrock outcrops. Iron-manganese concretions are the most abundant as they completely cover about 10% of the bottom of the Pacific Ocean where there are ore concentrations. The concretions occupy from 20-50% of the bottom and up to 80-90% on separate submarine rises. Such concretions are found in different types of bottom deposits, from abyssal red clays to terrigenous muds, but they occur most widely in red clays and quite often in carbonate muds. Their shape and their dimensions are very diverse and change from place to place, from station to station, varying from 0.5-20 cm. They may be oval, globular, reniform, or slaggy and often they are fiat or isometric concretions of an indefinite shape. The concretions generally have nuclei of pumice, basalt fragments, clayey and tuffaceous material, sharks' teeth, whale ossicles, and fossil sponges. Most concretions have concentric layers, combined with dendritic ramifications of iron and manganese oxides.