Concretion of iron and manganese oxides which also contains copper, nickel, and cobalt. Nodules are variable in size, shape, and composition, and are layered internally. The average composition is: manganese 30%; iron 24%; nickel 1%; copper 0.5%; cobalt 0.5%. Manganese nodules are widely distributed on the sea floor of every ocean and in some temperate lakes. They are found in areas of negligible sedimentation and/or strong bottom currents, e.g. on the N. Pacific abyssal plain at depths of 3500–4500 m, and on the shallow, current-swept Blake Plateau off the east coast of the USA. Submarine mining of these deposits is now considered viable to recover copper and other scarce metals. Growth of manganese nodules apparently reached a peak in the early Palaeogene period.