That manganese nodules and adjacent deep-sea sediments are accumulating manganese at almost the same rate has been established by thorium-230 dating of both sediments and nodules of known manganese content. The rate of manganese deposition is nearly constant over the world oceans. A relatively simple model of nodule evolution explains the distribution of manganese nodules between sediment column and sediment-water interface; the model appears to apply to other trace elements such as copper, nickel, and cobalt.