Tertiary geological map of Flanders (Belgium), scale 1/50.000. It is a subcrop map of the Tertiary formations under the Quaternary erosion surface.

Polymetallic nodules occur in abyssal plains (~4000 – 6000 mm water depth) of all major oceans as two-dimensional deposits, formed on or just below sediment-covered seafloor, rich in metals of economic interest such as manganese (Mn), nickel (Ni), copper (Cu), cobalt (Co), molybdenum (Mo), titanium (Ti), lithium (Li), and rare earth elements (REE). The nodules are generated in marine environments where the sedimentation rates are low, commonly less than 10 centimeters (cm) per thousand years. The nodules consist of micro-layers of Mn oxides and Fe oxy-hydroxides concentrically precipitated around a nucleus. Nodules are generally about golf ball sized, most commonly 1–12 cm in diameter, but can vary in diameter from millimeter-size (micronodules) to as large as 20 cm. The polymetallic nodules are formed by metals precipitation either from ambient seawater (hydrogenetic formation), from pore-waters in the sediments (diagenetic formation), from hydrothermal derived fluids (Bonatti and Nayudu, 1965; Bau et al., 2014; Kuhn et al., 2017) and the formation processes that represent a mix of these different end-member processes. The formation mechanisms control the general chemical composition of the nodules, e.g. the hydrogenetic precipitation leads to an enrichment of Co and REE while the diagenetic precipitation favours enrichment of Ni and Cu. Hydrogenetic nodules grow remarkably slow (1 to 5 mm per million years), whereas diagenetic nodules grow at rates up to 250 mm per million years.