An authigenic deposit of iron-manganese oxides enriched with cobalt. These crusts may contain potentially commercial quantities of manganese (20% to 30%), copper, nickel, and cobalt (less than 3% combined), but are primarily evaluated on the basis of their cobalt content. They are found as encrustations on exposed rocky seabeds on island slopes, seamounts, or submerged plateaus in water depths between 800 m and 2,400 m. The crusts may be up to 40 cm thick, but are more commonly 3 to 5 cm. They often occur in association with platinum and phosphorite.