Manganese crusts are hard, layered deposits rich in manganese, iron, and other metals that form on the ocean floor, particularly on seamounts and ridges. These crusts are significant for their potential as a resource for critical metals and for studying the geochemical processes of the ocean.
Reference: Hein, J. R., Koschinsky, A., & Halbach, P. (2000). “Deep-Ocean Ferromanganese Crusts and Nodules.” In: Cronan, D. S. (Ed.), Handbook of Marine Mineral Deposits. CRC Press, pp. 239-279.