Quartz is the most resistant and constitutes much of the beach sands. Basalt and other ferromagnesian rocks weather to clays and iron oxides. These are from coverings or shells made by organisms. Coral reef material and stromatolites are other forms of calcareous deposits. The pump is the process by which carbon dioxide is incorporated into organic carbon by photosynthesis and also by which organisms secrete carbonate shells.
Hydrogenous sediments form slowly by chemical reactions on the ocean floor.
The precise mechanisms are poorly understood. They have been known since the Challenger expedition to be abundant in all major ocean basins. They are mostly manganese dioxide and iron oxide, but they also contain economically significant concentrations of copper, cobalt, and nickel. Bacteria may be involved in nodule formation. They are found in upwelling areas, especially where the oxygen minimum zone intersects the ocean floor.