Cobalt crusts are created when metal ions in the water react with oxygen to form oxides, which are deposited on the rock surfaces at seamounts. Oxides, and thus cobalt crusts, can only form where sufficient oxygen is present in the sea water.
Paradoxically, however, the thickest cobalt crusts on seamounts are located near the ocean depth layer with the least oxygen. This oxygen minimum zone generally has a thickness of several hundred metres, and in most places it is located at a depth of around 1000 metres. It is produced by the bacterial breakdown of sinking dead biomass, a process that consumes oxygen in the water. Because the water here is not mixed by storms and waves, very little oxygen penetrates to this depth. So, theoretically, it would seem that neither oxides nor cobalt crusts should be formed.
This apparent contradiction, however, can be explained as follows: Because there is very little oxygen within the oxygen minimum zone, the metal ions can form few oxides, so the ions are enriched in the oxygen-poor water. At higher elevations of the sea floor such as seamounts, however, oxygen-rich deep water flows upward from the bottom. This can be sea water, for example, that cools intensely at the South Pole, sinks to the sea floor and spreads through the deep ocean. At the seamounts, this Antarctic deep water introduces oxygen to the oxygen-poor waters enriched in metal ions, and as a result metal-rich oxides are formed that subsequently precipitate onto the rock surfaces over time to produce crusts.