7 
manganese-oxide-reducing bacteria cause dissolution of manganese, 
copper, cobalt, and nickel, but not iron from the nodules. 
INORGANIC ORIGINS 
Earlier hypotheses favored inorganic formation of ferromanganese 
nodules. A recent genetic classification of ferromanganese deposits re- 
views these early theories and describes four types of deposits: (1) 
Hydrogenous deposits which are formed by slow precipitation of iron 
and manganese from “normal” sea water; (2) hydrothermal deposits 
in which the elements are supplied by hydrothermal activity on the sea 
floor, in areas of high heat flow frequently associated with volcanism ; 
(3) halmyrolytic deposits in which the elements are, at least in part, 
supplied by submarine weathering (halmyrolysis), generally of ba- 
saltic material; (4) diagenetic deposits in which the elements are sup- 
plied partly by their postdepositional redistribution within the sedi- 
ment column.° 
Another hypothesis of inorganic formation of manganese nodules 
is linked to data from the Deep Sea Drilling Project. This hypothesis 
proposes that hot intrusive rocks could have raised the temperature in 
overlying sediments thereby increasing the solubility of metals in 
interstitial water. This could cause leaching of metals out of sedi- 
ments and outward migration of waters trapped in the sediment. 
When these warm interstitial waters with higher than normal trace- 
metal concentrations reach the more oxidizing interface with the over- 
lying ocean water, rapid precipitation of manganese, iron, copper, and 
nickel would result. 
The role of iron as the mechanism for manganese nodule formation 
is the basis of another hypothesis. Evidence has been found indicating 
colloidal iron oxides may deposit onto suitable nuclei in cavities under 
conditions of locally high acidity, followed by deposition of manganese 
oxides and trace metals by a catalytic process.” 
In any event, the formation of manganese nodules is a poorly under- 
stood and complex process. It is likely that no single hypothesis fully 
expresses all the mechanisms involved, but portions or combinations of 
several of the hypotheses mentioned may be valid. Present evidence 
seems to indicate that the nodules can be formed in a variety of ways 
depending on local conditions. 
GEOGRAPHICAL DisTRIBUTION 
Manganese nodules are found on the ocean floor in many areas of the 
world. Some world-wide oceanographic expeditions have recovered 
them at nearly every station. Although only about 3 percent of the 
ocean’s floor has been extensively surveyed, there is abundant evidence 
indicating that deposits of manganese nodules exist in potentially 
commercial quantities. The advent of bottom photography and deep 
5 Bonatti, H., T. Kraemer, and H. Rydell. Classification and genesis of submarine iron- 
manganese deposits. In Ferromanganese Deposits on the Ocean Floor. Horn, D. R. ed., 
IDOE, National Science Foundation, Washington, D.C., 1972: 149-166. 
6 Raab, op. cit., p. 46. 
_7 Burns, R. G., and B. A. Brown. Nucleation and mineralogical controls on the compo- 
sition of manganese nodules. Ferromanganese Deposits on the Ocean Floor, Horn, D. R., 
ed., IDOE, National Science Foundation, Washington, D.C., 1972: 51-61. 
