Deep Sea Minerals - Vol 3 - Cobalt-rich Ferromanganese Crusts

1.1

The formation and occurrence of cobalt-rich ferromanganese crusts

Cobalt-rich ferromanganese crusts, precipitate onto nearly all rock surfaces in the deep ocean. Their thickness varies from less than 1 millimetre to about 260 millimetres. They occur only where the rock surfaces are free of sediment. There, they form pavements of intergrown manganese and iron oxides. Ferromanganese crusts may also coat rock pebbles and cob- bles. They form at water depths of 600 to 7 000 metres on the flanks of seamounts (undersea mountains with a height above 1 000 metres), knolls (heights of 200 to 1 000 metres), ridges, and plateaus. Crusts with sufficient mineral contnet to be of economic interest commonly occur at depths of about 800 to

2 500 metres (Hein et al . 2000, 2009). In the Pacific Ocean, there are more than 11 000 seamounts (57 per cent of the glob- al total) and 41 000 knolls (Yesson et al . 2011, estimated from the latest global bathymetry), and many more might exist in uncharted waters (Wessel et al . 2010). Many seamounts are within the Exclusive Economic Zones (EEZs) of Pacific Island countries (Figure 2). The Atlantic Ocean has fewer seamounts, and the cobalt-rich crusts that occur there are usually associ- ated with hydrothermal activity at sea-floor-spreading centres (see box over page), with the exceptions of the northeast and northwest continental margin areas.

Location of Seamounts

160°E

170°E

180°E

170°W

160°W

150°W

140°W

130°W

120°W

110°W

10°N

0°

10°S

20°S

30°S

Seamounts with summits between 800 and 2500 metres depth Seamounts with summits above 800 and bellow 2500 metres depth

Land areas

Seabed from 2500 to 4000 metres depth

Seabed from 0 to 800 metres depth

Seabed from 4000 to 6000 metres depth

Outer boundary of States economic exclusive zones (EEZ)

Seabed from 800 to 2500 metres depth

Seabed from 6000 metres and deeper

Figure 2. Bathymetric map of the Pacific showing the location of seamounts.

COBALT-RICH FERROMANGANESE CRUSTS 8