The Central Indian Ocean Basin (CIOB) extends from 0 S to 25 S latitudes and 70 E to 90 E longitudes( Fig 7). The major portion of CIOB is an abyssal plain and the plains are believed to be developed by the Ganges Fan turbidity currents which distribute the sediments to the far south. Four sedimentary facies are reported in CIOB. Terrigenous sediments dominating above 5 S latitude, siliceous sediments between 5 S adn 15S latitudes with isolated patches of calcareous sediments within, and pelagic sediments south of 15 S latitude.

Prior to launching of the project, very little data was available on the Indian Ocean nodules compared to those of Pacific. This chapter summarises the findings of the project staff as far as CIOB nodules are concerned.

BATHYMETRY IN CIOB

The CIOB is punctuated by numerous uncharted seamounts and hills( fig. 8). The prominent seamounts are at 13 15' S and 74 12E( height- 1200 m); 13 30'S and 80 30 E; 14 10'S adn 81 30'E( height- 1000m) and 13S and 75 45'E (height-1000m). It is observed that the seamounts are generally trending north-south. Apart from the seamounts there are a number of abyssal hills (height 100-600 m) and valleys (depth upto 500 m). It is found that extensive plain area comprises the major portion of the CIOB between 10-15 S and 77- 80 E. There are no prominent fracture zones in the basin. Morphometric and slope angle studies have been carried out on a part of the basin. It is found that gentle slopes ( 0 to 3) predominate in the area. 91.8% of the study area have slope angles in this range whereas only 8.2% of the area has angles higher than 3. These results are relevant to problems relating to the development of mining systems

MORPHOLOGY AND INTERNAL FEATURES OF THE CIOB NODULES

Shape and size of the nodules vary to a large extent and depend upon the geometry and size of the mucleating agents like basaltic fragments, clay lumps, bones and shark teeth. The CIOB nodules in general range in size from 2 to 6 cm and the smaller nodules < 4 cm are subspheriodal to spheroidal in shape. The studies also indicated that polynucleation is more abundant in larger size class and near the vicinity of oceanic ridge system. The roughness of the nodules is related to the thickness of the acoustically transparent layer of the underlying sediments. The layer comprises the major thichness of the zone of the diagnetic remobilization of the metals. It is reported that the highest abundance is associated with the rugged topography like submarine hills, slopes and valleys but with poor metal values (Cu+Ni+Co%) whereas the high metal value nodules occur on the plains . In terms of size, the 2-4 cm size nodules have highest trace metal content. Morphological variations and the internal structures of nodules also show considerable variation with topography. Nodules from the abyssal hills show parallel laminations while the nodules from abyssal plain show well developed columnar structures and in turn are related to the depositional environments.