The ocean floor
The ocean floor can be divided into the continental margin and the deep-sea floor. The continental margin consists of the continental shelf, the continental slope, and the continental rise (Fig. 9). The continental shelves have a flat topography (0.1° gradient), the average depth being ~130 m. They are sites for the deposition of abundant land-derived sediments and biogenic carbonates. The continental shelf is separated from the continental slope by the shelfedge or shelf-break at approximately 200 m depth. The gradient of the slope (4° on an average) is much higher than that of the continental rise (1°). The continental shelf and slope together cover ~15% of the total ocean floor, and the continental rise covers about 5%. Abyssal plains are sea floors with a slope of only 0.001o for hundreds of kilometres. About 42% of the deep-sea floor area shows relatively gentle relief.
Seamounts are elevations on the deep-sea floor exceeding 1 km in height. Both flat-topped (guyot) and peaked seamounts are known to occur. Fossil corals, phosphorites, and cobalt-rich manganese crusts may be found at their summits. Mid-ocean ridges are elevated physiographic features of the ocean basins and are the sites of formation of new oceanic lithosphere. The mantle material upwells, and as a result of seafloor spreading and plate movement, seawater comes in contact with fresh magmatic material (Fig. 5 and Fig. 6). The seawater circulates within the newly formed hot rocks, and this forms ‘hot springs’ similar to those on land. Hot water, whose temperature can reach 400°C (in contrast to the ambient seawater temperature of 1-3°C), gushes out of the cracks. These are called hydrothermal vents. The global system of mid-ocean ridges is about 74000 km in length. Trenches are deep, V-shaped valleys on the ocean floor (below the continental slope). They generally occur at the subduction zones where oceanic crustal plates collide with continents or island arcs. Water depth in the famous Mariana Trench (~11 km) located in the Pacific Ocean exceeds the height of Mt. Everest.
The North Indian Ocean has two major submarine fans. These fans are the Bengal fan and the Indus fan (Fig. 10). The basins are filled with sediments that are mostly derived from the continents through river systems. The Bengal fan in the Bay of Bengal is the largest deep-sea fan in the world. Its total area is ~3.0 x 106 sq. km. It is ~3000 km in length, 1430 km at its maximum width, and 20 km at its maximum thickness. The sediments of the fan are largely eroded from the Himalayas and transported by the Ganga-Brahmaputra River system. The sediments making up this deep-sea fan at times were deposited at a rate of 35 cm /1000 yr, a rate comparable to that of deposition in shallow shelves (20-30 cm /1000 yr). By investigating the sediments of the Bengal fan it is possible to identify different phases of the Himalayan uplift history. The Indus fan (1.1 x 106 sq. km area, 1500 km length, 960 km maximum width, >10 km maximum thickness) in the Arabian Sea not only receives sediments from the Himalayas, but also from the alluvial soils of Pakistan and the arid soils of Arabia.