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.
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