Question

midsem_2017_Ls4208

4. What is the depth range of a typical deep-ocean basin? Which types of geological features are commonly encountered in deep-ocean basin and why?[1+5 marks]

Answer 1

1. The depth of a typical deep-ocean basin ranges approximately from 3000-4000 metres beneath the surface of the oceans. Deep ocean basins usually refer to ocean floor lying more than  3,200 meters (10,500 feet) below the surface of the oceans. The depth of the four main ocean basins are given:

• The Pacific Ocean basin : ~4,300 meters (14,000 feet)
• The Atlantic Ocean basin: ~3,660 meters (12,000 feet )
• The Indian Ocean basin: ~3,885 meters (12,750 feet)
• The Arctic Ocean basin: ~1,190 meters (3,900 feet)

2. The typical geological features found in deep-ocean basin are mountainous mid-ocean ridges, accompanied by jagged and linear fracture zones, abyssal plains and hills, deep-sea trenches, seamounts, guyots and aseismic ridges.

• Mid-oceanic ridge: The long, continuous chains of volcanic mountain ranges on the ocean floor, extending for more than 64,000 kilometres on the surface of the Earth, forming a global mountain system under the sea and also constituing the longest topographical feature on Earth. These ridges are elevated, rising above the ocean floor and marking those areas on the ocean basin where the oceanic crust are being pulled apart from each other. As this happens, hot magma from beneath the crust emerges as lava to fill the crack created. The lava cools down and settles, associating itself with generation of new oceanic crust- a process commonly called seafloor spreading. More lava is added up by volcanoes erupting along the surface of these ridges. The mid-oceanic ridges are usually below sea level but at some places they extend above sea level, forming volcanic islands (e.g. Iceland). The ridges are separated by a width of 30-50 kilometres by rift valleys, which dissect into the ridge system at depths ranging from 1000-3000 metres.
• Fracture zones: The mid-oceanic ridges are offset at certain places, which are caused by fractures, faults or cracks in Earth's crust. Here the oceanic crust on either side of the fracture or fault zone slides along opposite direction, forming very deep, linear indentations or grooves within the ridges.
• Abyssal plains and hills: Sedimentary deposits from the left overs of marine life along with rock debris (grave, sand, silt) washed away from the surface of the continents for hundreds and thoudands of years, accumulate in layers on the flat areas of the deep-ocean basin, typically found between the continental margins and mid-oceanic ridges. At places, where abyssal plains build up on irregularities of the ocean floor, gently sloping low hills arise, called abyssal hills, which are of volcanic origin.
• Deep sea trenches: These are tectonic areas, characterised by long narrow depressions on the ocean floor, similar to canyons. These features mark the transition between the ocean basins and continents where segments of the oceanic crust are in motion and sliding under segment of the continental crust.
• Seamounts: These are secluded volcanic mountains, which rise to a height of 1000 metres above the ocean basin and sometimes extend above sea level. Seamounts are normally found in the vicinity of mid oceanic ridges or above hotspots of volcanic activity. E.g.: Hawaiian islands.
• Guyots: When the volcanic activity associated with a seamount ceases, this geological feature is subject to erosion, eventually collapsing back into the ocean. For such a seamount that extends above the sea level, it is exposed to wave turbulence and weathering action and the top is eroded flat. Such a submarine isolated volcanic mountain with a flat summit is termed as guyot or tablemount.
• Aseismic ridges: These are long, continuous mountain systems extending across the ocean floor, formed by volcanic activity in and around hot spots. Earthquakes do not occur here on these ridges, a characteristic of this topographical feature, and distinguish them from the seafloor spreading areas on the ocean floor associated with mid-oceanic ridges. E.g.: The Hawaiian- Emperor chain is the best characterised aseismic ridge, where earthquakes occur only at areas of high volcanic activity lying at the end of the ridge that is typically towards the south east end of the Hawaiian islands (seamounts).