The underlying lithosphere extends from 0 m, i.e. the conventional sea level, to the bottom of the Marianne Trench 11 034 m (10 911 +/- 40 m or 10 994 m – depending on the source) below sea level. The average depth of the World Ocean is 3794 meters. Takes various forms.

Construction of the seabed

The seabed consist of:

• continental shelf
• edge of shelf
• continental slope
  • steep slope
  • gentle slope
• abyssal plain
• mid-oceanic ridges with rifts
• oceanic trenches
• island arches

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Abisal plain is the most common bottom type. These are relatively flat spaces extending at depths of 3000-6000 m below sea level.

Undersea mountains are mid-oceanic ridges. They occur at the interface of ocean plates, where magma builds the lithosphere. They reach a length of thousands of kilometres and a height of 2000-4000 m above the bottom. Sometimes they are high enough to form islands, e.g. Iceland, Azores, Bermuda Islands. Between one ridge belt and the other, a rift valley is formed, with a depth of 1600-2000 m and a width of 30-60 km. Here are the youngest rocks on the planet.

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Oceanic trenches occur at depths below 6000 m, in subduction zones, where the ocean plate hides deep into the Earth. They are several hundred to several thousand kilometres long and only 30-140 kilometres wide, so they are very steep.

Island arches and coastal mountains arise near trenches, where the ocean plate descending in the subduction zone undermines the ocean (arches) or continental (mountains) plates. Separating the water fragment from the ocean, the arches from marginal seas, e.g. the Japanese Islands and the Sea of Japan.

Other elements such as shelves and slopes already belong to continental plates. The Continental shelf is really a flooded, shallow part of the coast with a slope of up to 8 degrees and a depth of up to 200 meters. Then it goes to the continental slope, usually gentle, with an inclination of up to 15 degrees, stretching for many kilometres up to the abyssal plain. There are also steep slopes with slopes up to 40 degrees and more. Then the shelves are absent or very narrow.

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