The Ocean Floor

The floor of the ocean is not a flat, featureless basin. Hidden beneath the water lies a dramatic landscape: gently sloping shelves, steep slopes, vast flat plains, towering underwater mountains and trenches deeper than the Himalayas are tall. This relief of the ocean floor is divided into a few major zones, and knowing them is essential physical geography.

The submerged edge of a continent is the continental margin, made of three parts:

• The continental shelf: the shallow, gently sloping margin of the continent, submerged to about 100 fathoms (600 feet). It is the most important zone. It holds about 20% of the world's oil and gas and supports the richest fishing grounds (such as the Grand Bank and Dogger Bank). Its width varies greatly: narrow off steep coasts, very wide off the Indian Ocean's west.
• The continental slope: a much steeper drop beyond the shelf, marking the true edge of the continent.
• The continental rise: a gentle wedge of sediment where the slope meets the deep ocean floor.

The point where the gentle shelf gives way to the steep slope is called the shelf break.

Beyond the margins lies the deep-sea plain or ocean basin, covering nearly three-quarters of the ocean floor at an average depth of about 4 km. Its key features:

• Abyssal plains: extremely flat, sediment-covered expanses, among the flattest places on Earth.
• Seamounts: isolated underwater mountains, often old volcanoes, rising from the floor. Those that break the surface become islands.
• Guyots: flat-topped seamounts, their tops worn flat by waves before they sank.

The deep ocean also has the planet's most extreme relief:

• Ocean trenches: long, narrow, very deep gashes. They form at subduction zones, where one tectonic plate slides beneath another. The Mariana Trench in the Pacific is the deepest point on Earth (the Challenger Deep, ~11 km).
• Mid-ocean ridges: great underwater mountain chains. Plates spread apart here and new crust forms. The Mid-Atlantic Ridge is part of the longest mountain range in the world.
• Submarine canyons: steep-sided valleys cut into the shelf and slope (e.g. the Hudson Canyon).

So the ocean floor records the same plate movements (spreading at ridges, sinking at trenches) that shape the land above.

Exams often ask which ocean a named trench lies in. Learn this set:

• Mariana Trench: western Pacific Ocean. Its Challenger Deep is the deepest point on Earth.
• Aleutian Trench: North Pacific Ocean, along the Aleutian Islands south of Alaska.
• Kermadec Trench: South Pacific Ocean, northeast of New Zealand.
• Sunda Trench: Indian Ocean, south of Java and Sumatra. It is also called the Java Trench.
• Diamantina Trench: southeastern Indian Ocean, off the coast of Western Australia. It is also called the Diamantina Deep.
• South Sandwich Trench: South Atlantic Ocean, near the South Sandwich Islands.

Note the pattern. Most great trenches ring the Pacific, but the Indian Ocean has the Sunda and Diamantina trenches, and the South Atlantic has the South Sandwich Trench.