Divisions of the ocean floor

The ocean floor. Oceans fill the great depressions of the earth's crust and merge into one another. Geographers name five oceans: the Pacific, Atlantic, Indian, Southern and Arctic; seas, bays and gulfs are parts of them. Most of the ocean floor lies 3-6 km below sea level. Like land, it has mountain ranges, trenches and plains, made by tectonic, volcanic and depositional processes.

Four major divisions:

• Continental shelf: the drowned edge of each continent, under shallow seas and gulfs; the shallowest part of the ocean, with a gradient of 1° or less. It ends at a steep drop, the shelf break. Average width is about 80 km; it is almost absent off Chile and the west coast of Sumatra, while the Siberian shelf in the Arctic Ocean, the largest, is about 1,500 km wide. Depth ranges from about 30 m to 600 m. Shelves collect thick sediments from rivers, glaciers and wind over long periods, which become sources of fossil fuels.
• Continental slope: joins the shelf to the ocean basins; gradient 2-5°, depth 200-3,000 m. Its edge marks the end of the continents. Canyons and trenches occur here.
• Deep sea plain: gently sloping areas of the ocean basins, the flattest and smoothest regions of the world, 3,000-6,000 m deep, covered with fine clay and silt.
• Oceanic deeps (trenches): the deepest parts: steep-sided, narrow basins about 3-5 km deeper than the floor around them. They lie at the foot of continental slopes and along island arcs, with active volcanoes and strong earthquakes, so they are key to studying plate movement. Of 57 deeps explored, 32 are in the Pacific, 19 in the Atlantic and 6 in the Indian Ocean.

Example (new): a ship sailing straight out from a wide, gently shelving coast would cross tens of kilometres of shallow shelf where fishing boats work, then pass over a steep slope where the water deepens fast, and finally reach a deep, flat plain kilometres down.