Unit 6: Ocean Floor Structure

Sea Floor Features: Earth's rocky surface is divided into two types: oceanic crust, with a thin dense crust about 10 km thick, and continental crust, with a thick light crust about 40 km thick. The deep, lighter continental crust floats higher on the denser mantle than does the oceanic crust. The diagram below illustrates the basic set up of the ocean floor. Remember, however, that this is just an overall view of what exists.

Continental Shelves zones adjacent to a continent (or around an island) and extending from the low-water line to the depth, usually about 120 m, where there is a marked or rather steep descent toward great depths. The volume of the water in the ocean exceeds the volume of the ocean basins, and some water spills over on to the low lying continental shelves, forming shallow seas.

Continental Shelves Some, such as the South China Sea, are more than 1100 km wide. Most are relatively shallow, with typical depths of m. A few of the more important continental shelves are: the East China Sea, the Bering Sea, the North Sea, the Grand Banks, the Patagonian Shelf, the Arafura Sea and Gulf of Carpentaria, and the Siberian Shelf.

Continental Shelves The shallow seas help dissipate tides, they are often areas of high biological (living) productivity, and they are usually included in the exclusive economic zone of adjacent countries

Continental Slope extends from the continental shelf at an average depth of about 135 meters. Both the continental shelf and slope are considered structurally part of the continents, even though they are below the sea surface.

Ridges Ridges are long, narrow, continuous mountain chains on the sea floor. Individually, ocean ridges are the largest features in ocean basins. Collectively, the ocean ridge system is the most prominent feature on the Earth’s surface. The East Pacific Rise is the largest oceanic ridge

Seamounts isolated or comparatively isolated elevations rising 1000 m or more from the sea floor and with small summit area. These are extinct underwater volcanoes that are cone shaped and often flat- topped. They rise abruptly from the abyssal plain to heights at least 3300 feet above the ocean floor. The Indian Ocean contains the most number of seamounts than any of the other oceans.

Guyots a seamount with a flat top created by wave action when the seamount is extended above sea level.

Trenches long, narrow, and deep depressions of the sea-floor, with relatively steep sides that form at subduction zones. As ocean plates move toward continents, they are drawn under, forming deep trenches. There are 26 oceanic trenches in the world: 3 in the Atlantic Ocean, 1 in the Indian Ocean, and 22 in the Pacific Ocean.

Trenches Generally, the trenches mark the transition between continents and ocean basins, especially in the Pacific basin. Trenches are also the tectonic areas.

Island arcs volcanic island chains that form on the opposing edge of a subducted plate. Examples include the Japanese islands and the Aleutian islands of Alaska.

Canyons are relatively narrow, deep furrows with steep slopes, cutting across the continental shelf and slope, with bottoms sloping continuously downward.

Canyons V-shaped canyons cut into the continental slope to a depth of up to 1200 meters. The submarine canyons are cut perpendicular to the running direction of the continental slope. Many canyons are associated with major rivers such as the Congo, Hudson, and others

Abyssal Plains flat, gently sloping or nearly level regions of the sea- floor. These plains may cover almost a third of Earth’s surface -- about as much as all the exposed land combined. They’re found between the edges of the continents and great underwater mountain ranges. Abyssal plains consist of beds of volcanic rock topped with sediments that are up to thousands of feet thick. Most of the sediments wash off the continents, and are carried to the depths by dense currents. Over time, the sediments spread out to provide a smooth, level surface. Abyssal plains are most common in the Atlantic; in the Pacific, deep trenches around the continents trap most of the sediment before it reaches the open ocean.

Abyssal Plains At depths of thousands of feet, there’s absolutely no light. The water is near freezing, and the pressure is hundreds of times greater than at the surface. Even so, many species have adapted to the harsh conditions. They eat a “snow” of dead organic matter that falls from the upper layers of the ocean. Because of the cold, they have slow metabolisms, so they don’t need to eat very often -- generally, only once every few months.

Basins deep depressions of the sea floor of more or less circular or oval form. Sub-sea features have important influences on the ocean circulation. Ridges separate deep waters of the ocean into distinct basins separated by sills. Water deeper than the sill between two basins cannot move from one to the other. Tens of thousands of isolated peaks, seamounts, are scattered throughout the ocean basins. They interrupt ocean currents, and produce turbulence leading to vertical mixing of water in the ocean.