The Physiography of the Ocean Floor

Bathymetry Scientists use this term to refer to submarine topography. This study did not begin in earnest until after World War II, when researchers began to compile great numbers of depth soundings.

The deep-sea bottom is not flat and featureless as was once believed. It is as rugged and variable as the familiar topography of the land. It was not until the 1950’s that the final chapter in exploring the earth’s physiography was completed by the discovery of the largest mountain belt on the planet, the great chain of midocean ridges

Bathymetric Provinces:

Continental Margins The edges of the oceans are covered by large accumulations of sand and mud that has been eroded from the nearby continents, transported to shores by rivers, and shaped by ocean processes into thick sedementary wedges, or continental margins. Each continental margin consists of the following topographic elements:

Continental Shelf A nearly flat plain that borders the continent and slopes gently toward the ocean basin A continental shelf is terminated on its ocean side by a pronounced change in the degree of the slope called the shelf break Most continental shelves were formed when thick buildups of sedimentary rock were deposited along the continental edges

Continental Slope The ocean side of the continental shelf is a steeper continental slope Drops at an angle of approx 4° to water depths of 2-3 km Submarine canyons are cut into many continental slopes which serve as chutes for sediments to drop to the deep ocean

Continental Rise At the toe of each continental slope, the ocean floor flattens out to a gradient of only 1º This underwater plain of sediment is called the continental rise

The floor of each ocean basin has the following bathymetric features: Deep-Ocean Basins The floor of each ocean basin has the following bathymetric features:

Abyssal Plains The abyssal plain is the flattest area found anywhere on earth, with a slope of less than a meter per kilometer. They are broad areas where land sediments have buried the original ocean floor. These plains usually form at water depths of 3 – 5 km.

Deep-Sea Trenches Relatively steep-sided , narrow basins some 3-5 km deeper than the surrounding ocean floor Occur at the bases of continental slopes Most are found in the Pacific Some are found far from a continent; others lie alongside landmasses

Abyssal Hills The abyssal hill is a low dome or elongated hill no more than 900 m high Composed of volcanic rocks and may be covered by a thin layer of fine-grained sediment that has settled from above

Seamounts Most seamounts are extinct volcanoes with steep sides that rise more than 900 m above the ocean floor Can be found alone or in clusters Guyots are flat-topped seamounts that had the tops eroded by waves before being submerged

MIDOCEAN RIDGES The chain of midocean ridges is the most striking feature of the ocean floor It is the longest, most continuous mountain belt on earth, extending 60,000 km Occupies almost one-third of the ocean floor The summit of each ridge is either pointed or occupied by a rift valley

Midocean Ridges Continents are in black. Ridges are shown by yellow line

Mid-Atlantic Ridge: Horizontal lines represent fracture zones. Circles represent hydrothermal vent areas.

Midocean Ridges cont’d The system of midocean ridges is characterized by frequent, shallow earthquakes and active volcanoes All of the peaks are formed from volcanic rock The center of the ridges are sliced perpendicularly by fracture zones