Submarine canyon

Submarine canyons are more common on the steep slopes found on active margins compared to those on the gentler slopes found on passive margins. They show erosion through all substrates, from unlithified sediment to crystalline rock. Canyons are steeper, shorter, more dendritic and more closely spaced on active than on passive continental margins. ==Examples==

• Avilés Canyon, off the coast of Asturias, Spain, reaches depth of about . • Amazon Canyon, extending from the Amazon River • Baltimore and Wilmington Canyons, East Coast of Maryland and Delaware States • Bass Canyon, off the coast of eastern Victoria, Australia • Blanes Canyon in the coast of Catalonia, in de Mediterranean • Bering Canyon, in the Bering Sea • Congo Canyon, the largest river canyon, extending from the Congo River, is long, and deep • Delgada Submarine Canyon, off the coast of California • Hatteras Canyon, off the coast of North Carolina • Hudson Canyon, extending from the Hudson River • Ganges Canyon, extending from the Ganges • Indus Canyon, extending from the Indus River • Kaikōura Canyon, extending offshore from the Kaikōura Peninsula, New Zealand • La Jolla and Scripps Canyon, off the coast of La Jolla, Southern California • Mona Canyon, off the coast of western Puerto Rico • Monterey Canyon, off the coast of central California • Nazaré Canyon, off the coast of Portugal, reaches abyssal depths of • Perth Canyon, off the coast of Perth, Western Australia, reaches a depth of and contains the world's largest plunge pool • Pribilof Canyon, in the Bering Sea • Whittard Canyon, Atlantic Ocean off southwest Ireland • Zhemchug Canyon the deepest and joint-widest submarine canyon in the world, in the Bering Sea ==Formation==

Different mechanisms have been proposed for the formation of submarine canyons. Their primary causes have been subject to debate since the early 1930s. An early and obvious theory was that the canyons present today were carved during glacial times, when sea level was about below present sea level, and rivers flowed to the edge of the continental shelf. However, while many (but not all) canyons are found offshore from major rivers, subaerial river erosion cannot have been active to the water depths as great as where canyons have been mapped, as it is well established (by many lines of evidence) that sea levels did not fall to those depths. The major mechanism of canyon erosion is thought to be turbidity currents and underwater landslides. Turbidity currents are dense, sediment-laden currents which flow downslope when an unstable mass of sediment that has been rapidly deposited on the upper slope fails, perhaps triggered by earthquakes. There is a spectrum of turbidity- or density-current types ranging from "muddy water" to massive mudflow, and evidence of both these end members can be observed in deposits associated with the deeper parts of submarine canyons and channels, such as lobate deposits (mudflow) and levees along channels. Mass wasting, slumping, and submarine landslides are forms of slope failures (the effect of gravity on a hillslope) observed in submarine canyons. Mass wasting is the term used for the slower and smaller action of material moving downhill. Slumping is generally used for rotational movement of masses on a hillside. Landslides, or slides, generally comprise the detachment and displacement of sediment masses. It is now understood that many mechanisms of submarine canyon creation have had effect to greater or lesser degree in different places, even within the same canyon, or at different times during a canyon's development. However, if a primary mechanism must be selected, the downslope lineal morphology of canyons and channels and the transportation of excavated or loose materials of the continental slope over extensive distances require that various kinds of turbidity or density currents act as major participants. In addition to the processes described above, submarine canyons that are especially deep may form by another method. In certain cases, a sea with a bed significantly below sea level is cut off from the larger ocean to which it is usually connected. The sea which is normally repleted by contact and inflow from the ocean is now no longer replenished and hence dries up over a period of time, which can be very short if the local climate is arid. In this scenario, rivers that previously flowed into the sea at a sea level elevation now can cut far deeper into the bottom of the bed now exposed. The Messinian salinity crisis is an example of this phenomenon; between five and six million years ago, the Mediterranean Sea became isolated from the Atlantic Ocean and evaporated away in roughly a thousand years. During this time, the Nile River delta, among other rivers, extended far beyond its present location, both in depth and length. In a cataclysmic event, the Mediterranean sea basin was flooded. One relevant consequence is that the submarine canyons eroded are now far below the present sea level. ==See also==