Abyssal plains

Abyssal plains are flat or very gently sloping areas of the deepocean basin floor. They are among the Earth's flattest and smoothest regions and the least explored. Abyssal plains cover approximately 40% of the ocean floor and reach depths between 2,200 and 5,500 m (7,200 and 18,000 ft). They generally lie between the foot of a continental rise and a mid-oceanic ridge.

There are several distinct abyssal plains across the world's oceans. Each abyssalplain starts at a continental rise and continues until it reaches a mid-oceanic ridge, resuming on the other side. Mid-oceanic ridges are huge underwater mountain chains marking major plate boundaries. These ridges are also the primary source of sea floor spreading since they are slowly pulling apart. Since the continental slope and the ridges essentially form the edge of a deep bowl, some people refer to the abyssal plain as the ocean basin. Overall, the abyssalplain represents around 40% of the ocean floor.

Other components of abyssal plain sediment include wind-blown dust, volcanic ash, chemical precipitates, and occasional meteorite fragments. Abyssal plains are often littered with nodules of manganese containing varying amounts of iron, nickel, cobalt, and copper. These pea to potato-sized nodules form by direct preciption of mineralsfrom the sea-water onto a bone or rock fragment. Currently, deposits of manganese nodules are not being mined from the sea bed, but it is possible that they could be collected and used in the future.

Of the 15 billion tons of river-carried clay, sand, and gravel that are washed into the oceanseach year, only a fraction of this amount reaches the abyssal plains. The amount of biological sediment that reaches the bottom is similarly small. Thus, the rate of sediment accumulation on the abyssal plains is very slow, and in many areas, less than an inch of sediment accumulates per thousand years. Because of the slow rate of accumulation and the monotony of the topography, abyssal plains were once believed to be a stable, unchanging environment. However, deep ocean currents have been discovered that scour the ocean floor in places. Some currents have damaged trans-oceanic communication cables laid on these plains.

Although they are more common and widespread in the Atlantic and Indian ocean basins than in the Pacific, abyssal plains are found in all major ocean basins. Approximately 40% of the planet's ocean floor is covered by abyssal plains. The remainder of the ocean floor topography consists of hills, cone-shaped or flat-topped mountains, deep trenches, andmountain chain such as the mid-oceanic ridge systems.

The abyssal plains do not support a great abundance of aquatic life, though some species do survive in this relatively barren environment. Deep sea dredges have collected specimens of unusual-looking fish, worms, and clam-like creatures from these depths.