The abyssal plain covers 40% of the ocean floor, is flat due to slow sediment accumulation, and is sparsely populated by bottom feeders. Hydrothermal vents, cold seeps, and whale falls provide oases for small ecosystems. Less than 0.1% of the abyssal plain has been explored by humans.
The ocean floor off the continental shelf is known as the abyssal plain. The abyssal plain is between 2,200 and 5,500 m (7,200 and 18,000 ft) deep and covers about 40% of the ocean floor. These areas are among the flattest and least explored on the earth’s surface. Less than a tenth of 1% of the abyssal plain has been explored by humans, mostly using deep-sea robots.
The ocean floor is flat due to sediments constantly accumulating there at an even rate. These sediments are divided into three types: siliceous oozes (from silica shells), calcareous oozes (from calcite shells), and red clays (from windblown sand and micrometeorites). The sediments accumulate very slowly, a few centimeters per millennium.
Life on the ocean floor is very scarce, making it look like a vast desert. Marine organisms prefer shallow waters where the energy of photosynthesis is abundant, providing the foundation of the food chain. There are several oases on the ocean floor: hydrothermal vents, cold seeps, and whale waterfalls. Hydrothermal vents release sulfide-rich minerals, which can be processed by chemotrophic bacteria that act as the keystone for small ecosystems. In cold seeps, a methane-filled brine seeps out of cracks in the sea floor, also providing energy for the bacteria. A whale tumble is a whale carcass that falls from a height. Because scavengers are so scarce on the ocean floor, it could take decades or centuries for a whale to be consumed. Scientists estimate the frequency of whale falls at about one every 25 km (15 mi).
The ocean floor is populated by bottom feeders that look somewhat like blobs. The blobfish, first discovered off the coast of Australia, is one example. Due to the extreme pressure, these fish have developed a gelatinous texture of the flesh with a slightly lower density than sea water. This allows them to float just above the surface without spending any effort on swimming. Many bottom feeders have a lower jaw, which allows them to scoop up nutrient-rich slime from the ocean floor.
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