Wiktionary
n. a large expanse of very flat and smooth ocean floor, usually found at depths of 4,600 to 5,500 meters (15,000 to 18,000 feet).
Wikipedia
An abyssal plain is an underwater plain on the deep ocean floor, usually found at depths between and . Lying generally between the foot of a continental rise and a mid-ocean ridge, abyssal plains cover more than 50% of the Earth’s surface. They are among the flattest, smoothest and least explored regions on Earth. Abyssal plains are key geologic elements of oceanic basins (the other elements being an elevated mid-ocean ridge and flanking abyssal hills). In addition to these elements, active oceanic basins (those that are associated with a moving plate tectonic boundary) also typically include an oceanic trench and a subduction zone.
Abyssal plains were not recognized as distinct physiographic features of the sea floor until the late 1940s and, until very recently, none had been studied on a systematic basis. They are poorly preserved in the sedimentary record, because they tend to be consumed by the subduction process. The creation of the abyssal plain is the end result of spreading of the seafloor (plate tectonics) and melting of the lower oceanic crust. Magma rises from above the asthenosphere (a layer of the upper mantle) and as this basaltic material reaches the surface at mid-ocean ridges it forms new oceanic crust. This is constantly pulled sideways by spreading of the seafloor. Abyssal plains result from the blanketing of an originally uneven surface of oceanic crust by fine-grained sediments, mainly clay and silt. Much of this sediment is deposited by turbidity currents that have been channelled from the continental margins along submarine canyons down into deeper water. The remainder of the sediment is composed chiefly of pelagic sediments. Metallic nodules are common in some areas of the plains, with varying concentrations of metals, including manganese, iron, nickel, cobalt, and copper. These nodules may provide a significant resource for future mining ventures.
Owing in part to their vast size, abyssal plains are currently believed to be a major reservoir of biodiversity. The abyss also exerts significant influence upon ocean carbon cycling, dissolution of calcium carbonate, and atmospheric CO concentrations over timescales of 100–1000 years. The structure and function of abyssal ecosystems are strongly influenced by the rate of flux of food to the seafloor and the composition of the material that settles. Factors such as climate change, fishing practices, and ocean fertilization are expected to have a substantial effect on patterns of primary production in the euphotic zone. This will undoubtedly impact the flux of organic material to the abyss in a similar manner and thus have a profound effect on the structure, function and diversity of abyssal ecosystems.
Usage examples of "abyssal plain".
The climb from the abyssal plain to the continental shelf had worn them out, and only the two-day exploration of the city had allowed them to recover.
Apart from the fact that there was sand rather than ooze underfoot, this place reminded him of the abyssal plain.
The eruption is believed to be taking place on the abyssal plain near a deep sea landmark known as the Scotia Ridge, in water that is approximately two miles deep.
The valley ends just about where the Abyssal Plain begins, at seven hundred miles off the coast, and seventeen thousand feet down.
Maybe Max packed his treasure in an airtight, pressure-resistant container and ditched his plane in the ocean over the mid-Pacific abyssal plain, where it sank in two miles of water.
Steep slopes rose up for thousands of meters above a dark, abyssal plain far below.
Then the machines penetrated the dark basalt of the deep-ocean crust below the Tagus Abyssal Plain.
Besides that, tankers do less damage if they sink in deep water rather than break up on a coast, and we're over the Iberian Abyssal Plain here.
Rather, it's a dry undersea canyon, the kind that slices through a continental shelf down to the edge of the abyssal plain flooring the ocean.
Disruption and a great upweiling commenced on the abyssal plain below.
In a few more hours the bottom would drop away to an abyssal plain, allowing them to go much deeper.
Twelve thousand feet below them was the floor of the Barents Abyssal Plain.