Abyssal plains are sediment-covered portions of the deep ocean floor. With surface slopes of less than one foot of elevation difference for each thousand feet of horizontal distance, they are the flattest areas on Earth.
Abyssal plains occur at depths greater than 6,500 ft (2,000 m) below sea level and are underlain by oceanic crust composed primarily of basalt, a dark colored volcanic rock rich in iron- and magnesium-silicate minerals. The basalt is covered by layers of sediment deposited by deep ocean turbidity currents, the shells of marine plants and animals that have fallen down from the ocean’s upper levels, or a combination of both. Turbidity currents, which are often triggered by earthquakes or submarine landslides, are denser than the surrounding ocean water because they carry large amounts of sediment being transported from continental slopes to abyssal plains. Other minor components of abyssal plain sediment include wind-blown dust, volcanic ash, chemical precipitates, and occasional meteorite fragments.
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 Earth’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, and mountain chains such as the mid-oceanic ridge systems.
Abyssal plains are often littered with manganese nodules containing varying amounts of iron, nickel, cobalt, and copper. The pea- to potato-sized nodules form by direct precipitation of minerals from the sea-water onto a bone or rock fragment.
Only a fraction of the 15 billion tons of clay, silt, sand, and gravel that is washed into the oceans each year 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 low, and in many areas less than an inch (2.5 cm) of sediment accumulates per thousand years. Because of the slow rate of sediment accumulation and the flat topography, abyssal plains were once thought to be stable and unchanging environments. Deep ocean currents, however, can periodically scour the ocean floor in places and have damaged transoceanic communication cables laid across abyssal plains.
Abyssal plains do not support abundant aquatic life, but deep sea dredges have collected specimens of unusual fish, worms, and clam-like creatures from these depths.