Abyssal plain

Abyssal plains are vast, underwater expanses located on the deep ocean floor.

These plains are among the flattest and smoothest regions on Earth. They typically lie between the foot of a continental rise and a mid-ocean ridge. These geologic features cover more than 50% of the entire Earth's surface. Because they are so large and deep, they are also some of the least explored areas on our planet. They serve as key components of oceanic basins, which also include elevated mid-ocean ridges and flanking abyssal hills.

The formation of an abyssal plain is a multi-step process driven by plate tectonics. It begins with decompression melting at mid-ocean ridges. In this process, magma rises from the asthenosphere, which is a layer of the upper mantle. As this basaltic material reaches the surface, it cools and solidifies to form new oceanic crust. This crust is then pulled sideways by the spreading of the seafloor. Initially, this new crust has a very rugged and uneven topography. The roughness depends on the spreading rate of the ridge. Fast-spreading ridges move at more than 100 mm/yr, while slow-spreading ridges move at less than 20 mm/yr. Slower spreading often results in rougher crust due to faulting.

To become a flat plain, the uneven crust must be covered by sediment. This process acts like a thick blanket over the rough seafloor. Fine-grained sediments, such as clay and silt, settle over the basaltic rock. Much of this material arrives via turbidity currents. These are currents that travel through submarine canyons from the continental margins into deep water. Other sediments are pelagic, meaning they consist of dust blown from land or the remains of marine organisms. This material sinks from the upper layers of the ocean. In remote areas, the total sediment deposition rate is roughly two to three centimeters per thousand years.

Abyssal plains are located within specific vertical layers of the ocean called zones. Most plains sit in the abyssal zone, which ranges from 3,000 to 6,000 meters deep.

This zone is part of the larger aphotic zone, a region of perpetual darkness. Above the abyssal zone lies the bathyal zone, which extends from 1,000 to 3,000 meters. Below the abyssal zone is the hadal zone, which contains the deepest oceanic trenches. The deepest point on Earth is the bottom of the Mariana Trench, reaching approximately 11,034 meters.

Unlike the surface photic zone, these deep layers do not receive sunlight for photosynthesis.

Life on the abyssal plains is shaped by extreme environmental conditions. The water is very dark and the pressure is immense. At these depths, pressure can reach 750 times the atmospheric pressure at the surface.

Oxygen levels are also quite low in these waters. Much of the dissolved oxygen found there actually originated in polar regions long ago. Because of this scarcity, the plains are inhospitable for many organisms that thrive in oxygen-rich surface waters. However, the plains are believed to be major reservoirs of biodiversity. Ecosystems here depend on the flux of food sinking from the productive layers above. Some deep-sea coral reefs can even be found at depths of 3,000 meters or more.

Humans did not recognize these plains as distinct geographic features until the late 1940s. Before this time, they had not been studied on a systematic basis. Much of our early understanding comes from the HMS Challenger expedition. This British Royal Navy mission took place from December 1872 to May 1876.

It provided essential bathymetric data about the deep sea. Today, we know that the age of the crust on these plains is a function of its distance from the mid-ocean ridge. The crust becomes older, cooler, and denser as it moves away from the ridge.

These plains are also chemically complex environments. In certain areas, metallic nodules are common on the seafloor. These nodules contain various metals, including manganese, iron, nickel, cobalt, and copper. The sediments also contain carbon, nitrogen, phosphorus, and silicon. These elements come from organic material that settles and decomposes.

Abyssal plains also play a role in global systems. They influence ocean carbon cycling and the dissolution of calcium carbonate. Over timescales of a hundred to a thousand years, they can even affect atmospheric CO2 concentrations. This connects the deep, dark seafloor to the very air we breathe.