An abyssal plain is one of the most extensive yet least explored geographic features on Earth, defining the vast, deep-sea floor. This underwater plain is characterized by its remarkable flatness, with a gradient often less than 1:1,000, making it flatter than most continental plains. Abyssal plains typically lie at depths between 3,000 and 6,000 meters, stretching between the continental rise and the mid-ocean ridge. Covering over 40% of the entire ocean floor, these regions form a massive, continuous habitat that plays a significant role in global oceanic processes.
Formation of the Underlying Oceanic Crust
The process begins with the constant creation of new oceanic lithosphere at mid-ocean ridges, driven by seafloor spreading. As tectonic plates diverge, magma rises from the mantle, cools rapidly upon contact with the seawater, and solidifies to form new basaltic crust. This newly formed crust is inherently rough and irregular, marked by numerous fractures, fault blocks, and small volcanic features known as abyssal hills. As the newly formed crust moves away from the spreading center, it cools and subsides, carrying its rough surface into deeper waters. This foundational bedrock is the initial environment upon which the abyssal plain will eventually form. The rough, basaltic surface of the young crust must be entirely buried by a thick blanket of material for the characteristic flatness to emerge. In areas like the Pacific Ocean, where sediment input is limited, this original rugged topography remains exposed or only thinly covered.
Sources of Sediment Supply
The vast blanket that covers the rough crust comes from two distinct primary origins: land and the ocean itself. The first source is terrigenous sediment, which consists of material weathered and eroded from continental landmasses. Rivers transport this mixture of mud, silt, and clay to the ocean, where it is deposited on the continental shelf and slope. This land-derived material provides the volume of fine-grained particles necessary to fill the great depths of the abyssal plain. The second type of input is pelagic sediment, also known as biogenous ooze, which originates within the water column. This sediment is composed mainly of the microscopic skeletal remains of plankton, such as diatoms and foraminifera, that slowly sink from the surface waters. The accumulation rate of pelagic sediment is slow, often measured in millimeters per thousand years. While pelagic particles form a continuous fine layer, the formation of a thick, smooth abyssal plain relies on the higher volume delivery of terrigenous material from nearby continents.
Sediment Deposition and Smoothing
The smoothing of the underlying rugged crust is primarily accomplished by powerful, gravity-driven currents. These events, known as turbidity currents, are dense, fast-moving underwater avalanches of sediment-laden water that surge down the continental slope and rise. They are triggered by geological disturbances, such as earthquakes or the slumping of unstable sediment piles at the shelf edge, and can travel for hundreds of kilometers across the sea floor. These currents channel massive volumes of terrigenous sediment from the continental margin out into the deep ocean basins.
When a turbidity current reaches the relatively flat abyssal plain, its velocity slows dramatically, causing the suspended material to settle out. The sediment settles in a characteristic pattern known as graded bedding, forming a distinct layer called a turbidite. In this layer, the coarsest, heaviest particles, like sand, settle first, creating the base, while the finest clay and silt particles drift down last, forming the top layer. Repeated sequences of these turbidite layers accumulate over millions of years, building the abyssal plain from the bottom up.
The continuous deposition of these horizontal layers systematically infills the low-lying areas, effectively burying the rough abyssal hills and valleys of the original basaltic crust. This blanketing process ultimately creates the signature flatness of the abyssal plain, as the sediment surface becomes level regardless of the topography beneath. The resulting sedimentary cover on a mature abyssal plain is substantial, often averaging one kilometer in thickness, which is far more than enough to completely obscure the original features of the crust below.