Abyssal plains form when thick layers of sediment settle over rough, uneven ocean floor, gradually burying the underlying terrain until it becomes extraordinarily flat. These plains sit at depths of roughly 3,000 to 6,500 meters (10,000 to 21,000 feet), cover about 70% of the ocean floor, and rank as the largest habitat on Earth. Their formation involves two major processes working together over millions of years: the creation and cooling of oceanic crust, and the slow but relentless accumulation of sediment from above and from the continents.
It Starts With New Ocean Floor
The story begins at mid-ocean ridges, underwater mountain chains where tectonic plates pull apart and magma wells up to create fresh oceanic crust. This new rock is hot, buoyant, and relatively elevated. As tectonic forces push it away from the ridge, the crust gradually cools, becomes denser, and sinks deeper into the underlying mantle. This cooling and subsidence is the fundamental reason deep ocean basins exist at all. Magmatic and tectonic processes define the shape of ocean basins and, through plate cooling and subsidence, set the stage for abyssal depths to develop.
The young ocean floor near ridges is anything but flat. It’s riddled with abyssal hills, small volcanic mounds and fault-created bumps that form during the chaotic process of crustal creation. These hills are actually the most common landform on Earth’s surface, though almost nobody sees them. Left exposed, they’d make the deep ocean floor look rugged and uneven. What transforms this rough terrain into a plain is sediment.
Sediment Buries the Rough Terrain
Over millions of years, material rains down and flows across the ocean floor, filling in the valleys between abyssal hills and smoothing out the topography. This sediment comes from two main sources.
The first is biological and chemical material that sinks from the waters above. Tiny organisms like plankton die and drift to the bottom in what oceanographers call “marine snow.” Volcanic ash, wind-blown dust, and fine clay particles from the atmosphere also settle onto the seafloor. This pelagic sediment accumulates extremely slowly, often just a few centimeters per thousand years, but over tens of millions of years it adds up to layers hundreds of meters thick.
The second source is land-derived sediment carried by turbidity currents. These are underwater avalanches of sediment-laden water that race down continental slopes and across the ocean floor at high speed. Triggered by earthquakes, storm waves, or simply the collapse of unstable sediment piles at the edge of continental shelves, turbidity currents are the primary carriers of terrestrial sediment to the deep sea. They transport massive volumes of sand, silt, and mud far out onto the ocean floor, spreading material across enormous areas. Each turbidity current deposits a thin layer, and over geological time these layers stack up, filling low spots first and progressively leveling the terrain.
The result is a surface so flat it’s almost hard to believe. Abyssal plains typically have a gradient of less than 0.05 degrees. To put that in perspective, if you walked across one for a full kilometer, you’d gain or lose less than a meter in elevation. They are, by a wide margin, the flattest places on Earth.
Why the Atlantic Has More Than the Pacific
Not all ocean basins develop abyssal plains equally. The Atlantic Ocean has the most extensive and well-developed examples, while the Pacific has far fewer. The reason comes down to plumbing.
In the Pacific, deep ocean trenches line the edges of the basin where tectonic plates collide. These trenches act like gutters, intercepting sediment flowing off the continents and trapping it before it ever reaches the open ocean floor. The sediment that would otherwise spread across the deep Pacific gets swallowed into subduction zones instead.
The Atlantic lacks this ring of trenches. Its continental margins are mostly passive, meaning sediment from rivers like the Amazon, Mississippi, and Congo flows off the shelves and travels via turbidity currents directly onto the deep ocean floor. With nothing to intercept it, that sediment spreads freely, creating some of the flattest and most extensive abyssal plains on the planet. The Sohm Abyssal Plain in the western North Atlantic, for example, stretches across an area roughly the size of Western Europe.
A Process Measured in Millions of Years
Abyssal plain formation is extraordinarily slow by human standards. The oceanic crust beneath these plains needs time to cool, sink, and move far enough from the mid-ocean ridge for sediment to accumulate in meaningful thickness. The oldest ocean floor sediments ever recovered date to the Jurassic Period, roughly 200 million years ago. No oceanic crust survives longer than that because it eventually gets recycled back into the Earth’s interior at subduction zones.
This means abyssal plains are constantly being created and destroyed on geological timescales. New ocean floor forms at ridges, cools and subsides over tens of millions of years, gets buried in sediment, and eventually slides back into the mantle at a trench. The plains we see today are snapshots of an ongoing cycle. Some areas of the Atlantic basin have had over 100 million years to accumulate sediment, which is why those plains are so thick and flat. Younger ocean floor closer to mid-ocean ridges still has its abyssal hills poking through, not yet buried.
What Lies Beneath the Surface
If you could drill straight down through an abyssal plain, you’d pass through distinct layers. The top consists of fine, soupy sediment, often clay and the silica or calcium carbonate shells of dead microorganisms. Below that, the sediment becomes progressively older and more compacted. Turbidity current deposits show up as repeating sequences of coarser material at the bottom grading into finer material at the top, each layer representing a single event. Deeper still, you’d hit the original basaltic ocean crust with its buried hills and valleys, the rough foundation that all that sediment worked so hard to hide.
The total sediment thickness varies enormously depending on proximity to continents and the age of the underlying crust. Near continental margins with active rivers feeding sediment into the ocean, deposits can be several kilometers thick. In the central Pacific, far from any land and shielded by trenches, sediment cover can be startlingly thin, sometimes just a few tens of meters over crust that is itself tens of millions of years old.