Atlantic Ocean Floor: Ridges, Basins, and Deeps Explained

The Atlantic Ocean floor is far from a flat, featureless plain. Stretching across the second-largest ocean basin on Earth, it displays a complex landscape of ridges, basins, and trenches that tell the story of plate tectonics and geological history. Understanding these relief features helps us grasp how our planet’s surface continues to reshape itself even today.

The continental shelf: where land meets ocean 🔗

The continental shelf is the submerged extension of continents, forming a relatively shallow platform before the ocean floor drops away dramatically. In the Atlantic, these shelves display remarkable variation in width. Some areas feature narrow shelves measuring as little as 2 kilometers wide, while others extend up to 400 kilometers into the ocean.

This dramatic variation depends largely on coastal geography. Along mountainous western coasts like those of the Americas, the continental shelf measures only about 32 kilometers wide, whereas the eastern Atlantic coast can extend more than 120 kilometers. The shelves provide rich fishing grounds because sunlight penetrates these shallow waters, supporting abundant marine life.

Marginal seas and productive waters 🔗

The Atlantic hosts several significant marginal seas that sit atop these continental shelves. The Mediterranean Sea stands out as one of the most prominent, formed where the continental shelf widens between Europe and Africa. Other notable marginal seas include the North Sea, Baltic Sea, and Caribbean Sea. These semi-enclosed bodies of water support diverse ecosystems and have played crucial roles in human maritime history.

The Mid-Atlantic Ridge: Earth’s longest mountain range 🔗

Running down the center of the Atlantic Ocean like a massive underwater spine, the Mid-Atlantic Ridge extends for about 10,000 miles in a curving path from Iceland in the north to near the southern tip of Africa. This S-shaped formation represents one of the most striking features of the ocean floor.

The ridge has an average spreading rate of about 2.5 centimeters per year, constantly creating new oceanic crust as tectonic plates pull apart. While most of the ridge remains submerged, several peaks thrust above the ocean surface, forming islands like Iceland, the Azores, and Tristan da Cunha.

Major branches extending from the ridge 🔗

The Mid-Atlantic Ridge doesn’t run in a simple straight line. Instead, it sends off several significant branches that connect to various landmasses. The Newfoundland Rise branches off near 50° latitude and extends northwest toward Newfoundland. This elevated feature influences ocean currents and marine habitats in the northwestern Atlantic.

Further south, the Azores Rise bifurcates from the main ridge south of 40°N latitude. This rise represents an area of thickened oceanic crust that formed over approximately 20 million years, created by the interaction between the Mid-Atlantic Ridge and volcanic hotspot activity. The Azores archipelago itself sits atop this plateau, with nine volcanic islands extending across nearly 500 kilometers.

Near 40°S latitude, the ridge sends off another major branch called the Walvis Ridge. This large volcanic feature extends northeast and merges with the African continental shelf. Along with its conjugate partner, the Rio Grande Rise on the South American side, the Walvis Ridge formed as volcanic edifices when a hot spot erupted at the Mid-Atlantic Ridge during the Late Cretaceous period.

Ocean basins: deep depressions divided by the ridge 🔗

The Mid-Atlantic Ridge effectively divides the Atlantic Ocean into two sets of deep basins. These basins, formed by the continuous spreading of the seafloor, range between 3,700 and 5,500 meters in depth and represent some of the flattest areas on Earth.

Western Atlantic basins 🔗

On the western side of the ridge, several major basins dominate the seafloor. The Labrador Basin sits between Greenland and Canada, where depths reach approximately 3,400 meters. This basin plays a crucial role in ocean circulation, contributing significantly to the formation of North Atlantic Deep Water.

The North American Basin extends along much of the western Atlantic, stretching from the continental shelf eastward to the Mid-Atlantic Ridge. These basins accumulate sediments transported by ocean currents and receive material from the continents through submarine canyons and turbidity currents.

Eastern Atlantic basins 🔗

On the African side of the ridge, the Cape Verde Basin stands as one of the most significant features. This submarine depression has an average depth of more than 23,600 feet below sea level. Unlike many ocean basins covered with calcareous oozes, the Cape Verde Basin contains red clay, indicating unique sedimentation patterns influenced by deep ocean currents.

Other eastern basins include the Canary Basin to the north and the Angola Basin to the south. Together with their western counterparts, these basins form a network of deep-sea environments that host unique ecosystems adapted to extreme pressure, darkness, and cold temperatures.

Atlantic deeps: few but significant 🔗

Unlike the Pacific Ocean, which is ringed by numerous deep ocean trenches, the Atlantic contains relatively few deep trenches. This difference stems from fundamental geological characteristics of the two oceans.

The Puerto Rico Trench: Atlantic’s deepest point 🔗

The Puerto Rico Trench represents the deepest point in the Atlantic Ocean, with depths between 8,376 and 8,740 meters. Located roughly 75 miles north of Puerto Rico, this 810-kilometer-long trench marks where the North American plate subducts beneath the Caribbean plate.

The trench’s deepest point, known as the Milwaukee Deep, was first reached by a crewed submersible in 2018 when explorer Victor Vescovo descended to 8,376 meters. The extreme depth creates an environment with pressures more than 1,000 times greater than at the surface, yet scientists have discovered unique life forms adapted to these conditions.

Why the Atlantic has fewer deeps 🔗

The scarcity of deep ocean trenches in the Atlantic relates directly to plate tectonic processes. Ocean trenches form primarily at subduction zones where oceanic plates plunge beneath continental or other oceanic plates. The Pacific Ocean, surrounded by active subduction zones forming the “Ring of Fire,” contains dozens of trenches.

In contrast, the Atlantic represents a younger ocean with passive continental margins along most of its coasts. Rather than active subduction, the Atlantic formed through the rifting apart of continental plates and continues to widen as new crust forms at the Mid-Atlantic Ridge. This divergent boundary creates the ridge itself but not the deep trenches characteristic of convergent boundaries.

The few trenches that do exist in the Atlantic, like the Puerto Rico Trench and South Sandwich Trench, occur where localized subduction takes place. These represent exceptions rather than the rule in Atlantic geology, making them particularly interesting to scientists studying plate tectonics and deep-sea ecosystems.

What do you think? How might the different geological characteristics of the Atlantic versus the Pacific Ocean affect marine life distribution and ocean circulation patterns? How do you think the continuous spreading of the Mid-Atlantic Ridge might change the Atlantic Ocean over the next few million years?

References

1. https://www.noaa.gov/education/resource-collections/ocean-coasts/ocean-floor-features
2. https://www.britannica.com/science/continental-shelf
3. https://www.britannica.com/place/Mid-Atlantic-Ridge
4. https://en.wikipedia.org/wiki/Mid-Atlantic_Ridge
5. https://www.geographynotes.com/oceanography/bottom-reliefs-of-various-oceans-oceanography-geography/2592
6. https://en.wikipedia.org/wiki/Azores_hotspot
7. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020GC009390
8. https://en.wikipedia.org/wiki/Labrador_Sea
9. https://www.britannica.com/place/Cape-Verde-Basin-Atlantic-Ocean
10. https://en.wikipedia.org/wiki/Puerto_Rico_Trench
11. https://www.britannica.com/science/deep-sea-trench