Seafloor Topography: Shelves, Trenches, and Abyssal Plains

Seafloor topography is the shape of the ocean bottom, which includes the continental margin (shelf, slope and rise), the flat abyssal plains of the deep ocean floor, the mid-ocean ridges where new crust forms, and the deep trenches where plates are subducted. In brief: the average depth of the ocean is about 3,700 m, and the deepest known point, the Challenger Deep in the Mariana Trench, is about 10,935 m below sea level. The continental shelf is a shallow, gently sloping platform extending to about 130 m depth on average, and the abyssal plains lie at 3,000 to 6,000 m and are among the flattest places on Earth. Most of the seafloor shapes are produced by plate tectonics.

The seafloor was almost unknown until the 20th century, and it is still less mapped than the surface of Mars. This guide explains the main seafloor features and how they form, how the seafloor is mapped, why they matter, compares the features in a table, works through calculations of depth, pressure and spreading, outlines the history of ocean exploration and corrects common mistakes.

What features does the continental margin have?

• Continental shelf: the submerged edge of a continent, with a very gentle slope (usually less than 0.1°) and a width from a few kilometers to more than 1,000 km (in the Arctic Ocean off Siberia). It ends at the shelf break, typically at a depth of about 130 m, though this varies. The shelf is the most productive part of the ocean for fishing, and it holds much of the oil and gas extracted at sea. During the ice ages, when sea level was about 120 m lower, much of the shelf was dry land.
• Continental slope: a steeper descent (on average a few degrees, and locally much more) from the shelf break to the deep ocean floor, often cut by submarine canyons, some of which, such as the Monterey Canyon in California, are comparable to the Grand Canyon. Turbidity currents (fast underwater avalanches of sediment) carve these canyons.
• Continental rise: a gentle apron of sediment at the base of the slope, built up from material that has slid down the slope, at depths of about 3,000 to 4,000 m.

At passive margins (such as the Atlantic coasts of the Americas, Africa and Europe), the margin is wide and covered with sediment. At active margins (such as the Pacific coasts of South America), the shelf is narrow, the slope is steep and a deep trench lies close to the coast.

What features does the deep ocean floor have?

• Abyssal plains: vast, flat areas at depths of 3,000 to 6,000 m, covering about half of the ocean floor. They are flat because fine sediment, such as clay and the remains of tiny organisms, has buried the uneven basaltic crust. Their slope is less than 1 part in 1,000.
• Mid-ocean ridges: underwater mountain chains about 65,000 km long, where new oceanic crust is created at divergent plate boundaries, as at the Mid-Atlantic Ridge. They rise 2,000 to 3,000 m above the abyssal plains. Ridges have a central rift valley, and hydrothermal vents (“black smokers”) release hot, mineral-rich water, with ecosystems based on chemosynthesis (energy from chemicals) and not from sunlight.
• Ocean trenches: long, narrow, very deep valleys at subduction zones, where one plate descends under another. The Mariana Trench in the western Pacific reaches about 10,935 m at the Challenger Deep. Other deep trenches include the Tonga, Kermadec and Philippine trenches, which are mostly in the Pacific. Trenches are linked to strong earthquakes, tsunamis and volcanic arcs.
• Seamounts and guyots: underwater volcanoes that rise from the seafloor, in the case of guyots with flat tops that were eroded at sea level before the seafloor subsided. Seamounts are biodiversity hotspots.
• Volcanic islands and island arcs: chains such as Hawaii (above a hot spot) and the Aleutians (above a subduction zone).
• Fracture zones: long fault lines offsetting ridges, formed by transform faults.

The age of the oceanic crust increases away from the ridges: the youngest is at the ridge and the oldest, about 180 million years, is in the western Pacific, much younger than continental rocks, because the old crust is recycled at trenches.

How is the seafloor mapped?

Bathymetry is the measurement of ocean depth. Early sailors used weighted ropes (lead lines). Since the 1920s, echo sounders measure the time taken by a sound pulse to travel to the bottom and back, and multibeam sonar, used since the 1970s and 1980s, scans a swath of seafloor beneath a ship, which produces detailed maps. Satellite altimetry measures small bumps in the sea surface that reflect the gravity of underwater features and can give a global but lower-resolution picture. Autonomous underwater vehicles and remotely operated vehicles take close-up measurements, and submersibles carry people to great depths. The Seabed 2030 project aims to map the entire ocean floor in high resolution by 2030, and a quarter of the seafloor had been mapped to modern standards by the early 2020s.

Seafloor features compared

Worked example: depth, pressure and spreading

1. Echo sounding: sound travels through seawater at about 1,500 m/s. A pulse returns after 4.0 seconds. The depth is d = v × t ÷ 2 = 1,500 × 4.0 ÷ 2 = 3,000 m, because the sound travels down and back.
2. Pressure in the Challenger Deep: pressure from water is P = ρ × g × h = 1,025 kg/m³ × 9.81 m/s² × 10,935 m = about 110,000,000 Pa, which is 110 MPa, or about 1,085 times the atmospheric pressure at sea level (101,325 Pa). The real pressure is slightly higher because water is compressed with depth.
3. Gradient of the shelf: the shelf break lies 70 km from the coast at a depth of 130 m. The gradient is 130 ÷ 70,000 = 0.00186, which is an angle of arctan(0.00186) = 0.11°.
4. Gradient of the slope: the seabed falls from 130 m to 3,000 m over a horizontal distance of 40 km. The change is 2,870 m, so the gradient is 2,870 ÷ 40,000 = 0.072, an angle of about 4.1°, more than 30 times steeper than the shelf.
5. Seafloor spreading: new crust moves away from a ridge at 2.5 cm per year on each side. In 40 million years (40,000,000 years), the distance is 0.025 m × 40,000,000 = 1,000,000 m, or 1,000 km on each side. The ocean basin is therefore widening by 5 cm a year (2 × 2.5 cm).
6. Age of the crust: crust 1,500 km from a ridge that spreads at 3 cm per year is 1,500,000 m ÷ 0.03 m per year = 50,000,000 years old.

How did ocean exploration develop?

The British HMS Challenger expedition of 1872–1876 took hundreds of depth soundings and discovered a very deep area in the Pacific, laying the foundations of oceanography. In the 1950s, Marie Tharp and Bruce Heezen used echo-sounding data to produce the first maps of the ocean floor and revealed the mid-ocean ridge with its central rift, which supported the theory of seafloor spreading, proposed by Harry Hess in 1960 and confirmed by the magnetic stripes of the seafloor in the 1960s. In 1960, Jacques Piccard and Don Walsh descended in the bathyscaphe Trieste to the bottom of the Challenger Deep, and in 2012, film director James Cameron made a solo dive. Hydrothermal vents were found in 1977 near the Galápagos Islands. Today, multibeam sonar, satellites and autonomous vehicles continue to reveal new features, and mapping the whole ocean floor is a global effort.

Which mistakes are common?

Mistake 1: Picturing the ocean floor as a flat, featureless bowl

It has mountain ranges, volcanoes, canyons, trenches and plains. The Mid-Ocean Ridge is the longest mountain chain on Earth.

Mistake 2: Confusing the continental shelf with the continental slope

The shelf is the gently sloping shallow platform. The slope is the steeper descent beyond the shelf break.

Mistake 3: Thinking that the deepest ocean is the middle of the ocean

The deepest points are in narrow trenches near the edges of the oceans, along subduction zones. The central parts are mostly abyssal plains and ridges.

Mistake 4: Assuming that the seafloor is old and unchanging

Oceanic crust is young compared to continents, with a maximum age of about 180 million years, because it is created at ridges and destroyed in trenches.