Ocean floor glossary: 54 terms explained
Plain-English definitions of the words used about the seabed, how it is mapped and how sea level has changed. Every term with a page of its own shows what it means on the map and uses real figures from the GEBCO grid.
41 of the 54 terms have a page of their own. The rest are defined here, with a link to the page that covers them where there is one.
Shape of the seabed
- Abyssal hillAn abyssal hill is a small rise on the floor of an abyssal plain, generally no more than a few hundred metres high.
- Abyssal plainAn abyssal plain is a flat, underwater plain on the deep ocean floor, usually between the foot of a continental rise and a mid-ocean ridge.
- Abyssal zoneThe abyssal zone is the part of the ocean's water column from about 4,000 m down to about 6,000 m, where no sunlight reaches.
- BathymetryBathymetry is the measurement of the depth and shape of the floor of the ocean, lakes and rivers.
- Continental riseThe continental rise is the gently sloping apron of sediment at the foot of the continental slope, where the seabed levels out towards the abyssal plain.
- Continental shelfThe continental shelf is the shallow, gently sloping part of the seabed that surrounds a continent, ending at the shelf break where the ground drops away steeply.
- Continental slopeThe continental slope is the steep part of the seabed that falls away from the edge of the continental shelf towards the deep ocean.
- Hadal zoneThe hadal zone is the deepest part of the ocean, below about 6,000 m, and is found almost entirely in ocean trenches.
- Ocean basinAn ocean basin is a large depression in the Earth's surface that is filled with seawater.
- Ocean floorThe ocean floor, or seabed, is the solid surface of the Earth beneath the ocean.
- Ocean zonesThe ocean zones are layers of water stacked by depth: sunlight, twilight, midnight, abyss and trenches.
- Oceanic crust vs continental crustOceanic crust is the thin, dense, basalt-rich rock that floors the oceans, while continental crust is the thicker, lighter rock that forms the continents.
Undersea features
- AtollAn atoll is a ring-shaped island made of a coral rim that encircles a lagoon.
- Bank and shoalA bank is a broad, shallow rise of the seabed, usually flat-topped, with enough water over it for ships. A shoal is a shallow spot, often sand or gravel, that can be a hazard to navigation. Both are named on charts, and many sit on continental shelves.
- Fracture zoneA fracture zone is a long, narrow line of steep valleys and ridges across the ocean floor, hundreds or even thousands of kilometres long.
- GuyotA guyot is an isolated underwater volcanic mountain with a flat top that lies more than 200 metres below the sea surface.
- HotspotA hotspot is a small, long-lived, very hot region beneath the Earth's plates that feeds volcanoes.
- Hydrothermal ventA hydrothermal vent is a hot spring on the seafloor.
- KnollA knoll is a small, rounded rise on the seabed, lower and less steep than a seamount. Hydrographers use the word for a modest hill on the sea floor that is easy to miss on a chart but may still matter to ships and sub-sea cables.
- Mid-ocean ridgeA mid-ocean ridge is a long chain of undersea volcanic mountains that runs along the boundary where two tectonic plates pull apart.
- Ocean trenchAn ocean trench is a long, narrow, steep-sided valley in the seabed, and the deepest kind of place in the ocean.
- Oceanic plateauAn oceanic plateau is a large, relatively flat area of the seabed that stands higher than the ocean floor around it, with one or more steep sides.
- Seafloor spreadingSeafloor spreading is the process by which new ocean floor forms at mid-ocean ridges.
- SeamountA seamount is an underwater mountain with steep sides that rises from the seafloor but does not reach the surface.
- Subduction zoneA subduction zone is a place where one tectonic plate sinks beneath another and descends into the Earth.
- Submarine canyonA submarine canyon is a steep-sided valley cut into the seabed of the continental slope.
- Transform faultA transform fault is a plate boundary where two plates slide sideways past each other.
- Undersea feature namesUndersea feature names are the official names given to seabed features such as trenches, ridges and seamounts. They are kept in the GEBCO Gazetteer of Undersea Feature Names, which is maintained by an international subcommittee so that charts and maps use the same names.
- Undersea mountain rangeAn undersea mountain range is a long chain of mountains rising from the ocean floor.
- Volcanic island arcA volcanic island arc is a curved chain of volcanic islands and undersea volcanoes that forms beside an ocean trench, where one oceanic plate sinks beneath another.
Sea level and the past
- Below sea levelBelow sea level means lower than the zero height used for the ocean's surface.
- Beringiamap pageBeringia is the name for the Ice Age land that joined north-east Asia to North America across what is now the Bering Strait and the shallow seas around it.
- Doggerlandmap pageDoggerland is the name given to the land that once joined Britain to continental Europe, now under the southern North Sea.
- Eustatic sea level changeEustatic sea level change is a change in the level of the whole world's ocean, caused by a change in the amount of water in it or the size of the ocean basins.
- IsostasyIsostasy is the balance between Earth's crust and the denser mantle beneath it, with the crust floating at a height that depends on its thickness and density.
- Land bridgeA land bridge is a strip of dry land that joins two landmasses normally separated by water.
- Last Glacial MaximumThe Last Glacial Maximum was the time, roughly 26,000 to 20,000 years ago, when ice sheets were at their largest.
- Sea levelSea level is the height of the ocean's surface, used as the zero point for measuring how high land is and how deep the sea is.
- Sundalandmap pageSundaland is the name for the Ice Age landmass that joined the Malay Peninsula, Sumatra, Java and Borneo across what are now shallow seas.
- What if the oceans drainedmap pageDraining the oceans means lowering the sea until the seabed shows.
Mapping the ocean
- Arc-secondAn arc-second is one 3,600th of a degree, a tiny angle used to measure position on Earth.
- Depth soundingA depth sounding is a single measurement of how deep the water is at one spot. It was once made by lowering a weighted line and is now made with sound, and charts print the figures as a way of showing the depth.
- Digital elevation modelA digital elevation model, or DEM, is a grid of numbers in which each cell holds the height of the ground.
- Echo sounderAn echo sounder is an instrument on a ship that measures water depth by sending a pulse of sound down and timing how long the echo takes to come back from the seabed.
- FathomA fathom is an old unit of water depth equal to six feet, or about 1.8 metres. Sailors used it for centuries when measuring depth with a weighted line, and it still appears on some charts. This map uses metres.
- GEBCOGEBCO, the General Bathymetric Chart of the Oceans, is the international project that produces the main free global map of the ocean floor.
- HillshadeHillshade, also called shaded relief, is a way of drawing terrain by working out how light from one direction would light and shade each slope.
- How much of the ocean is mappedThe whole seafloor has a satellite-based map, but only a minority has been surveyed in high resolution.
- Multibeam echosounderA multibeam echosounder is a ship-mounted sonar that sends many sound beams out in a fan at once. It measures depth across a wide strip of seabed on each pass, which makes it the main tool for high-resolution ocean floor mapping.
- Nautical chartA nautical chart is a map made for ships. It shows depths, hazards, channels and navigation marks so a vessel can travel safely. This ocean floor map is for learning, not navigation, and should never replace an official chart.
- Satellite-derived bathymetrySatellite-derived bathymetry is a way of estimating seabed depth from space instead of with ships.
- Seabed 2030Seabed 2030 is a project of the Nippon Foundation and GEBCO that aims to produce a definitive map of the whole ocean floor by 2030 and make it freely available. Its regional centres gather survey data for the GEBCO grid.
- Side-scan sonarSide-scan sonar is a sonar that looks sideways from a ship or towed vehicle to make a picture of the seabed. It is used to find wrecks, rocks and other objects on the bottom, rather than to measure depth.
- SonarSonar, short for Sound Navigation and Ranging, uses sound waves to detect and measure things underwater.