abyssal plainA vast level expanse of mud-covered deep sea bed. Abyssal plains are among the flattest, smoothest places on Earth. Most lie between 3,000 and 6,000 meters down, and together they cover more than half of the planet's surface.
continental marginA continent's whole underwater edge, in three parts. It has three parts. Going out from the shore they are the continental shelf, the continental slope and the continental rise.
continental riseThe gentle ramp of sediment at the slope's foot. It sits between the abyssal plain and the continental slope, and covers about a tenth of the ocean floor.
continental shelfThe shallow underwater ledge that rims a continent. Shelves make up about 7 percent of the ocean's area. The widest, off Siberia, reaches 1,500 kilometers out to sea.
continental slopeThe steep underwater hillside below the shelf break. It starts at the shelf break, which almost everywhere lies about 140 meters down, and runs down to the continental rise.
guyotA seamount whose summit is flat like a table. Waves wore its top flat when it stood at the surface, and it later sank. The flat top can be more than 10 kilometers across.
Show all 36 terms (30 more)
Ocean floor features, continued
mid-ocean ridgeA volcanic mountain range strung along the sea bed. New sea floor forms along it. The ridges join into one system nearly 65,000 kilometers (about 40,000 miles) long, the longest mountain range on Earth.
ocean basinA vast low region of the Earth that seawater fills. On average the sea bed lies 3,682 meters (12,080 feet) below the surface.
rift valleyA valley that opens where the crust splits apart. One runs along the crest of the Mid-Atlantic Ridge. It is about as deep and as wide as the Grand Canyon.
seamountA sea-bed peak that stops short of the surface. Most seamounts are dead volcanoes. To count as one, a peak must stand 1,000 meters (3,300 feet) or more above the sea floor around it. There are thought to be more than 100,000.
submarine canyonAn undersea gorge carved down the continental slope. Muddy currents rush down these canyons and carry sediment to the deep sea. The Hudson Canyon lies off New York.
trenchA narrow chasm far deeper than the sea bed around it. There are about 50,000 kilometers of trench in the world, most of it around the Pacific Ocean. A single one may run for thousands of kilometers.
How the ocean floor forms 12
basaltThe common dark rock of sea floors and lava flows. It is fine-grained, and it forms when lava cools. More than 90 percent of all the volcanic rock on Earth is basalt.
hot spotA long-lived patch of extra-hot rock below a plate. The Hawaiian Islands formed over one in the middle of a plate, some 3,200 kilometers or more from the nearest plate boundary.
hydrothermal ventA hot spring that flows from a crack in the sea bed. The first was found in 1977 near the Galapagos Islands. The water can come out at more than 700 degrees Fahrenheit (370 Celsius), and its minerals build chimneys called smokers.
island arcA bow-shaped line of volcanoes that stand in the sea. Arcs rise where one plate of sea floor sinks beneath another. The Aleutian Islands of Alaska are an island arc.
manganese noduleA lump of metal ores lying on the deep sea bed. Nodules grow about a centimeter in several million years, one of the slowest of all geological processes.
oceanic crustThe outer shell of rock that lies beneath the oceans. It is usually less than 10 kilometers thick: thinner than the crust of the continents, but heavier for its size.
pillow lavaRounded lumps of lava that cooled under water. Pillow lava forms wherever lava comes out under water, as it does along the mid-ocean ridges. One pillow is commonly up to a meter across.
seafloor spreadingThe making of new sea floor as two plates move apart. The Mid-Atlantic Ridge spreads 2 to 5 centimeters a year. The East Pacific Rise is faster, at 6 to 16 centimeters a year.
sedimentLoose grains that sink and pile up on the bottom. Deep-sea clay builds up by only about a millimeter in a thousand years. Sand and mud carried off the land can pile up a thousand times faster.
subductionThe slide of old sea floor back down into the mantle. It happens where two plates meet and one bends down under the other. A trench marks the line.
turbidity currentA rush of muddy water down an underwater slope. In 1929 one set off by an earthquake near the Grand Banks broke undersea cables.
volcanic islandAn island that eruptions built from the sea bed up. It is also called a high island. A low island is built of sand or coral instead. The Hawaiian Islands are volcanic through and through.
Mapping the ocean floor 12
bathymetryThe measuring and mapping of the sea bed's shape. A bathymetric map shows the sea floor the way a topographic map shows land, with colors and lines for depth.
benthosAll the living things found on the sea bed. Benthos are also called bottom dwellers. Many are fixed to the bottom for life.
Challenger DeepThe deepest known point in the ocean. It lies in the Mariana Trench, at its southern end, southwest of Guam, about 10,935 meters (35,876 feet) down. It is named for the British ship HMS Challenger, which first measured the trench's depth in 1875.
fathomTwo yards, as a measure of how deep water is. It began as the width of a man's arms held out to the sides. Charts in the United States still give depths in fathoms.
lead lineA weighted rope lowered to measure the water's depth. Marks tied along the rope let a sailor read the depth by eye. Sounding this way goes back to ancient Greece and Rome, and boaters still do it.
oceanographyThe branch of science that deals with the seas. It takes in the ocean's physics, chemistry, biology and geology, from currents and waves to the sea floor.
remotely operated vehicleAn underwater robot steered by cable from a ship. It is called an ROV for short. Most carry lights and cameras, and they range from the size of a small computer to the size of a small truck.
sediment coreA long column of sea-floor mud pulled up in a tube. The tube may be drilled into the bottom or simply dropped so that its own weight drives it in. Cores show how climate and sea life changed over long ages.
sonarSound pulses and echoes used to see under water. Sonar is short for sound navigation and ranging. It works because sound travels farther in water than light or radar.
soundingOne reading of the water's depth at one spot. The word comes from the Old English sund, meaning sea or swimming. It has nothing to do with noise.
submersibleA diving craft launched from a mother ship. That is what sets it apart from a submarine, which can work on its own at sea for a long time. Some submersibles carry people and some do not.
topographyThe shape of a surface, with its heights and hollows. Bathymetry has been called submarine topography: a map of the hills, valleys and plains that lie under water.
How this list was compiled: Term list chosen for this site on 2026-10-12. Each meaning is written in our own words and was checked that day against the article on the term in the English Wikipedia (https://en.wikipedia.org/), whose text is under the Creative Commons Attribution-ShareAlike license: Oceanic basin, Continental margin, Continental shelf, Continental rise, Submarine canyon, Abyssal plain, Mid-ocean ridge, Rift valley, Oceanic trench, Seamount, Guyot, Oceanic crust, Basalt, Seafloor spreading, Subduction, Hotspot (geology), Volcanic island, Island arc, Hydrothermal vent, Pillow lava, Sediment, Marine sediment, Pelagic sediment, Turbidity current, Manganese nodule, Bathymetry, Sonar, Depth sounding (sounding and the lead line), Fathom, Submersible, Remotely operated underwater vehicle, Core sample, Oceanography, Challenger Deep, Benthic zone and Topography. Sounding, fathom, guyot and benthos were also read in Wiktionary, the free dictionary (https://en.wiktionary.org/). Seamounts and guyots, the mid-ocean ridge and its rift valley, the rates of seafloor spreading, hydrothermal vents, bathymetry, sonar, remotely operated vehicles, the average depth of the ocean, the Challenger Deep and the share of the sea floor that has been mapped were checked a second time against pages of the U.S. National Oceanic and Atmospheric Administration: the ocean facts of NOAA Ocean Exploration (https://oceanexplorer.noaa.gov/facts/: What is a seamount?, What is a mid-ocean ridge?, What is bathymetry?, What is an ROV?, How much of the ocean has been explored?) and of the National Ocean Service (https://oceanservice.noaa.gov/facts/: What is bathymetry?, What is sonar?, What is a hydrothermal vent?, How deep is the ocean?). Hot spots and the Hawaiian Islands were checked against This Dynamic Earth of the U.S. Geological Survey (https://pubs.usgs.gov/gip/dynamic/hotspots.html). The fact after each meaning is from the same pages. Licence: own work
How it works
This ocean floor vocabulary worksheet prints twelve features of the sea bed, among them continental shelf, continental slope, continental rise, abyssal plain, mid-ocean ridge, rift valley, trench, seamount and guyot, beside their meanings in another order. A key follows.
Read the full guide
How the ocean floor forms
Set opens on Ocean floor features. The second set has oceanic crust, basalt, seafloor spreading, subduction, hot spot, volcanic island, island arc, hydrothermal vent, pillow lava, sediment, turbidity current and manganese nodule.
Mapping the ocean floor
The third set has bathymetry, sonar, sounding, lead line, fathom, submersible, remotely operated vehicle, sediment core, oceanography, Challenger Deep, benthos and topography.
The sunlight, twilight and midnight zones of the water above are on the ocean zones worksheet.
Questions and answers
How do scientists map the ocean floor?
Mostly with sound. An echo sounder on a ship sends a pulse of sound down and times its return: the longer the echo takes, the deeper the water. Satellites have mapped the whole sea floor too, but only as a general picture.
How much of the ocean floor has been mapped?
All of it roughly, from satellite data, and 28.7 percent in sharp detail by sonar as of April 2026, according to NOAA Ocean Exploration. People have seen far less with their own eyes or cameras: under 0.001 percent of the deep sea floor.
What is the ocean floor made of?
Basalt, a dark volcanic rock, with a blanket of sediment over most of it. Near the continents the sediment is sand and mud washed off the land. Far out it is fine clay and the shells of tiny drifting sea life.