Underwater landscapes

Everyone has their own mental image of what the bottom of the ocean looks like...but did you know that the seafloor topography is as varied and rugged as that on land?

When you think about the seafloor, what sort of image comes into your head? Flat stretches of gently rippled sand under a shallow blue tropical sea? A dark, cold expanse of sludgy marine mud? In truth, the topography of the seafloor is as varied as we see on land.

Abyssal plains form vast flat areas in the deepest parts of the oceans, and are the flattest places on Earth. In fact, they are so flat that even on the steepest plains you'd have to walk for several kilometres just to gain 2 metres of height!

Submarine eruption of lava near Hawaii (photo: NGDC)

In complete contrast, the submarine mountains and volcanoes of the mid ocean ridge system form the most dramatic features of the seafloor. This chain of active and inactive volcanoes, formed by magma pushing up through the seafloor, is the longest mountain chain on Earth, stretching nearly 60,000 km around the globe. In addition, seamounts and guyots formed by isolated hotspots of volcanic activity also create enormous peaks of volcanic rock. Mauna Kea, part of the Hawaiian volcanic island chain, is the largest mountain on Earth, measuring over 10,000m from its base on the seafloor to its peak which rises some 4000m above sea level. Because Mauna Kea is composed of basaltic rock, the profile of the volcano is very wide and low; although it is extraordinarily tall, its diameter is over 100km. The volcano is so heavy, it has pushed the underlying seafloor down by 8km!

Lava rivers from Kilauea, Hawaii (photo: NGDC)

On the edges of some continents, plate tectonics have created incredibly deep trenches called subduction trenches, where one crustal plate is being dragged back down into the Earth. The Marianas Trench in the western Pacific Ocean is the deepest place in the oceans - at 11,304 metres deep, it could easilty swallow up Mauna Kea!

But what happens between these two topographic extremes? If you've read the page on ocean basins, you'll know a bit about how the shape of the seafloor changes as you move away from the edge of a continent towards the middle of the ocean. Many of the physical processes occurring under the sea are similar to those which shape the land. Think about how rivers carve their way through soil and rock to create large valleys - viewed from the air, you can clearly see patterns of streams and drainage channels feeding into one main river which eventually reaches the sea. The same features are formed under the sea, created by strong currents carving away the sediment to create a deep, wide channel across the continental shelf. In many places, these channels are a continuation of river channels and they act as a giant funnel for the sediment pouring into the sea from river mouths. The channel provides an easy way for the sediment to be transported from the coastline down into the deep sea. Steeper ravines and canyons are commonly cut deep into the continental slope by strong currents, and sediment avalanches (turbidity currents) regularly cascade down them.

In the same way as deltas build up at the mouth of a large river, similar features can form where a submarine channel spews out its sediment load onto the seabed. These are called submarine fans and can build up into enormous deposits of sand and mud. The Amazon Fan, offshore Brazil, and the Bengal Fan in the Indian Ocean are two of the largest modern submarine fans known.

Left: A typical fan delta at the mouth of a river in Greenland.
Photo by John Simmons, © The Geological Society of London

With so much sediment building up on the continental shelf, it is inevitable that sometimes the seafloor becomes unstable. Submarine landslides are quite common, triggered either by a sudden event such as an earthquake or simply by the sheer weight of sediment building up on the continental shelf. The size of such landslides varies hugely; larger landslides leave behind huge scars in the seafloor - steep cliffs rising even higher than those seen on land. Downslope from the scar, you might find evidence of the slide - enormous blocks of rock scattered across the seafloor like giant building blocks.

Landslide scar resulting from an arthquake-induced landslide in California. Photo courtesy USGS.

3D image of the El Golfo landslide on the island of El Hierro in the Canary Islands. You can clearly see the landslide scar and the remains of the landslide debris on the seafloor. Image courtesy R.Wynn/D.Masson, SOC.

3D image of the Storegga Slide scar, offshore Norway. This slide is thought to be one of the biggest ever, triggered by release of hydrocarbons under the sediment 7000 years ago.

But it isn't all about dramatic, violent forces shaping the seafloor. Powerful yet slow-moving currents wash around the continental slope and rise, gently sculpting sand and mud into giant ripples and dunes. These bedforms (as they are called) are larger versions of the sand sculptures seen closer to the coastline, and are very similar to the dunes created by the wind in deserts.

Dunes of wind-blown sand in Death Valley, Califonia. Similar features appear on the seabed, created by ocean currents. Photo by John Simmons, © The Geological Society of London

Satellite image of sandy bedforms and seaweed on the Bahamas coast.Tides and ocean currents in the Bahamas sculpted the sand and seaweed beds into these multicolored, fluted patterns in much the same way that winds sculpted the vast sand dunes in the Sahara Desert. Image courtesy NASA Earth Observatory [click image to enlarge]

Photographs showing ripples in sediment on the floor of the Faeroe-Shetland Channel, NW UK. The water depth in this area is around 1150 metres. The width of each image represents about 1 metre.
Images courtesy R. Wynn, SOC.

The Great Barrier Reef off Australia is the most famous of coral reefs, which form prominent features on the seafloor. Photo courtesy NASA Earth Observatory [click iamge to enlarge].

Prominent seafloor features can also be created by biological processes. One of the most famous of these is the Great Barrier Reef (left), which stretches almost the entire length of the Queensland coastline, Australia. The Great Barrier Reef is made up of 2,900 coral reefs, 900 islands, 600 continental islands and 300 sand cays, and stretches a total distance of about 2000km. Coral communities form mounds and reefs on the seafloor all around the world where the environmental conditions are right. Recent scientific research has resulted in a protection order being placed on coral mounds in the Rockall Trough, NW of the UK to prevent damage being caused to them by fishing trawlers.

Mounds composed of Lophelia corals in the Rockall Trough are now protected. Image courtesy D. Masson, SOC.

Find out more about marine geology:

Underwater landscapes
The ocean basins
Where does all the sediment go?
Mud, mud, glorious mud...
What is a turbidity current?

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February 2007