What is a turbidity current?

Imagine an underwater sediment avalanche the size of
Britain cascading down the continental slope

The seafloor environment is a place of constant change, with sediment being pushed around the ocean floor by tides and currents. But the movement of sand and mud on the seafloor isn't always the slow, steady process many people imagine it to be. In many places on the continental shelf and slope, massive underwater landslides can create enormous scars on the seabed as millions of tons of sediment go crashing down to the deep sea.

The easiest way to describe a turbidity current is an underwater avalanche - imagine the cloud of snow and ice that accompanies a snow avalanche as it cascades down a mountainside (left). Turbidity currents are a bit like that cloud, but they are composed of sediment grains and occur under the sea. When a submarine landslide is triggered, some of the seafloor sediment is picked up by turbulent currents created by the landslide. This current (known as a turbidity current because of its turbulent nature) races downslope towards the deep ocean, carrying its sediment load along with it. As the current begins to slow down, it no longer has the energy to carry its load of sediment, and the particles start to settle out onto the seafloor, forming a tubidite deposit.

What triggers a turbidity current?

There are many events which might trigger a tubidity current. Around the Canary Islands, many turbidity currents in the past have been triggered by landslides along the flanks of the island. As the rocks and debris crashed into the sea, sand and mud on the seafloor was disturbed and formed a dense underwater cloud of sediment which rushed down the continental shelf towards the abyssal plain. [Click here to read more about the Canary Islands landslides].

Other triggers could be volcanic activity, earthquakes or the release of gas from hydrocarbon deposits under the seafloor. Even just the buildup of sediment on the edge of the shelf can create a turbidity current - if more and more sediment is added to the pile on the edge of the shelf, eventually the pile becomes unstable and avalanches down the slope, kicking up a cloud of sediment as it goes. A turbidity current is always the result of a slope becoming unstable.

What happens to the sediment in a turbidity current?

As turbidity currents slow down, they do not have enough energy to carry all the sediment, so particles start to drop out of the flow and form a deposit of sand and mud on the seafloor, known as a turbidite deposit. These deposits are very distinctive in deep sea sediments because they are usually composed of very different material compared to the very fine mud normally deposited in deep sea.

When turbidite sediments are deposited, they usually show a distinctive sequence of layers which shows how the turbidity current has slowed down and lost energy. In a perfect sequence (called a Bouma Sequence, after the scientist who devised the classification), the layers laid down first are composed of much coarser (larger) grains than layers laid down later on by the same turbidity current. This is because of the way that turbidity currents lose their energy as they slow down: the first particles to drop out of the flow are the biggest and the heaviest, but the flow still has enough energy and turbulence to carry the smaller particles. As the flow's energy continues to decrease, finer and finer particles settle out, with the smallest grains settling out last.

It's quite rare to find a complete Bouma Sequence which shows a continual sequence from coarse grains at the bottom to the very finest grains at the top. This is often because subsequent higher energy turbidity currents erode away the finer top layers.

How do we know so much about turbidity currents?

Turbidity currents can happen at a range of scales and can travel fast or slow. Large, fast-moving flows can cause damage to structures on the seafloor such as submarine communications cables. However, expensive as they are to repair, cable breaks allow us to calculate the speed of a turbidity current. If we know the exact time cable breaks occur in two or more cables of a known distance apart, a simple calculation can tell us how fast the flow was travelling. As an example, an earthquake in 1929 off the US coast caused a turbidity current to go cascasding into the Atlantic. Many years later, a scientist named Maurice Ewing calcuated the speed of the flow based on the times that the transatlantic telegraph service was interrupted. On the steepest parts of the seafloor, Ewing calculated a speed of around 65mph, slowing to around 25mph where the seafloor gradient was shallower.

Scientists today can recreate small-scale turbidity currents in the laboratory using a special tank of water called a flume tank. By running experiments in which they vary the type of sediment, the angle of the 'seafloor' and so on, they can get a much better picture of the forces which trigger and drive turbidity currents.

In the deep sea, marine geologists can look at turbidity currents by removing columns or tubes of sediment from the seafloor known as sediment cores. Looking at the layers of sediment allows them to identify turbidite events, and relate them to other geological events such as earthquakes which may have been the trigger. It also allows them to get an idea of how big the turbidity current was: this is done by looking at factors such as the thickness of the turbidite deposit, how coarse the grains in it are and so on. Very large turbidity currents can transport mud and sand over huge distances. The 1929 earthquake in the States triggered a turbidity current which spread a 100cm-thick deposit of sediment over 280,000 square km of the seafloor.

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