Turbidity Current

A turbidity current is a current of rapidly moving, sediment-laden water moving down a slope through water, or another fluid. The current moves because it has a higher density than the fluid through which it flows—the driving force of a turbidity current derives from its sediment, which renders the turbid water denser than the clear water above.

Turbidity currents are an example of density or gravity currents, which include: oceanic fronts, avalanches, lahars, pyroclastic flows and lava flows. Seafloor turbidity currents are commonly used to describe underwater currents in lakes and oceans, which are usually triggered by earthquakes, slumping and sediment-laden rivers. They are characterized by a well-defined front, also known as head, followed by a layer known as the body of the current.

Turbidity currents are characteristic of areas where there is seismic instability and an underwater slope, especially submarine trench slopes of convergent plate margins, continental slopes and submarine canyons of passive margins.

With increasing continental shelf slope, current velocity increases, as the velocity of the flow increases, turbulence increases, and the current draws up more sediment. The increase in sediment increases the density of the current, and thus its velocity, even further. Turbidity currents can reach speeds up to half the speed of sound.

Turbidity currents are traditionally defined as those sediment gravity flows in which sediment is suspended by fluid turbulence. However, the term 'turbidity current' was adopted to describe a natural phenomenon whose exact nature is often unclear. The turbulence within a turbidity current isn’t always the support mechanism that keeps the sediment in suspension; however it is probable that turbulence is the primary or sole grain support mechanism in dilute currents (<3%). Definitions are further complicated by an incomplete understanding of the turbulence structure within turbidity currents, and the confusion between the terms turbulent (i.e. disturbed by eddies) and turbid (i.e. opaque with sediment). Kneller & Buckee, 2000 define a suspension current as 'flow induced by the action of gravity upon a turbid mixture of fluid and (suspended) sediment, by virtue of the density difference between the mixture and the ambient fluid'. A turbidity current is a suspension current in which the interstitial fluid is a liquid (generally water); a pyroclastic current is one in which the interstitial fluid is gas.