Turbidity currents are immense, underwater avalanches in the deep ocean. These flows consist of dense mixtures of water and sediment that race down continental slopes under the influence of gravity. They are the primary mechanism transporting massive amounts of terrestrial material from the continental shelf out into the deep ocean basins.
What Makes Up a Turbidity Current?
A turbidity current is defined by its density, which is greater than the surrounding seawater due to the massive volume of suspended sediment it carries. This density contrast is the driving force, allowing gravity to pull the mass downslope along the seabed. The mixture is turbulent, meaning the water is constantly churning. This turbulence keeps the sediment particles suspended within the flow rather than allowing them to settle out immediately.
The material composing these currents is a blend of fine sand, silt, and clay mixed with water, often originating from river mouths or shelf deposits. As the dense flow moves, it can accelerate rapidly, sometimes reaching speeds exceeding 15 miles per hour (about 7 meters per second). This high velocity grants the current significant erosional power, allowing it to scour the seafloor and pick up more sediment, increasing its density and momentum.
A current can travel for hundreds of kilometers across the relatively flat abyssal plain after leaving the steep continental slope. The massive scale of these flows means they can transport millions of cubic meters of sediment in a single event. While in motion, the sediment is supported by the turbulence of the water.
How Turbidity Currents Are Triggered
The initiation of a turbidity current requires a trigger event that destabilizes a large volume of sediment resting on a slope, converting the static pile into a dynamic flow. One primary trigger is seismic activity, where an earthquake shakes the seafloor and causes sudden, widespread slope failure. For instance, the 1929 Grand Banks earthquake triggered a flow that broke multiple transatlantic communication cables, providing clear evidence of the destructive power and speed of these currents.
Submarine landslides, or mass wasting events, are another common mechanism, occurring when accumulated sediment becomes unstable and collapses under its own weight. This initial slump can transform into a fast-moving turbidity current as the sliding sediment mixes rapidly with the surrounding water, creating the necessary dense, turbulent mixture.
Other triggers involve the rapid input of sediment-laden water from above, such as hyperpycnal flows. These occur when a major river flood dumps large amounts of sediment into the ocean. If the river water is denser than the receiving seawater, it plunges below the surface and flows down the slope. Intense storms or tsunamis can also disturb and resuspend unconsolidated sediment on the continental shelf, sending pulses of turbid water into deeper water.
The Geological Footprint
When a turbidity current slows down, either because the slope decreases or the current loses momentum, the suspended sediment begins to settle out, leaving behind a distinctive record on the seafloor. The primary deposit is a rock layer called a turbidite, which is the lithified product of a single flow event. Turbidites are characterized by graded bedding, where the coarsest and heaviest particles settle first, followed by progressively finer material.
This vertical change in grain size, from coarse sand or gravel at the base to fine silt and clay at the top, is a diagnostic characteristic of a turbidite. This pattern reflects the waning energy of the flow, as the heaviest sediment drops out first. Geologists use the full sequence of sedimentary structures within a turbidite, often referred to as the Bouma sequence, to reconstruct the dynamics of the ancient flow.
Over geological time, the repeated deposition of turbidites at the base of continental slopes builds up immense, fan-shaped structures known as submarine fans. These fans are vast accumulations of sediment extending for hundreds of kilometers across the abyssal plain. Turbidite layers within these fans are of economic interest because the coarse-grained sand layers often become excellent reservoirs for oil and natural gas.