Turbidity current

Dirty water can slide down underwater hills.

This water is full of sand and mud. It moves very fast. It is like a flash flood under the sea.

Gravity pulls the heavy, dirty water down. As it flows, it picks up even more mud. This makes the water even heavier. It moves even faster as it goes.

Sometimes, an earthquake starts these flows. A river pouring into the sea can do it too. These flows can happen in lakes, as well.

When the water reaches flat ground, it slows down. The sand and mud sink to the bottom. This leaves behind a new layer of ground. This layer is called a turbidite.

These flows can even break underwater cables. They are powerful and change the ocean floor.

Imagine a flash flood happening deep under the sea. This is a turbidity current. It is a fast flow of water filled with sediment. Sediment is stuff like sand and mud.

These currents move down slopes. Gravity pulls them down because the muddy water is heavy. This heavy water is denser than the clear water around it. As the current moves, it stirs up the seafloor. It picks up even more mud. This makes the flow even heavier and faster.

Many things can start these flows. Earthquakes often shake the seafloor and start them. Sometimes, a river pours too much mud into the ocean. This can create a special flow called a hyperpycnal plume.

When the current reaches flat ground, it slows down. The sand and mud sink to the bottom. This leaves a new layer on the seafloor. We call this layer a turbidite. These flows are very strong. They can even break cables on the ocean floor.

A turbidity current is a fast-moving flow of water filled with sediment. This sediment includes things like sand and mud. These currents usually travel down slopes in the deep ocean. They can also happen in very deep lakes. These flows are important because they move material across the seafloor. They can even shape the ground by scouring and eroding it as they pass.

How does this flow work? It starts because the water is heavy with sediment. This makes the water more dense than the clear water around it. Gravity pulls this heavy mixture down a slope. As it moves, it often has a snowballing effect. The current stirs up the ground beneath it. This picks up even more particles and makes the flow heavier. This extra weight makes the current move even faster.

Scientists have studied these flows for a long time. In 1965, J.E. Sanders wrote about the structures these currents leave behind. Researchers also look at how they start. Some scientists believe the flow starts with sediment on the seafloor. They think the water flow is just the tail-end of the process. In 2018, researchers from the Monterey Bay Aquarium Research Institute found that water-saturated sediment might be the main actor. They saw sediment move rapidly over the seafloor.

Many different events can trigger a turbidity current. Earthquakes are a common cause because they shake the seafloor. Large river outflows can also start them. When a river pours too much mud into the sea, it creates a hyperpycnal plume. For example, the Haile River in China has a high sediment load. It stays at about 40.5 kg/m3. This is enough to create these special flows. In lakes, the amount of sediment needed is much lower, at only 1 kg/m3.

When the current reaches flat ground, it slows down. The particles settle out and form a layer called a turbidite. These currents are very powerful. In 1929, a massive earthquake at the Grand Banks caused a current that broke submarine cables. This event showed how much energy these flows hold. You can think of them like underwater flash floods. They move quickly and leave a lasting mark on the ocean floor.

A turbidity current is a powerful, rapid flow of sediment-laden water moving down a slope. These currents are a type of sediment gravity flow. This means gravity is the main force moving the material. While they are most common in the deep ocean, they can also occur in deep lakes. These flows are vital because they transport massive amounts of material across the seafloor. They can also shape the landscape by scouring and eroding the ground.

The mechanism behind these currents is based on density. When water contains a high concentration of suspended solids, it becomes denser than the surrounding water. Gravity pulls this heavy mixture down any available slope. As the current moves, it often experiences a "snowballing effect." The flow stirs up the ground beneath it, gathering more sedimentary particles. This extra sediment increases the density of the current even further. This higher density causes the current to move even faster.

Scientists have different views on how these currents begin. Traditionally, they were defined as flows where turbulence keeps sediment in suspension. However, recent research from the Monterey Bay Aquarium Research Institute in 2018 suggests a different starting point. They found that water-saturated sediment might be the primary actor. They observed a layer of sediment moving rapidly over the seafloor. This movement then mobilized the upper meters of the existing seafloor. In this view, the water flow is actually the tail-end of the process.

There are several ways these currents are triggered. Earthquakes are a major cause due to tectonic disturbances. The physical shaking and the displacement of the continental crust can initiate a flow. Another trigger is the arrival of sediment-laden river outflows. When river water is denser than seawater, it forms a hyperpycnal plume. This happens when sediment concentrations reach 35 to 45 kg/m3 in marine environments. In contrast, freshwater lakes only need 1 kg/m3 to trigger such a flow. Some rivers, like the Haile River in China, maintain a high concentration of 40.5 kg/m3.

Turbidity currents can also be caused by slumping. This happens when too much sediment piles up at the top of a continental slope. This overloading can cause the sediment to slide down submarine canyons. Once a current enters a canyon, it can become self-sustaining through canyon-flushing. This process allows the current to pick up even more sediment from the canyon walls. As a result, the final volume of the current can be many times larger than the original slide.

When the current finally reaches the flat abyssal plain, it loses energy. The water slows down, and the suspended particles settle out of the water column. This sedimentary deposit is known as a turbidite. These events can be incredibly destructive. For example, a massive turbidity current following the 1929 Grand Banks earthquake broke submarine cables. This event highlighted the immense power and energy these underwater flows carry.

Understanding these currents helps scientists study the history of our planet. By examining turbidites, researchers can investigate past seismic activity. They have used these records to study earthquakes in the Cascadia subduction zone and the Northern San Andreas Fault. They also study them in European, Chilean, and North American lakes. These flows connect many different geological systems, from river mouths to the deepest parts of the ocean floor.