Density currents work because a volume of fluid (like water or air) becomes denser than the surrounding fluid, causing it to flow downwards (if denser) or upwards (if less dense) due to gravity.
Understanding Density Currents
Density currents are driven by density differences within a fluid. This density difference can arise from variations in:
- Temperature: Colder fluids are generally denser than warmer fluids.
- Salinity: Saltier fluids are denser than fresher fluids.
- Suspended Sediment: Fluids with higher concentrations of suspended sediment are denser than cleaner fluids.
The Process
- Density Difference Creation: A localized area of fluid experiences a change (e.g., cooling, increased salinity, sediment input) that makes it denser than the surrounding fluid.
- Gravitational Force: Due to the higher density, gravity exerts a stronger force on the denser fluid.
- Downward Flow: The denser fluid begins to sink or flow downwards along a sloping surface, such as the bottom of a lake or ocean. This downward movement creates a current.
- Entrainment and Mixing: As the density current flows, it can entrain (draw in) surrounding fluid, mixing and potentially modifying the current's properties.
- Dissipation: Eventually, the density current may dissipate due to mixing with surrounding fluids, changes in the underlying slope, or other factors that reduce the density difference.
Examples in Nature
- Subaqueous Currents: These are density currents that flow along the bottom of oceans or lakes. They can be caused by cold water sinking from polar regions, highly saline water forming in evaporation basins, or sediment-laden water from rivers entering a lake or ocean.
- Turbidity Currents: A type of subaqueous current, turbidity currents are primarily driven by suspended sediment. These can be very powerful and are a major force in shaping the seafloor.
- Atmospheric Density Currents: Cold air drainage is a prime example, where cold, dense air flows down slopes and valleys at night. Sea breezes and katabatic winds also operate on density differences.
Factors Affecting Density Current Behavior
Several factors influence how density currents behave:
- Density Difference: A larger density difference leads to a stronger, faster current.
- Slope: A steeper slope accelerates the flow.
- Fluid Viscosity: More viscous fluids create slower currents.
- Entrainment Rate: The amount of surrounding fluid entrained affects the current's volume, density, and velocity.
Importance
Density currents play significant roles in:
- Ocean Circulation: Contributing to the global thermohaline circulation, which distributes heat around the planet.
- Sediment Transport: Moving large amounts of sediment across the seafloor and shaping underwater landscapes.
- Nutrient Distribution: Transporting nutrients to different regions of oceans and lakes.
- Waste Disposal: Understanding density currents is important for designing effective wastewater outfalls.
