The word “gravity” refers to the difference in density between two fluids. Gravity currents occur under the influence of gravity upon a horizontal flow that is driven by this density difference.  This happens when fluid of one density flows into fluid of another density.  Flow connotes a fluid or gas; however solid matter is often involved as it is carried by hot expanding gas.  There are a number of phenomena is nature that possess this gravity current property.  Gravity currents are a subject of fluid mechanics.

Gravity currents typically display a low aspect ratio, that is a low height compared to long length.  The action seems to be in the leading edge of the current.  Due to its nature, gravity currents can move matter through a great distance.There are many examples of gravity currents in nature.  Some examples are pyroclastic flows which occur because there is a difference in temperature between the ejected gas and debris and the surrounding air  into which it is injected, sea breeze fronts as a result of a difference in temperatures. 

It occurs at the mouth of a river because salt water has a different density than fresh water.

One special type of gravity current is a pyroclastic flow or surge.  This is fluidized masses of rock fragments that flow very fast from a volcano eruption.  Under the influence of gravity, the flow behaves more like a fluid because there is very little friction between individual particles.  This very hot mass mixes with cooler air.  Pyroclastic flows are the most dangerous part of a volcanic eruption.  They can be at temperature as high as 800 degrees C, and they can travel as fast as 1,000 kilometers an hour.

Sometimes land will slide into the ocean causing particles to be suspended in the water.  These particles can travel great distances because they are laced into a gravity current.

Wind can cause microburst to spread along the ground in the form of a ring vortex.  This is a very dangerous condition for aircraft.

Avalanches are airborne particles of powder snow and as such they also are considered a  gravity current .

Gravity currents occur in outer space and have been observed to be occurring on the different planets.  Gravity currents also occur in the magnetosphere especially after magnetic reconnection.  Because different areas of the magnetosphere have different plasma densities, when they mix the leading edge of the gravity currents has swirls in it called Kelvin Helmholz billows.

REFERENCE:

http://en.wikipedia.org/wiki/Gravity_current