What are convection currents and how do they promote cycling of matter?

Convection currents is the movement of a fluid or a gas from one particular location to another location. This motion is created by the difference in the density or the temperature of the fluid or gas in the two locations. It could also be used in the movement of energy from one location to another due to the temperature gradient. For convection current in the atmosphere, it could be describe in the rising of warm air towards the upper portion of the atmosphere while the colder air goes under it. This happens because warmer air would have less density as compared to the colder air. 

81 is a composite number