Most commonly, convective flows are generated by heterogeneities in density or the effects of gravity. When the cause of the convection is unspecified, convection due to the effects of thermal expansion and buoyancy can be assumed. Convection frequently occurs in fluids, but may also take place in soft solids or mixtures where particles can flow.

A material transport process which arises spontaneously due to physical heterogeneities in a portion of a material and forces which act throughout its volume.