Convection is a kind of fluid flow driven by internal buoyancy. In general, the buoyancy that drives convection derives from horizontal density gradients. In the mantle, the main sources of density gradients are horizontal thermal boundary layers. Convection is driven when the buoyancy (positive or negative) of a thermal boundary layer causes it to become unstable, so that fluid from it leaves the boundary of the fluid and rises or falls through the interior of the fluid. This statement may seem to be labouring the obvious, but there has been a lot of confusion about the nature of mantle convection, and much of this confusion can be avoided by keeping these basic ideas clearly in mind.

In general the buoyancy driving convection may be of thermal or compositional origin. We will be concerned mainly with thermal buoyancy, but compositional buoyancy is also important in the mantle. It is best to consider first thermal convection, that is convection driven by thermal buoyancy. Some aspects of compositional buoyancy will be considered in Chapter 14.

Here I describe sources of buoyancy, give a simple example of thermal convection, and show how there is an intimate relationship between convection and the surface topography that it produces. This establishes some basic concepts that will be applied more explicitly to the mantle in subsequent chapters.

In the course of doing this, I show how convection problems scale, how the Rayleigh number encapsulates this scaling, why convection occurs only if the fluid is heated or cooled strongly enough, and how the mode of heating (from below or internally) governs the nature of the thermal boundary layers.