Mantle plumes are buoyant mantle upwellings that are inferred to exist under some volcanic centres. In Chapter 8 I stated the basic idea that convection is driven by thermal boundary layers that become unstable, detach from the boundary and thereby drive flow in the interior of a fluid layer. In Chapter 10 we looked at plates as a thermal boundary layer of the convecting mantle, driving a distinctive form of convection in the mantle that I called the plate mode of mantle convection.

Here we look at the evidence that there is a mode of mantle convection driven by a lower, hot thermal boundary layer, at the expected form of such a mode, and at the consistency of the evidence with that expectation. Since it will become clear that the form and dynamics of such upwellings, or plumes, are quite different from the downwellings of lithosphere driving the plate mode, I will call the plumes and the flow they drive the plume mode of mantle convection.

Volcanic hotspots and hotspot swells

In Chapter 3 I described Wilson's observation that there are, scattered about the earth's surface, about 40 isolated volcanic centres that do not seem to be associated with plates and that seem to remain fixed relative to each other as plates move around (Figure 11.1). Their fixity (or at least their slow motion relative to plate velocities) is inferred from the existence of ‘hotspot tracks’, that is of chains of volcanoes that are progressively older the further they are from the active volcanic centre.