An important feature is that plumes must be initiated at some thermo-mechanical boundary layer within the Earth, for instance at the core – lower-mantle boundary [2900km depth] or at the shallower 700km discontinuity:
It is at these boundaries where either compositional differences (core-mantle) or phase differences (700km discontinuity) permit denser but hotter material to exist beneath lighter cooler rock, so that any instabilities in the convective patterns across the boundary layer may spawn a hot plume. At initiation, such plumes can be several hundred degrees hotter than the surrounding mantle, and will rise, the excess heat then causing a lowering of the viscosity of the surrounding mantle (or even melting it slightly) so allowing it to be entrained into the mushroom head of the plume. in this way the plume head gradually enlarges itself and becomes cooler (relatively), whereas the tail of the plume is narrower, but hotter, the hot material continually rising up into the head.
These features have been described by Campbell, Griffiths, Hill & Co., and in simplified form can be summarised as follows: