High beat-flow foci on the Earth have been named ‘hot-spots’ and are commonly correlated with ‘mantle-plumes’ in the deep. A mantle plume may be described as a portion of mantle material with a higher heat content than its surroundings. The intrusion of a mantle-plume is inferred to be similar to the intrusion of a salt diapir and the process of diapirism is discussed. The theoretical mechanistic and thermal effects of hot diapirs and the tectonic and metamorphic implications are discussed. Two sets of diapirs, i.e. a first order diapir equal to a mantle-plume and, originating from it, second order diapirs causing hot spots, are invoked to give a reasonable explanation for the Palaeozoic evolution of the continental lithosphere of Western Galicia (NW Spain); the heat-flow pattern in the Rio Grande rift is also elegantly explained in a similar way. The sources of heat that might cause a mantle-plume are discussed but no one can be singled out as the most plausible. Mantle-plumes may be held responsible for the creation of a zone of weakness that is essential to initiate seafloor spreading, but certainly not every mantle-plume will play that role. The behavioural parameters of a mantle-plume are briefly explored and it is inferred that the heat content might be the most important one.