Rapid lithospheric thinning by mantle plumes has not been achieved in
numerical experiments performed to date. Efficient thinning depends on
small-scale instabilities that convectively remove lithospheric mater
ial. These instabilities are favored by hotter plumes or stronger temp
erature dependence of viscosity, and a simple scaling independent of r
heology controls their onset. This scaling allows extrapolation of the
results of numerical experiments to the Earth's mantle. Mantle plumes
between 100 and 150 kelvins hotter than the background mantle should
exhibit small-scale convective rolls aligned with the plate motion. Th
e unusual variation in heat flow across the Hawaiian swell may be due
to such instabilities. It was found that the spreading of the plume cr
eates a downwelling curtain of material that isolates it from the rest
of the mantle for distances of at least 1000 kilometers from the plum
e origin. This isolation has important consequences for the geochemica
l heterogeneity of the lithosphere and upper mantle.