The magmatism occurring when a hot convective mantle plume is sited beneath a lithospheric plate may be more complex if the latter is continental, rather than oceanic, because of the characteristic local physical inhomogeneity of continents. Thus, the surface volcanic expression of the plume (hotspot) may be displaced from immediately above its rising stem, if the continent has been previously locally thinned nearby. The magmatism of the British Tertiary Igneous Province and Parana basin, South America, appears to fit this model, and it may also explain why the Miocene Columbia River basalts of the NW United States overlie an Early Tertiary sedimentary basin.

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