Mafic magmas rising into the middle crust are typically trapped in wall rocks of similar density. Upon crystallization their densities increase significantly, and the resulting plutons have a large negative buoyancy. Calculations indicate that plutons of even modest size will sink at rates on the order of several kilometres per million years in typical middle and lower crust. This foundering process accounts for the widespread stable density stratification of the crust in spite of magmatic processes that should lead to density inversions.

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