COMPOSITION DIFFERENTIATION IN A LAVA FLOW

Abstract

Particles (crystals or bubbles) suspended in a flowing medium may be displaced by mechanisms other than gravity. Such mechanisms may involve the wall effect, the Manus effect, or the R.A. Bagnold effect. Flow differentiation can be described to good accuracy by neglecting the other effects and using only the semiempirical Bagnold-Barrier formulas that define the Bagnold effect [6-9]. The displacement of the particles from their paths depends on their size, the distance traveled by the medium, and the extent to which the behavior of the flow is non-Newtonian, and is independent of the flow viscosity and velocity. Therefore flow differentiation can be used to explain structures arising not only in magma, but also in cold plastic flow of rocks. The overall composition of the lava responds to the redistribution of the mineral phenocrysts. Two series of silicate analyses of samples of megaplagiophyre basalt were studied, and results are discussed.