THE FORMATION AND EVOLUTION OF LAYERED STRUCTURES IN POROUS-MEDIA

Horizontally layered structures can develop in porous or partially mol ten environments, such as magma chambers, the early Earth's mantle, an d hydrothermal systems. We have studied the generation and evolution o f these horizontally layered structures in a rigid porous medium by he ating a compositionally stably stratified fluid from below. Growth of a convective layer through entrainment, the formation of a vertical de nsity interface on top of this layer and destabilization of the next l ayer are closely coupled. It is shown that the growth of the first con vective layer stops once the thermal equilibrium is reached. Since hea t (solute) transfer across the thin density interfaces becomes purely diffusive (dispersive), the separately convecting layers can persist o n a compositionally dispersive timescale. While the number of layers t hat develop is determined by the magnitude of the thermal Rayleigh num ber, the final height of a newly formed layer is inversely proportiona l to the buoyancy ratio. Two dynamical mechanisms which lead to sudden interface disappearance determine the vertical layer scales, rather t han the initial evolution toward the thermal equilibrium. Finally, the results are discussed with respect to the above mentioned geophysical scenarios.