Mechanisms of formation of igneous layering

Layering is a common, almost ubiquitous, feature of gabbroic and syenitic intrusions. Individual layers, or layered sequences, however, vary greatly in such features as thickness and length, the nature of layer boundaries, internal vertical and lateral variations within layers, and the relationships to other nearby layers. A wide variety of layer-forming mechanisms has been proposed. Some operate during the initial filling of a magma chamber, as a result of the settling of crystals carried in suspension, flow segregation during magma transport, magma chamber recharge, or magma mixing. Other proposed mechanisms operate in response to continuous, intermittent, or double-diffusive convection. Layering may also form as the result of mechanical processes, such as gravity settling, crystal sorting by magma currents, magmatic deformation, compaction, seismic shocks, or tectonic deformation.