Magma chambers, particularly those of basaltic composition, are often replenished by an influx of magma whose density is less than that of the resident magma. This paper describes the fundamental fluid mechanics involved in the replenishment by light inputs. If &rgr; denotes the uniform density of the resident magma and &rgr;-Δ&rgr; that the input, the situation is described by the reduced gravity g¿=gΔ&rgr;/&rgr;, the volume flux Q, and the viscosities of the resident and input magmas ve and vi, respectively. The (nondimensional) Reynolds numbers, Ree=(g¿Q3) 1/5ve and Rei=(g¿Q3)1/5/vi and chamber geometry than completely specify the system. For sufficiently low values of the two Reynolds numbers (each less than approximately 10), the imput rises as a laminar conduit. For larger values of the Reynolds numbers, the conduit may break down and exhibit either a varicose or a meander instability and entrain some resident magma. At still larger

Reynolds numbers, the flow will become quite unsteady and finally turbulent. The values of the Reynolds numbers at which these transitions occur have been documented by a series of experiments with water, glycerine, and corn syrup. If the input rises as a turbulent plume, significant entrainment of the resident magma can take place. The final spatial distribution of the mixed magma depends on the geometry of the chamber. If the chamber is much wider than it is high, the mixed magma forms a compositionally stratified region between the roof and a sharp front above uncontaminated magm. In the other geometrical extreme, the input magma is mixed with almost all of the resident magma. If the denisty of the resident magma is already stratified, the input plume may penetrate only part way into the chamber, even though its initial density is less than that of the lowest density resident magma. The plume will then intrude horizontally and form a hybrid layer at an intermediate depth. This provides a mechanism for preventing even primitive basaltic magmas of minimum density from erupting at the surface. By conducting an experiment using aqueous solutions, we show that entrainment can lead to crystallization of the magma in the input plume by making it locally supersaturated. All these effects are discussed and illustrated by photographs of laboratory experiments.