Geological and physical observations and constraints are applied to the development of a modelof the ascent and emplacement of basaltic magma on the earth and moon. Mathematical models of the nature and motion of gas/liquid mixtures are developed and show that gas exsolution from terrestrial and lunar magmas commonly only occurs at shallow depths (less than 2km); thus the ascent of bubble-free magma at depth can be treated separately from the complex motions caused by gas exsolution near the surface. Magma ascent is related to dike or conduit width; a lower limit to width is determined by the presence of a finite magma yield strength or by excessive magma cooling effects related to magma viscosity. For terrestrial basalts with negligible yield strengths and viscosities greater than 10²Pa s, widths in the range 0.2-0.6m are needed to allow eruptions from between depths of 0.5-20km. Fissure widths of about 4m would be needed to account for the output rates estimated for the Columbia River flood basalt eruptions.-Authors