Ascent and Eruption of Basaltic Magma on the Earth and Moon

Geological and physical observations and constraints are applied to the development of a model of 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 2 km); 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 ter-restrial basalts with negligible yield strengths and viscosities greater than 102 Pa s, widths in the range 0.2-0.6 m are needed to allow eruptions from between depths of 0.5-20 km. Fissure widths of about 4 m would be needed to account for the output rates estimated for the Columbia River flood basalt eruptions. As the magma nears the surface, bubble coalescence will tend to occur, leading to intermittent explosive strombolian-style activity. For commonly occurring lunar and terrestrial basalts the magma rise speed must be greater than 0.5-1 m/s if strombolian activity is to be avoided and relatively steady fire fountain-ing is to take place. Terrestrial fire fountain heights are dictated by the vertical velocity of the magma/ gas dispersion emerging through the vent, increasing with increasing magma gas content and mass erup