Lunar Volcanic Eruptions: Range of Eruption Styles and Implications for Magma Ascent and Emplacement

Introduction: A wide variety of morphologic features representing a range of eruption styles has been documented on the Moon [e.g., 1-2]. We have characterized the nature of numerous steep-sided domes, small shields, cones, dark halo craters of internal origin, dark mantle deposits, linear rille-related deposits, and sinuous rille-related deposits on the Moon using Clementine multis-pectral, Apollo, and Lunar Orbiter data [2-7]. We have also shown that the main path of the ascent and eruption of magma from mantle source regions is through magma-filled cracks or dikes [2, 8-9]. We have been analyzing additional landforms and synthesizing these results into an overall assessment of the relationship between the nature of dike intrusion to shallow depths within the crust and the resulting landforms and deposits (Fig. 1). These data and this synthesis will be of importance to the general reanalysis of the models of the ascent and eruption of magma [10]. Analysis: We have revised our theoretical treatment for the penetration of magma-filled cracks (dikes) to the vicinity of the lunar surface (Fig. 1) [10], and we outline here the predicted range of tectonic and associated volcanic features and processes from our analyses [7, 9, 11]. The surface manifestation of a dike that does not actually reach the surface can take a range of forms. If the dike stalls at a sufficiently great depth, there will be some undetectably small amount of surface extension and uplift. If it penetrates to shallower depths there may still be no noticeable topographic effects at the scale of available images, but incipient failure or activation of pre-existing fractures may generate pathways along which gas (probably mainly carbon monoxide) formed by carbon-metal oxide "smelting" reactions [12-13, but see also 14] in magma in the shallowest parts of the dike can reach the surface. Still shallower penetration will lead to a larger volume of melt being exposed to the relatively low pressure environment near the surface and will encourage the generation of a greater mass of CO since the chemical reaction producing it is pressure-dependent. Subsequent loss of this gas, coupled with a magma volume decrease on solidification and cooling, may lead to collapse features (or even explosion craters) forming on the surface above the dike. Very shallow intrusion may lead to further development of a graben and will encourage the formation of small secondary intrusions and possible eruptions; we have developed criteria to distinguish between graben formed …