HR: 17:15h
AN: P34A-06    [Abstracts]
TI: Lunar lava flows and related eruption styles: Relation to dike emplacement processes
AU: * Wilson, L
EM: L.Wilson@lancaster.ac.uk
AF: Lancaster University, Environmental Sciences Dept, Lancaster, LA1 4YQ, United Kingdom
AU: Head, J W
EM: james_head@brown.edu
AF: Brown University, Dept of Geological Sciences, Providence, RI 02912, United States
AB: Lava flows are the traditional evidence for volcanism on the Moon, but the nature of volcanic vents can provide important information about the range of eruption conditions and the context in which effusive flows occur. The wide variety of morphologic features representing a range of eruption styles has been documented on the Moon, including steep-sided domes, small shields, cones, crater chains, dark halo craters of internal origin, dark mantle deposits, dark halo rings, linear rille-related deposits, extremely long lava flows, and sinuous rilles and their related deposits. No large Hawaii-like shield volcanoes have been observed. The main path for the ascent and eruption of magma from mantle source regions is through magma-filled cracks or dikes. We assess the relationship between the nature of dike intrusion to shallow depths within the crust and the resulting landforms and deposits. 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 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 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 by dike emplacement and those resulting from tectonic deformation alone. The shallow stalling of a dike wide enough to allow spontaneous convection to occur during the early stages of its cooling can expose so much magma to low pressure degassing that it leads to major gas buildup and propagation of a crack to the surface, resulting in an Io-like eruption plume and the formation of a dark ring deposit (such as the 154 km diameter feature surrounding a vent in southern Orientale). Also assessed is the deep generation of magmatic gas on the Moon at dike crack tips and the implications for pyroclastic eruptions and pyroclastic glass provenance. Models of the ascent of magma feeding steep-sided domes suggest high-viscosity magma, consistent with their distinctive shapes. Together, the relationship between the vent characteristics and models of the behavior of dikes provide important insight into the generation, ascent and eruption of magma on the Moon, and the role of lava flows. New data scheduled to be acquired in the next several years will provide important advances in these studies.
DE: 5480 Volcanism (6063, 8148, 8450)
DE: 8425 Effusive volcanism
DE: 8428 Explosive volcanism
DE: 8450 Planetary volcanism (5480, 6063, 8148)
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting