Abstract
Vents and deposits attributed to explosive volcanism occur within numerous impact craters on both the Moon and Mercury. Given the similarities between the two bodies it is probable that similar processes control this spatial association on both. However, the precise morphology and localization of the activity differs on the two bodies, indicating that the nature of structures beneath impact craters and/or volcanic activity may also be different. To explore this, we analyze sites of explosive volcanism within complex impact craters on the Moon and Mercury, comparing the scale and localization of volcanic activity and evidence for post-formation modification of the host crater. We show that the scale of vents and deposits is consistently greater on Mercury than on the Moon, indicating greater eruption energy, powered by a higher concentration of volatiles. Additionally, while the floors of lunar craters hosting explosive volcanism are commonly fractured, those on Mercury are not. The most probable explanation for these differences is that the state of regional compression acting on Mercury's crust through most of the planet's history results in deeper magma storage beneath craters on Mercury than on the Moon. The probable role of the regional stress regime in dictating the depth of intrusion on Mercury suggests that it may also play a role in the depth of sub-crater intrusion on the Moon and on other planetary bodies. Examples on the Moon (and also on Mars) commonly occur at locations where flexural extension may facilitate shallower intrusion than would be driven by the buoyancy of the magma alone.
Highlights
It has long been recognized that vents and deposits attributed to explosive volcanism frequently occur within complex impact craters on the Moon [e.g., Schultz, 1976; Head and Wilson, 1979; Coombs and Hawke, 1992]
V sin 2, g where g is gravitational acceleration and θ is the angle at which dispersal is greatest (45°). This gives a value of 284 m s-1 for the median and 468 m s-1 for the greatest ballistic range observed in the Mercury sample set, and 143 m s-1 for the median and 297 m s-1 for the greatest ballistic range observed in the lunar sample set
A comparison of the scale of vents and surrounding deposits attributable to pyroclastic volcanism within complex impact craters on the Moon and Mercury indicates that eruptions had a significantly higher average energy on Mercury
Summary
It has long been recognized that vents and deposits attributed to explosive volcanism frequently occur within complex impact craters on the Moon [e.g., Schultz, 1976; Head and Wilson, 1979; Coombs and Hawke, 1992]. It has been proposed that localization of explosive volcanism within impact craters results from density-trapping of magma in the brecciated zone below the crater [Head and Wilson, 1979] In this model, a vertically-propagating dike encounters the low density, weak material of the breccia lens beneath the crater floor and is diverted to form a sill because the density and rigidity contrast favors lateral propagation rather than continued vertical ascent [Schultz, 1976; Wichman and Schultz, 1995a]. The products of both of these styles of volcanism are observed at circumferential fractures in floor-fractured craters (FFCs) on the Moon, so this appears to be a good explanatory model
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