Abstract Ambrym is one of the largest volcanic islands of the Vanuatu arc. It has been the focus of exclusively mafic volcanism and has a structure dominated by a central 13 km-diameter caldera. Contained within the caldera are two major cone complexes, Marum and Benbow, which have been the locus of most historic eruptions. Vents within these are constantly in a state of strong degassing, with visible lava lakes periodically being observed in several subcraters. Vulcanian and strombolian explosive eruptions occur at least yearly, along with larger subplinian events every 20–30 years. The active vent systems are enclosed by several 100 m-deep vertical-walled pit craters that expose cross-sectional views through the transition zone between the conduit and the crater. Units include coherent magma bodies with interbedded pyroclastic successions. One of the Marum craters, Niri Taten, exposes portions of solidified lava lakes, magma pods that fed spatter cones, small shallow-level intrusions and larger sills that connect through a complex network of dykes to the surface and/or into the pyroclastic edifice. These features show that shallow-level infiltration of degassed and low-viscosity melts into pyroclastic-deposit-dominated volcanic sequences of Marum plays an important role in the growth of scoria and spatter cones. Once solidified, the large intrusive bodies apparently provide important buttressing of pyroclastic cones, but during emplacement they may also cause cone collapse and lateral escape of magma to form lava flows.

Central pit craters are common on ice-rich bodies, such as Mars, Ganymede, and Callisto. Mars and Ganymede represent the two end members regarding target characteristics (mixed ice and soil for Mars vs. almost pure ice for Ganymede). Comparisons of central pit craters on these two bodies can provide insights into the environmental conditions under which these craters form and provide constraints on the proposed formation models. This analysis includes 1604 central pit craters on Mars and 471 central pit craters on Ganymede. Martian central pit craters are divided into floor pits and summit pits, whereas all central pit craters on Ganymede are floor pits. Central pit craters form in similar-diameter ranges on both Mars and Ganymede when gravity differences are considered, and both bodies show no regional variations in pit crater distribution within the ±60° latitude zone. Martian floor pits are larger relative to their parent crater than summit pits, but the Ganymede pit/crater diameter ratio is larger than for either central pit type on Mars. Central pits have formed over the entire history of both bodies, and there is no indication that excavation depths have varied over time. Lack of crater floor updoming in Martian floor pit craters indicates that low concentrations of ice (estimated at ~20%) still allow production of central pits. The results of this study argue against central peak collapse as the formation mechanism for central pit craters. Excavation into a subsurface liquid layer cannot be ruled out but is difficult to support based on the distributions and consistencies in excavation depth on both bodies. These results support the model of vaporization and gas escape for central pit formation on both Mars and Ganymede.