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Volcano instability development: a planetary perspective

Abstract Analysis of the types and causes of volcano instability using the record seen on other planets, where volcanoes form in a wide variety of environments and are often better preserved, provides a perspective on the processes involved and deposits produced in edifices on Earth. The records on these bodies illustrate the potential significance of neutral buoyancy zones and rheological boundaries in relation to initial magma ascent and storage, related eruption styles, and subsequent edifice development and behaviour. On the Moon no Hawaii-like shield volcanoes are observed; the thick low-density anorthositic crust provided a density barrier to mantle plumes and the low frequency of dyke-emplacement events precluded development of shallow reservoirs and large shields. On Venus, the very high atmospheric pressure reduces volatile exsolution and fragmentation, inhibiting the formation of neutral buoyancy zones (NBZ) and shallow magma reservoirs near or below mean planetary radius; eruptions are predicted to be characterized by relatively high total volumes and effusion rates, and poorly developed edifices. Decrease in atmospheric pressure with elevation favours the development of NBZs and reservoirs at higher elevations. These and other factors result in low and broad volcanic edifices, with reservoirs predominantly in the substrate. Smaller loads, thicker lithospheres, and dry substrates combine to minimize large-scale vertical edifice growth and instability development. Exceptions include a wide array of collapse runout features associated with steep-sided domes. On Mars, low gravity and low surface atmospheric pressure cause density profile differences and flow length differences and massive volcanoes have been emplaced over long periods on a stable lithospheric plate and a volatile-containing upper crust. They exhibit a wide range of rift zone development, internal deformation related to lithospheric loading and flexure, flank and slope failure, and summit failure and caldera development. These planetary examples permit one to begin to isolate various factors in the development of volcano instabilities, including edifice geometry (height/width) and size, relation to lithospheric thickness as a function of time, potential role of neutral buoyancy zones and magma reservoir size and development, and the presence of fluids in the substrate. The extensive and well-preserved flank-failure deposits on Mars provide a significant areal perspective and analogue for the interpretation of less-well preserved deposits on Earth.