Steep-sided domes are one of the most striking volcanic landforms on Venus. They may also be key to determining the range of magmatic processes operating on Venus as, in contrast to all other volcanic landforms, they likely represent eruption of viscous lava. Although there have been various explanations for the presence of high-viscosity lavas on a planet dominated by effusive basaltic volcanism, it is often assumed that they are silica-rich. This would necessitate either periodic, large-scale, extensive fractionation of basaltic magma in the Venusian crust, or a mechanism for re-melting an already silica-enriched lower crust. As such, determining the origin of steep-sided domes is important in constraining magmatic processes on Venus, and for understanding geological evolution of stagnant lid regime planets generally. Here, we use observations from the Marki region of the Troodos ophiolite, Cyprus, to propose an alternative model where steep-sided domes form by eruption of crystal mush from the same magmatic systems which fed extensive basaltic terrains with which domes are associated. Steep-sided volcanic landforms near Marki represent extrusion of ‘un-eruptible’, extremely olivine-rich mush onto the palaeo-seafloor, following cessation of widespread basaltic volcanism. Field relations suggest that these bodies formed by localised, repeated extrusion of crystal mush, fed by extensional faults tapping crustal magma chambers. Differential stress enabled eruption of viscous, non-Newtonian magmas with crystal contents >50 vol%, which then built up volcanic edifices on the seafloor. A similar, much larger-scale mechanism can explain many features of steep-sided volcanic domes on Venus, including their intimate relationship with extensive, basaltic terrains, general morphology, and dome spatial and temporal clustering. This implies that domes share a common magmatic origin with the Venusian basaltic crust, rather than representing a discrete magmatic process, and that they represent periods of magmatic quiescence. It also implies that the contrasting morphology of these domes arises from a fundamental difference in eruptive style, from widespread effusive basaltic magmatism to localised, extensional fault-controlled extrusion of crystal mush. If correct, this mechanism might also explain formation of steep-sided volcanic edifices on other large, stagnant-lid regime planetary bodies.
- Crystal mush
- Steep-sided dome