Magma reservoirs and neutral buoyancy zones on Venus: Implications for the formation and evolution of volcanic landforms

We examine the production of magma reservoirs and neutral buoyancy zones (NBZs) on Venus and the implications of their development for the formation and evolution of volcanic landforms. The high atmospheric pressure on Venus reduces volatile exsolution and generally serves to inhibit the formation of NBZs and shallow magma reservoirs. For a range of common terrestrial magma volatile contents, magma ascending and erupting near or below mean planetary radius (MPR) should not stall at shallow magma reservoirs; such eruptions would be characterized by relatively high total volumes and effusion rates. For the same range of volatile contents at 2 km above MPR, about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones. In general, neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on Venus than on Earth. For a fixed volatile content, NBZs become deeper with increasing elevation: over the range of elevations treated here (−1 km to +4.4 km) depths differ by a factor of 2–4. These analyses reveal several factors that may help to account for the low height of volcanoes on Venus. Larger primary reservoirs cause the wide dispersal of conduits building edifices. Models of the position of the shallow NBZ reservoir during edifice growth show that for Earth the magma chamber center remains at a constant depth below the growing edifice summit, thus keeping pace with the increasing elevation, while on Venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude. Therefore neutral buoyancy zones and magma reservoirs on Venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all; the lower rate of vertical migration implies that magma reservoirs would tend to stabilize, undergo greater lateral growth, and become larger on Venus than Earth. The proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on Venus than Earth. Large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions. In large reservoirs, positively buoyant materials are likely to be produced from differentiation, substrate remelting, and volatile exsolution. Nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on Earth, and when this gas-enriched melt emerges at the surface, it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure. Therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations, leading to anomalous, more viscous rheological properties. Inhibition of disruption of volatile-rich magma for both basaltic and more evolved compositions can lead to the production of (1) more bubble-rich, vesicular flows characterized by higher viscosity and greater thicknesses, the Venus equivalent of effusions that would have lost much of their volatiles in the fire-fountaining process on Earth (2), high-volume, high discharge rate extremely gas-rich basaltic effusions that increase viscosity considerably upon surface extrusion and produce domes whose volumes are considerably higher than their dense rock equivalent, and (3) more evolved compositions that are gas rich but do not undergo disruption upon effusion, and produce domes and flows of high viscosity, the Venus equivalent of terrestrial ignimbrites. On the basis of these analyses we predict that intrusions and dike emplacement should be common, particularly around both shallow and deep magma reservoirs, and the tops of dikes should be manifested as radial fractures. There should be a range of diameters of circular features associated with the presence of both shallow (NBZ-related) and deep (plumerelated) magma reservoirs. Large-volume lava outpourings should be favored at lower elevations, and shallow reservoirs and edifices should be more common at intermediate elevations; at the highest elevations, magma reservoirs are predicted to be large and relatively deep with low associated edifices. Direct ascent of magma where neutral buoyancy zones are unlikely (low elevations) and the propensity for relatively larger magma reservoirs on Venus when they do form, are both factors which will tend to favor high-volume eruptions, independent of variations in magma temperature or chemistry. Individual volcanic features should be interpreted in terms of regional geologic setting and the dynamics of elevation changes commonly associated with various scales of mantle processes and magma emplacement in order to assess global crustal growth processes on Venus.