Steep‐sided domes on Venus: Characteristics, geologic setting, and eruption conditions from Magellan data

Distinctive steep‐sided flat‐topped domes have been discovered in the Magellan images of Venus. These domes have similar characteristics to terrestrial domes formed by viscous andesitic, dacitic, and rhyolitic lavas. A survey of more than 95% of the surface reveals 145 such features ranging from less than 10 km to almost 100 km in diameter, with a mean diameter of 23.8 km and a mean height of about 700 m. The domes are 10–100 times wider and have volumes (25–3400 km 3 ) orders of magnitude larger than analogous terrestrial features. The morphology of many of the domes suggests that they were emplaced in a single continuous mode rather than episodically. The domes are widely distributed over the planet (including one near the Venera 8 site) and occur preferentially in association with coronae and in plains adjacent to regional occurrences of tessera. They show no strong correlation with latitude and longitude, but are preferentially concentrated at altitudes near and just below the mean planetary radius; this altitude concentration may be related to the preferential development of neutral buoyancy zones and magma reservoirs at intermediate to higher altitudes. The common association of domes with coronae appears to be an indication of the importance of magma upwelling and magma reservoirs in the formation of domes. The probability that magma reservoirs on Venus will grow to unusually large sizes favors large‐scale differentiation processes in such magma reservoirs. This suggests that production of large‐volume magma bodies by in situ processes such as fractional crystallization or volatile enhancement is entirely plausible. The association with tesserae may be related to ab initio processes of melting of preexisting crust of evolved composition by stalling of basaltic magma at neutral buoyancy zones, comparable to the environment on terrestrial continents. The resemblance of the Venus domes to terrestrial rhyolite and dacite domes provides possible evidence for the emplacement of high viscosity magma on Venus. The extremely large volumes of the domes and evidence for their emplacement in a continuous mode has led us to develop two models for the emplacement of the Venus domes. In the compositionally evolved magma model, the high apparent viscosity is due to differentiation and evolution of basaltic magma to produce silicic magmas. Magmas with such compositions and viscosities can be erupted on Venus through fissures of plausible widths and at rates comparable to those observed on Earth; terrestrial rhyolite flows with volumes of 200 km 3 are known. A related possibility is that the domes are the undisrupted Venus equivalent of terrestrial ignimbrite‐producing eruptions. The second model is a basaltic bubble enhancement model, in which the high apparent viscosity is due to the extrusion of basaltic foams following volatile enhancement in the upper part of a magma reservoir. Large volumes and single eruptive phases are plausible because of low magma viscosity during ascent, high viscosity on the surface, and enhanced bulk volume (compared to magmatic volume) caused by the high bubble content.