Venus volcanism: Classification of volcanic features and structures, associations, and global distribution from Magellan data

A preliminary analysis of a global survey of Magellan data covering over 90% of the surface and designed to document the characteristics, location, and dimensions of all major volcanic features on Venus has revealed over 1660 landforms and deposits. These include over 550 shield fields (concentrations of small volcanoes <20 km in diameter), 274 intermediate volcanoes between 20 and 100 km diameter with a variety of morphologies, 156 large volcanoes in excess of 100 km diameter, 86 calderalike structures independent of those associated with shield volcanoes and typically 60–80 km in diameter, 175 coronae (annulus of concentric ridges or fractures), 259 arachnoids (inner concentric and outer radial network pattern of fractures and ridges), 50 novae (focused radial fractures forming stellate patterns), and 53 lava flood-type flow fields and 50 sinuous lava channels (all of which are in excess of 102–103 km in length). The vast majority of landforms are consistent with basaltic compositions; possible exceptions include steep-sided domes and festoons, which may represent more evolved compositions, and sinuous rules, which may represent more fluid, possibly ultramafic magma. The range of morphologies indicates that a spectrum of intrusive and extrusive processes have operated on Venus. Little evidence was found for extensive pyroclastic deposits or landforms, consistent with the inhibition of volatile exsolution and consequent disruption by the high surface atmospheric pressure. The large size of many volcanic features is evidence for the presence of very large magma reservoirs. The scale of resurfacing implied by individual features and deposits is typically much less than 125,000 km2. The areal distribution, abundance, and size distribution relationships of shield fields, arachnoids, novae, large volcanoes, and coronae strongly suggest that they are the surface manifestation of mantle plumes or hot spots and that the different morphologies represent variations in plume size and stage and thermal structure of the lithosphere. Maps of the global distribution of volcanic features show that they are broadly distributed globally, in contrast to the plate boundary concentrations typical of Earth. However, they are not randomly distributed on the surface of Venus. An observed deficiency of many volcanic features in several lowland areas of Venus may be due to an altitude-dependent influence of atmospheric pressure on volatile exsolution and the production of neutral buoyancy zones sufficient to form magma reservoirs; this would favor lava floods and sinuous channels at low elevations and edifices and reservoir-related features at higher elevations. A major concentration of volcanic features is observed in the Beta/Atla/Themis region, an area covering about 20% of the planet and centered on the equator. This region is unique in that it is the site of local concentrations of volcanic features with concentrations 2–4 times the global average, an interlocking network of rift and deformation zones, several broad rises several thousand kilometers in diameter with associated positive gravity anomalies and tectonic junctions, and evidence for volcanically embayed impact craters. Although the region as a whole does not appear to be anomalously older or younger than the rest of Venus, there is evidence that the most recent volcanic activity on the planet occurs here, and the presence of this series of concentrations suggests that the mantle in this region is anomalous. Analysis of the impact crater population shows that it cannot be distinguished from a completely spatially random population (Phillips et al., this issue), and several end-member models for this distribution are possible: (1) single production age or “spasmodic or catastrophic volcanism” model: craters have accumulated subsequent to a global volcanic resurfacing event about one-half billion years ago (Schaber et al., this issue); (2) vertical equilibrium or “leaky planet” model: craters are removed by slow accumulation of lava over the whole planet leading to a range of volcanic degradation states for craters; (3) regional resurfacing or “collage” or “cookie-cutter” model: craters are removed largely instantaneously by superposition of features and deposits; the horizontal scale of resurfacing does not exceed the horizontal scale of randomness of the crater population. Our data on the scale and location of resurfacing are consistent with the regional resurfacing model and with the catastrophic resurfacing model. The nature and abundance of impact craters definitely degraded by volcanism also favor these two models, although uncertainty exists as to whether all such craters have been detected. Although a process toward the regional resurfacing end-member model presently seems most plausible, distinction between the three models requires an understanding of the mode and timing of emplacement of the volcanic plains that make up the majority of the surface and which are not clearly related to the edifices and features mapped in this study. In addition, the resurfacing mechanisms involved in the catastrophic resurfacing models are not yet explicitly enough formulated to test with the existing data. An equilibrium resurfacing model implies a volcanic flux of 0.5 km3/yr, a value similar to the present rate of intraplate volcanism on Earth (0.3–0.5 km3/yr). This value is broadly comparable to that implied by the edifices and deposits on Venus mapped in this study. Geologically recent volcanism on Venus is dominated by features interpreted to be related to mantle plumes.