LOCATION AND FORMATION OF THOLI ON VENUS

Volcanism on Venus manifests itself in many ways including large lava flows, coronae, and smaller constructs called tholi. Defined as small domical hills or mountains; there are currently 57 catalogued tholi on Venus. Despite this seemingly large number of tholi, little work has addressed their geologic setting or formation. Here, we report our initial results of a campaign aimed at determining the geologic setting of Venusian tholi. This survey is part of a larger project that addresses tholi formation mechanisms. For this study, a planetary survey was conducted using satellite images and published maps from the NASA/ USGS Planetary Geologic Mapping program. Venusian tholi were located on these maps and then separated into categories according to the geologic setting in which they appear on the planet’s surface. Initially, tholi were broadly mapped using prints of the Venus maps to mark each individual tholi and its nearby volcanic constructs, or lack thereof. Once three main geologic setting classifications were established, we mapped example tholi from each setting more extensively. Our results show that tholi are typically found in one of three settings: 1) in conjunction with coronae, 2) in the vicinity of rift zones, and 3) as singular, isolated features. The first tholus setting is tholi in conjunction with coronae, which are large volcanic basins, formed from the collapse of a magma plume beneath the surface. These tholi tend to lie around the outside of the coronae, but on occasion are found within the basin itself. Rifting on Venus is similar to rifting on Earth and tholi are often found in the vicinity of large rift zones, both among the rifts as well as in the outskirts of the zone. The final setting for tholi was for those that are found with no surrounding volcanic constructs. These appear in relatively isolated regions of Venus and have no visible connection to other features. Tholi appear in a variety of diverse geologic settings on Venus, and likely form from equally varied geologic processes.