The Summit, Tectonic and Flank Characteristics of Large Venusian Volcanoes: a New Global Survey

Introduction: Large volcanoes on Venus are defined as topographically positive, broadly domical structures with lava flow aprons larger than 100 km in diameter surrounding a central volcanic vent [1]. Several surveys have been carried out since the first return of data from the Magellan mission, investigating the location and distribution of volcanoes over the venusian surface [1,2], their structure [3] and variation in size with altitude [4]. The number of volcanoes included in each survey varies from 123 [4] to 167 [2], but to date, only the 1997 survey by Crumpler et al. [2] has been published. Large volcanoes on Venus show a diversity of morphologies with some displaying similar characteristics to large coronae. It is not surprising therefore that many features included in the 1997 database [2] have been classified by others as coronae and included in other relevant corona databases [5-7]. The first task of this new study was re-examine the large volcano population to eliminate the overlap with other catalogues and to try to accurately determine the number of features on the planet. In addition to their earlier surveys, Head et al. [8] and Crumpler et al. [9] described nine fundamental types of large volcano suggested to represent a morphological classification describing individual features and associations with each edifice. Large volcanoes are structurally diverse with many different morphologies. The 9 classes describe a mix of both summit and tectonic characteristics meaning that an individual volcano may not be described accurately by any individual field or fields. The second task was to therefore devise a new classification scheme that would better describe the major morphological features of each edifice. Survey Procedure and Design: The new survey located and classified the population of large volcanoes using full resolution F-Maps and C1-MIDR mosaics. Magellan digital altimetry data was used to take at least two cross-sections over each volcano in order to measure the average diameter of the main edifice to the outer breaks of slope, the maximum altitude and the level of the surrounding plains. The average diameter of the flow apron and the size of any summit caldera were measured directly from F-Maps. Synthetic stereo images were used extensively and were invaluable in determining the morphological properties of each feature. The new classification scheme (Figure 1, Table 1) describes four types of summit structure: Simple, Caldera, Elongate and Multiple Summit, and four types of tectonic structure: Radially fractured summit, Radially fractured flanks, Rift/fracture zone related, Concentric fractures. Any number of these categories can therefore be used to describe each feature. We also analysed the abundance and location of small edifices (shields, cones or domes) on each large volcano and any associated lava flows erupted from vents on its flanks. The small edifice and flank vents category may be noted as multiple entries if different styles and locations of flank activity are observed. Results: The survey located and recorded the morphological characteristics of 134 large volcanoes. Volcano flow aprons were found to have diameters ranging from 100 – 1000 km, and edifice diameters from 50 –