Icarus 152, 75–95 (2001) doi:10.1006/icar.2001.6633, available online at http://www.idealibrary.com on Development of Large Volcanoes on Venus: Constraints from Sif, Gula, and Kunapipi Montes Ellen R. Stofan Proxemy Research, Laytonsville, Maryland, and Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, United Kingdom E-mail: ellen@proxemy.com and John E. Guest and Duncan L. Copp Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, United Kingdom Received May 10, 2000; revised February 7, 2001 Sif, Gula, and Kunapipi Montes are large volcanoes on Venus. They have differing morphologies reflecting theirdifferent eruptive histories and plumbing systems. All have a steeper upper cone sur- rounded by more shallowly sloping flanks, interpreted as resulting from histories of more frequent, shorter, lowereffusion rate sum- mit eruptions accompanied by infrequent, more voluminous flank eruptions. All three volcanoes have multiple flow field types (digi- tate, sheet, and fan), which probably reflect variations in effusion rate, eruption duration, and vent geometry. Sif Mons’summit has a large caldera, Gula Mons has summit calderas at eitherend of a summit rift, and the summit of Kunapipi Mons is composed of two volcanic centers on a summit plateau. The summit morphologies of Sif, Gula, and Kunapipi Montes are interpreted to reflect differ- ences in magma chamber location over time, and differing internal plumbing. We find no positive evidence that neutral buoyancy has governed magma chamber position. Instead, magma chambers at the three volcanoes appear to have changed position over time, with lithospheric stresses as well as neutral buoyancy affecting chamber location. All three volcanoes have had episodes of diking at their summits, followed by episodes of extrusion. We interpret this to be caused by variations in either magma supply rate or magma volatile content, or both, over time. Fractures on the flows of the volcano indicate that dike intrusion has occurred throughout the visible history of the volcanoes, indicating that the local stress field within the volcano favored intrusion as well as extrusion overmuch of its history. The overall shape and history of the venusian volca- noes is similar to the histories of terrestrial volcanoes. However, some aspects of these volcanoes, including the rift at Gula, the large size and very long flows, differsignificantly from terrestrial volcanoes. c  2001 Academic Press Key Words: Venus; surface; volcanism. INTRODUCTION Large volcanoes with basal diameters greater than 100 km are a characteristic form of volcanism on Venus. Venusian large volcanoes tend to have heights of 2–3 km, and complex summit regions (Head et al. 1992; Crumpler et al. 1997). The volumes of large volcanoes at venusian hotspot rises are comparable to vol- canic volumes produced at terrestrial hotspot island chains (Sto- fan et al. 1995a). On Mars and Venus, the lack of plate tectonics results in the formation of large individual volcanoes, rather than chains of volcanoes (e.g., Carr 1984). However, whereas on Mars volcanoes may achieve great heights, on Venus the low heights of the volcanoes may be the result of a relatively thin venusian lithosphere that progressively sags under the volcano’s weight as it grows (McGovern and Solomon 1997). Recent studies of large volcanoes on Venus have suggested that: (1) all large volcanoes may be relatively young (e.g., Price et al. 1996, Basilevsky et al. 1997); (2) that neutral buoyancy governs magma chamber loca- tion (Head and Wilson 1992); and (3) that most volcanoes exhibit a progression from sheet-like flows to digitate flows (Keddie and Head 1994, 1995). To understand further the implications of large volcanoes for the overall geologic history of Venus, there is a need to investi- gate further the volcanic processes that formed, by Earth stan- dards, such gigantic volcanoes, including: (1) what magma stor- age system has each volcano had?; (2) does the internal plumbing of the volcano change with time?; and, (3) do the volcanoes have a specific sequence of evolution? In order to address these ques- tions, we focus on Sif, Gula, and Kunapipi Mons (Fig. 1), three volcanoes studied as part of the Venus Geologic Mapping Pro- gram (Copp and Guest 2000, Stofan and Guest 2000). Sif Mons 75 0019-1035/01 $35.00 Copyright c  2001 by Academic Press All rights of reproduction in any form reserved.