Silicic Volcanism on Io? Evidence from Tohil Mons and Other Possible Volcanic Cones

INTRODUCTION Recent volcanic activity of Io generally does not produce conical mountains. Instead, their lava run out in thin, long flows. Volcanic centers are typically large, deep calderas cut into the interpatera plains. Bright lava flows on Io can mostly be associated with sulfur lavas, with no visible escarpments that would indicate higher viscosity material. From temperature measurements it is clear that silicate lavas are present on Io. However, they are ultramafic, low viscosity lavas [10, 11]. There are very few examples where the lava is thick enough to form positive relief features. Mountains of Io are of generally explained as tectonic features. One of the most geologically complex mountains is Tohil Mons [1]. Although its unique morphology suggests a volcanic origin, earlier interpretations explained its formation, similarly to other mountains, by tectonic and erosional (collapse) processes [12, 14]. We now see more evidence that its genesis, at least in part, is also due to viscous silicic volcanic activity. Moreover, from other smaller conical mountains, we postulate that silicic magmas are not unique on present Io – originating probably from other depths or magma chambers than those of the large calderas or sulfur flows.