Introduction

The structure and morphology of Martian calderas have been well studied through analysis of the Viking Orbiter images (e.g., Mouginis-Mark, 1981; Wood, 1984; Mouginis-Mark and Robinson, 1992; Crumpler et al., 1996), and provide important information on the evolution and eruptive styles of the parent volcanoes. Using Viking data it has been possible, for numerous calderas, to define the sequence of collapse events, identify locations of intra-caldera activity, and recognize post-eruption deformation for several calderas. Inferences about the geometry and depth of the magma chamber and intrusions beneath the summit of the volcano can also be made from image data (Zuber and Mouginis-Mark, 1992; Scott and Wilson, 1999). In at least one case, Olympus Mons, analysis of compressional and extensional features indicates that, when active, the magma chamber was located within the edifice (i.e., at an elevation above the surrounding terrain). The summit areas of Olympus and Ascraeus Montes provide evidence of a dynamic history, with deep calderas showing signs of having been full at one time to the point that lava flows spilled over the caldera rim (Mouginis-Mark, 1981). Similarly, shallow calderas contain evidence that they were once deeper (e.g., the western caldera of Alba Patera; Crumpler et al., 1996). Some of the best evidence for circumferential vents on Mars can be found on Pavonis Mons, where several sinuous rilles can be identified that must have originated from vents close to the rim (Zimbelman and Edgett, 1992).