UNDERSTANDING VOLCANIC FEATURES AT THE HIRISE SCALE

The ~30-cm ground sampling scale of the High Resolution Imaging Science Experiment (HiRISE) images has, for the first time, resolved many of the volcanic features that terrestrial volcanologists rely on for their interpretations of eruption and emplacement conditions. This makes the use of terrestrial analogs more useful as more volcanic processes can be discerned. The “basaltic ring structures” (BRSs) of the Channeled Scablands provide an illuminating example of using terrestrial field observations to understand features seen in HiRISE images. The BRSs are circular features unique to the portions of the Miocene Columbia River Basalt Group that were eroded by the catastrophic Pleistocene Missoula Floods. Features that looked similar to BRSs were seen in Mars Orbital Camera (MOC) images of Athabasca Valles. Since this part of Mars was thought to be composed of lava flows eroded by a catastrophic flood, the BRSs were seen as a likely terrestrial analog. Previous work on the BRSs in the 1970s suggested that they formed as the upper crust of the lava flow sagged underneath the weight of a rootless cone. The features were internal to the lava flow but were exposed by the flood erosion that selectively removed lava, based on the orientation and style of jointing. However, HiRISE images and stereo-derived topography revealed that the floor of Athabasca Valles is covered by an uneroded lava flow. If there was a catastrophic flood of water, it took place before the emplacement of the lava. Thus the BRSs cannot be a good analog for the features on Mars. However, the processes proposed for the formation of the BRSs provided a viable explanation for how the features on Mars formed. At the same time, new field studies revealed that the BRSs were not formed by these processes. Instead, they are the product of lava flow inflation around phreatovolcanic constructs, such as rootless cones. This example highlights the level of in-depth understanding of volcanic processes that HiRISE not just enables - but requires - for proper interpretations.