A unique volcanic field in Tharsis, Mars: Pyroclastic cones as evidence for explosive eruptions

Abstract

Based on theoretical grounds, explosive basaltic volcanism should be common on Mars, yet the available morphological evidence is sparse. We test this hypothesis by investigating a unique unnamed volcanic field north of the shield volcanoes Biblis Patera and Ulysses Patera on Mars, where we observe several small conical edifices and associated lava flows. Twenty-nine volcanic cones are identified and the morphometry of many of these edifices is determined using established morphometric parameters such as basal width, crater width, height, slope, and their respective ratios. Their morphology, morphometry, and a comparison to terrestrial analogues suggest that they are martian equivalents of terrestrial pyroclastic cones, the most common volcanoes on Earth. The cones are tentatively interpreted as monogenetic volcanoes. According to absolute model age determinations, they formed in the Amazonian period. Our results indicate that these pyroclastic cones were formed by explosive activity. The cone field is superposed on an old, elevated window of fractured crust which survived flooding by younger lava flows. It seems possible that a more explosive eruption style was common in the past, and that wide-spread effusive plain-style volcanism in the Late Amazonian has buried much of its morphological evidence in Tharsis.