An overview of explosive volcanism on Mars

Abstract

Decades of space exploration reveal that Mars has been reshaped by volcanism throughout its history. The range of observed volcanic landforms shows that effusive and explosive eruptions have occurred, albeit unevenly in time and space. Evidence for explosive volcanism—characterized as eruptions in which magma is disrupted by the expansion of dissolved gases or by an interaction of magma with external volatiles—is less common than evidence for effusive activity. Nonetheless, indications of explosive volcanism have been identified. Examples include, old, rimless depressions, termed paterae, often on the summits of broad topographic rises with very gentle flanks, are located mainly around the Hellas impact basin. Also, fields of kilometre-sized cones are interpreted as scoria cones, tuff rings and tuff cones, and extensive clusters of sub-kilometre pitted cones in the northern lowlands are proposed to be rootless cones, i.e. constructional features caused by accumulation of volcanic fragments. Finally, layered deposits widely spread in equatorial areas (e.g., the Medusae Fossae Formation), and layered stacks of ash and a putative volcanic bomb observed by rover, also point to a protracted history of explosive volcanism on Mars. Yet some of these interpretations remain a matter of scientific debate. The discovery of evidence for explosive volcanism on Mars triggered an interest in the theoretical aspects of such volcanism under gravitational and atmospheric conditions different from those on Earth. These studies indicate that explosive eruptions on Mars would behave differently from those on Earth. This is because a lower atmospheric pressure and gravity can affect all stages of the eruption including the ascent of magma, the process of degassing and magma fragmentation, the transport and deposition of the pyroclasts, and, in some cases, the formation of explosive volcanoes themselves. On Earth, explosive eruptions are responsible for the formation of most volcanoes on land, and so the relatively sparse occurrence of explosive volcanism on Mars is surprising, especially considering the martian environmental conditions as well as wide occurrence of external volatiles on Mars. This is because the lower atmospheric pressure than on Earth ought to favour magma fragmentation and hence the formation of pyroclasts and associated explosive volcanic edifices, even if lower volumes of dissolved gases were present in martian magma than is usual on Earth. The relative dearth of explosive activity on Mars therefore represents a gap in our understanding of martian volcanism, suggesting that there may be considerable compositional differences between Mars and Earth or that evidence of explosive volcanism on Mars manifests differently than on Earth. Understanding these differences is important, as explosive volcanism provides insight into the planet’s composition and plays a crucial role in the evolution of a planet’s atmosphere by the release of magmatic gases, which have the ability to affect geological and even biological processes operating on the surface of a planet. In this paper, we present an overview of explosive volcanism on Mars—from both observational and theoretical perspectives—and discuss the implications of explosive eruptions for the evolution of the Red Planet.