SUB-ICE VOLCANISM: HYDRAULICS, EDIFICE STRUCTURES AND IMPLICATIONS FOR MARS EXAMPLES

Eruptions beneath glaciers on Earth have unique characteristics that enable the resulting deposits and landforms to be used to document the presence, distribution and properties of glaciers over geological time. Many glacier characteristics are uniquely preserved in subglacial volcanic sequences, and the sequences are important repositories of palaeoenvironmental information. On Mars, the presence of polar ice caps and formerly more extensive glaciers makes it permissive that sub-ice volcanism has also played an important part in its history. Moreover, the characteristics of any subglacially erupted volcanoes on Mars can be used to deduce information on past environments otherwise currently unobtainable. Terrestrial examples of sub-ice eruptions are relatively well understood, with thermodynamic and hydraulic models already published. Comparable models do not yet exist for Mars. Moreover, without access to the kind of detailed lithofacies analysis available for terrestrial examples, the internal architecture of putative Mars edifices remains almost completely unknown. Planetary variables, such as low Mars gravity and atmospheric pressures, significantly modulate subaerial eruption dynamics but are less important in subglacial eruptions. Edifice morphology is particularly strongly affected by the simple presence of surface ice, its thermal regime and rheology, which determine the hydraulics associated with the eruptive system. Hydraulics, in turn, determines the sequence and types of lithofacies formed and their architecture. There are significantly different implications for edifices formed in association with both temperate and polar ice that can be used to predict and assess putative examples on Mars. For example, under polar ice conditions, 1) lava-fed deltas are likely to be quite short; 2) subaqueous tuff cones (tindars) will be taller, with essentially horizontal beds of fine grained tephra and few sequence discordances compared with their submarine or temperate ice equivalents; and 3) tuyas can exceed 1000 m in thickness (and may be diagnostic of eruptions in association with polar ice).