Subglacial and Subaqueous Monogenetic Volcanoes, Thermal and Morphological Contrasts

Tuyas, tindars (hyaloclastite ridges), sheets and various types of smaller mounds are the main types of monogenetic volcanic edifices formed in eruptions within glaciers. Similar landforms can be created in subaqueous to emergent eruptions. Differences in confinement by ice on the one hand and water on the other may provide a diagnostic tool to differentiate between the two environments. In marine or lake environments a stable water level can be assumed in most cases. Within glaciers the level of englacial lakes may wary considerably, depending on drainage mechanisms and possible shifts during eruption between subglacial and supraglacial drainage. A combined study of (1) the morphology of predominantly basaltic tuyas in Iceland, (2) thermal constraints on ice melting and size of depression in glacier surface around a growing volcano, (3) ice flow patterns of Pleistocene ice sheets as recorded by striations, and (4) ice flow modeling, provides insight into some of the characteristics unique to subglacially formed volcanoes. In the modeling it is assumed that the ice sheet in which the tuyas formed was warm-based, although it may well have been polythermal. The results show that the tuyas tend to be elongated in the direction of ice flow at the time of eruption. A progression towards lowering of the passage zone away from vent area in the down-flow direction is also indicated. These patterns are also predicted by the ice flow modeling, since inflow of ice on the up-slope side is much greater than from the sides or from the down-slope side, effectively preventing significant extension of a growing lava delta in the up-flow direction of the glacier. Another aspect of the large scale edifice morphology that may be a useful diagnostic is the level of erosion. Islands formed in monogenetic eruptions in the ocean or large lakes are eroded by wave action. Surtsey off the south coast of Iceland has a lava cap but has lost more than half of its area since formation in 1963-67. Several examples exist of islands that have briefly appeared in eruptions and then been quickly removed by the ocean. In many cases erosion is orders of magnitude slower in glaciers. This especially holds for subglacial volcanic edifices formed in the accumulation areas of ice sheets and ice caps, where ice flow is slow.