The processes, products and paleoenvironments of large volume, polygenetic, ice-confined volcanoes are very poorly known. Dyngjufjoll ("Askja"), in Iceland, is one of the largest and best exposed, formerly ice-confined,basaltic volcanoes on Earth, but previous work has concentrated on its more recent Holocene (i.e. ice-free) evolution. This project will focus on its older Pleistocene history by targeting one large massif (Austurfjoll) that preserves several hundred meters of glaciovolcanic rocks. There has been considerable interest recently in the role of volcanogenic, subglacial ice melting and drainage as a lubricant for rapid ice sheet movement, and its potential for sheet destabilization. Also, source area processes and products of large-volume volcanogenic meltwater floods (i.e.jokulhlaups)are a significant hazard to infrastructure (roads, bridges) and the local economy in south Iceland. The results of this study will help close some gaps in knowledge regarding the geologic and environmental impacts of volcano-ice interaction processes. Volcanoes like Dyngjufjoll are also the most relevant terrestrial analogs to constraining models of the interaction of large long-lived basaltic shield volcanoes and ice on Mars. The study will be conducted as part of an international collaboration with scientists from the UK and Iceland.
This project will offer insights into two relevant aspects of the meltwater source region, namely temporal variations in the overlying ice thickness and the nature of water drainage and ponding, including possible jokulhlaup (i.e. release of large-volume volcanogenic meltwater) events. There are no studies of the record of jokulhlaups at a volcanic edifice source. Volatile analysis of pillow rind glass at Austurfjoll will help constrain estimates of paleo-ice thicknesses in central Iceland, and models that link deglaciation with increased rates of volcanism in this area. The age of the Dyngjufjoll glaciovolcanic massifs are currently unknown, but up to 16 terrestrial glacial events have been recognized in Iceland since 2.5Ma, so there is potential for these rocks to record evidence of pre-late glacial maximum ice thicknesses. It is proposed to apply an integrated approach to stratigraphy using detailed quantitative glass texture and component analysis, geochemical analysis, 40Ar-39Ar dating, detailed structural and facies mapping and logging. This project and approach to correlation is particularly relevant to interpreting the very many (>1000) fissure-erupted, glaciovolcanic, clastic-dominated sequences (i.e. 'tindars') in Iceland, which represent an enormous database on the glacial history of Iceland. The results of this project will help refine the criteria by which the presence of former ice in glaciovolcanic (particularly clastic) sequences is recognized, and hence contribute to the growing use of such rocks as indicators of past climate.
This project focused on understanding what happens when large, long-lived basaltic volcanoes interact with their surrounding ice sheets, in terms of the volcanic products, their environments of deposition, the controls on volcanic explosions generated when magma meets water and evidence for the presence and thickness of former ice. Askja volcano in central Iceland is one of the largest volcanoes in the world to have been mostly constructed by eruptions beneath and through ice, but this aspect of its origin has not been previously studied. We have a reasonably good understanding of what happens when small volume, short-lived basaltic volcanoes erupt beneath ice, but not volcanoes like Askja. We discovered that the bulk of one of the largest massifs, the 800m thick Austurfjoll at Askja was erupted beneath 600-900m of ice sometime between 71 and 29 thousand years ago, from a series of overlapping concentric fissures, some of which parallel one of the main caldera faults. The base of the massif rests on an earlier glacially-eroded volcano that was emplaced in a dry (subaerial) environment, that was presumably largely ice-free. Deposits from glaciers are rare within the Austurfjoll sequence but are preserved near its base. Almost all of the products of volcano-ice interaction at Austurfjoll were deposited in water, with only the uppermost few metres of deposits displaying any evidence of deposition in a dry environment. These include deposits that are similar to typical Strombolian eruptive products but also include lava flows emplaced in a dry (subaerial) environment. The bulk of the deposits were generated by explosive magma-water interaction. We have some evidence that the at least some of these explosive magma-water interactions were initially triggered by dry magma explosions, i.e. expansion of steam bubbles within the magma rather than expansion of external water. Such results have important implications for our understanding of how hazardous magma-water explosions happen. Some sequences at Askja appear to record an in-situ transition from non-explosive (effusive) subaqueously emplaced lava flows to overlying explosive products, and hence represent an invaluable record of the onset of explosive magma-water interaction. Recognizing volcanic products that have formed in a glacial environment is often difficult, but it is essential that they can be confidently identified if we are to understand how volcanoes grow through ice. In this project, we discovered a type of igneous intrusion, which we called "coherent margined volcaniclastic dikes" that we believe are generated when magma intrudes sediments that are cemented by ice. The recognition of such dikes is an important addition to the range of volcanic products that indicate the presence of former ice, both here on Earth and on Mars, where intrusions through ice-cemented sediment must have been a common process.
