Facies analysis of proximal subglacial and proglacial volcaniclastic successions at the Eyjafjallajökull central volcano, southern Iceland
Published:January 01, 2002
S. C. Loughlin, 2002. "Facies analysis of proximal subglacial and proglacial volcaniclastic successions at the Eyjafjallajökull central volcano, southern Iceland", Volcano–Ice Interaction on Earth and Mars, J. L. Smellie, M. G. Chapman
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The long-lived (at least 0.78 Ma) Eyjafjallajökull volcano has been constructed during a period of dramatic climatic shifts, and has produced subaerial lavas and cinder cones, pillow lavas, hyaloclastite, monogenetic volcaniclastic sediments and polygenetic glacio-fluvial sediments. Subaqueous lithofacies are dominant and nine distinct cogenetic lithofacies associations are identified within the successions that are interpreted as the proximal deposits of subglacial eruptions. These associations are typically bound by unconformities and lie directly on thin diamictites or glaciated surfaces; epiclastic sedimentary rocks are absent. The lithofacies include subaqueous sheet lava flows, lobate flows, pillow lavas, breccias, hyaloclastite and hyalotuff generated by eruption of mainly basaltic lavas. Lavas associated with massive hyaloclastite breccias commonly lie on or intrude cogenetic redeposited hyalotuff, indicating rapid changes in style of activity from explosive to effusive. This suggests that the vent was initially subaqueous but became subaerial as meltwater drained away down-slope beneath temperate ice sheets. In other instances, probably under thicker ice, the eruptions were effusive throughout but lava was subjected to intense steam explosivity. There is abundant evidence that volcaniclastic material was transported downslope by mass flow, grain flow and traction currents (i.e. by running water). Lithofacies associations that ponded in ice-dammed water commonly comprise thick redeposited tephra overlain by a ‘passage zone’ of hyaloclastite and then subaerial lavas. These deposits are voluminous but conditions suitable for ponding of large volumes of water were not common in the evolution of this large volcano.
During the early stages of volcano growth, eruptive deposits were emplaced on the gentle slopes of the growing cone during both glacial and interglacial periods. Glacial erosion modified the growing edifice and, as a result, younger deposits (<600 ka) tend to be valley-confined. The unconformities produced during glacial advance represent significant time gaps within the succession. Observations support the hypothesis that there were higher volcanic production rates during periods of deglaciation, probably as a result of the rapid decrease in lithostatic pressure as ice melted.
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Volcano–Ice Interaction on Earth and Mars
This volume focuses on magmas and cryospheres on Earth and Mars and is the first publication of its kind to combine a thematic set of contributions addressing the diverse range of volcano-ice interactions known or thought to occur on both planets. Understanding those interactions is a comparatively young scientific endeavour, yet it is vitally important for a fuller comprehension of how planets work as integrated systems. It is also topical since future volcanic eruptions on Earth may contribute to melting ice sheets and thus to global sea level rise.
Papers included here are likely to influence the choice of sites for future Mars missions in exobiologically important areas. On Earth, snow and ice are widespread, not only in extensive icecaps but also as alpine glaciers at high elevations in tropical regions. By contrast, Mars today is an arid volcanic planet with only small polar ice-caps although an abundance of water is believed to be trapped in the cryolithosphere. It is also thought that the planet may have sustained extensive frozen oceans early in its history. The presence of a former hydrosphere, a cryosphere and coincident volcanism thus make Mars the likeliest prospect for the first discoveries of life away from Earth. Much research has assumed that terrestrial volcano-ice systems are plausible analogues for putative Martian examples, but until mankind finally sets foot on Mars, there is no simple test for that assumption.
Our hope is that the knowledge presented here will stimulate research among planetary geologists in this exciting, rapidly expanding field for many years to come.