Mars Polar Science 2000 4062.pdf

CHARACTERISTICS OF HYALOCLASTITE RIDGES UNDER WESTERN VATNAJÖKULL. K.A.Langley1, M.T. Gudmundsson1 and H.Björnsson1, 1 Science Institute, University of Iceland, Hofsvallagata 53, 107 Iceland.

The interaction between volcanoes and ice can result in morphology and known petrology. It was determined the creation of very distinctive landforms, including that the simple ridges do indeed correspond to individ- tuyas, hyaloclastite ridges and extensive hyaloclastite ual eruption units although, it is unlikely that all the beds and columnar jointed flows. Such features have units can be identified from the radio echo data alone. been observed in many places around the world al- though most abundantly in Iceland, the Canadian Cor- Of special interest were the width length ratios of the dillera and Antarctica. ridges. A particularly useful comparison was that of the width length ratios of other hyaloclastite ridges in Iceland affords one the most accessible and perhaps the now ice-free section of the Eastern Volcanic Zone one of the most interesting locations to study these (Jakobsson, pers. com.). This indicated that the sub- phenomena. Not only does it proffer numerous exam- glacial composite ridges are on average approximately ples of each, but also the occurrence of present day the same lengths as the ice-free ridges but in general subglacial and submarine volcanism offers the oppor- much wider. This difference could be due to the dif- tunity to witness and study the processes associated ferent erosional and depositional regimes one might with the formation of these products in situ. expect to find in the two environments. For example, it is evident that deposition of glaciofluvial sediments Vatnajökull, Europe’s largest ice cap, straddles an ac- and subaerial lavas in the ice-free areas have filled in tive section of the neovolcanic zone; the expression of valleys and covered the bases of many ridges. The the current plate boundary as it passes through the is- width of these ridges will be underestimated. Con- land. An extensive radio echo sounding survey, con- versely, in a subglacial environment such as that under ducted by the Science Institute of the University of Vatnajökull, glacial and fluvial erosion are more likely Iceland, has been carried out across the ice cap. The to dominate, especially with regards to the jökulhlaups data has been used to construct ice surface and bedrock and the surging behaviour of many of the outlets. topography maps as well as to describe the subglacial Where this erosion has caused the overdeepening of geology and define the ice and water drainage basins valleys, the width of the ridges will be overestimated. of Vatnajökull [1], [2], [3], [4], The length of the ridges correspond to the length of the fissures along which they were erupted and thus reveal The aim of the present work was to study the charac- information on the tectonics involved. teristics of the hyaloclastite ridges beneath the glacier using the radio echo data. Two types of ridge have References: been determined. The smaller shorter units termed [1] Björnsson, H. 1982. Nordic Hydrology 213-232, simple ridges are thought to correspond to single erup- [2] Björnsson, H. 1986. Annals of Glaciology. 8, 11- tion units. The larger scale morphological units, re- 18. [3] Björnsson, H. 1988. Hydrology of Ice Caps in ferred to here as composite ridges, may include several Volcanic Regions. Soc. Sci. Isl. Rit 45, Reykjavik, 139 of the simple ridges and are thought to have been pp., [4] Björnsson, H and P. Einarsson 1990. Jökull formed during a number of eruptions along single or 40, 147-168 parallel fissures. In attempt to validate this, a pseudo survey was conducted over an ice-free area of similar