| Summary: | A group of several basaltic volcanic edifices in Vonarskarð at the flank of Bárðarbunga volcano in Central Iceland, were observed to have flat tops characteristic of tuyas identified in Iceland and elsewhere. However, these edifices lack subaerial lava flows on their summits, which are generally used to explain the flat tops of tuyas. Closer analysis reveals that the Vonarskarð edifices more closely resemble flat-topped seamounts on the Kolbeinsey Ridge and elsewhere. These edifices formed a similar shape without surface or water-ice interaction, implying a different formation mechanism. Fieldwork was undertaken on the Vonarskarð flat-topped edifices and samples of pillow lavas were taken. Edifices in Vonarskarð and on the Kolbeinsey Ridge were mapped and measured via DEM. Profiles were measured along four axes of each edifice, and their elevation, diameter, slope, volume, eccentricity, aspect ratio, and flatness were measured. The morphologies of the two groups were very similar. Vonarskarð's edifices average 1444 meters in diameter, compared to 1704 meters on Kolbeinsey Ridge. Average elevations over their surroundings were 147 and 129 meters respectively, and average volumes were 0.183 km^3 and 0.201 km^3 respectively. The other morphological parameters were also comparable. The same parameters measured on several well-known Icelandic tuyas showed their morphologies to be significantly different. An analogue experiment using the extrusion of granular material without cohesion gives a possible formation mechanism for the flat-topped edifices studied. Extrusion of sand (an analogue for fragmenting pillow lavas) initially forms a pillow-mound shaped pile, but as it expands the top becomes relatively flat, while the flanks oversteepen and collapse repeatedly, creating a transition from flat top to angled slopes. The analogue edifices in this experiment bear a striking resemblance to the edifices mapped in the field and we propose this as a possible formation mechanism. Í austurhluta Vonarskarðs, í undirhlíðum ...
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