SURFACE MORPHOLOGY OF BASALT COLUMNS AT SVARTIFOSS, VATNAJÖKULSÞJOÐGARÐUR, SOUTHERN ICELAND

One of the best-known examples of columnar-jointed basalt in Europe occurs at Svartifoss (Black Falls) in the Vatnajökulsþjóðgarður (formerly Skaftafell National Park) of southern Iceland. The basalt columns are exposed in situ in the falls in a ~20 m exposure. Dislodged column fragments are mostly, but not entirely, hexagonal, and typically 0.6 m to 1.2 m in diameter. In addition to the regular vertical jointing, the columns are notable for a variety of surface features. The horizontal parting surfaces of the column fragments display a characteristic semicircular ring that inscribes most of the diameter of the interior of the column. The ring features alternately positive or negative relief against the perimeter of the column and most commonly exhibits a radiating fracture pattern originating at the center of the ring and extending to the margin. The jointed surfaces of the columns are horizontally corrugated at a spacing of 5 cm to 20 cm. The corrugations exhibit varying features including subhorizontal to evenly dipping laminations and crescentic fracture patterns, the orientation of which may vary between corrugations. Columns broken longitudinally display no apparent internal structure at outcrop scale. Similar surficial features are evident at other exposures of columnar basalt in Iceland, such as at Hrepphólar in the south (Mattsson et al., 2011: Nature Communications 2, 299, doi:10.1038/ncomms1298) and Dettifoss in the north.

Petrographic examination of thin sections cut in mutually perpendicular axes reveals that the tholeiitic basalt at Svartifoss contains a densely interlocking, two-dimensional network of plagioclase laths, similar to that described by Mattson et al. (2011), who proposed a late-stage melt-migration driven by a crystallization-induced pressure gradient to explain some of the macroscopic morphology of columnar-jointed basalts.