Surface Morphology of Basalt Columns at Svartifoss, Vatnaj？kulsTjóegareur, Southern Iceland

A spectacular example of columnar-jointed basalt occurs at Svartifoss in the Vatnaj？kull National Park of southern Iceland. The columns are notable for a variety of features on the vertical joint surfaces and the horizontal parting surfaces. The jointed surfaces of the columns display horizontal striations at a spacing of centimeters to decimeters. The individual striations exhibit crescentic hackles with a plumose pattern, the orientation of which varies between adjacent striations. Also present are gently dipping, millimeter-scale laminations not previously described. Horizontal parting surfaces of the columns display a circular ring that inscribes most of the diameter column. The ring features alternately positive or negative relief against the perimeter of the column and exhibits a radiating pattern of hackles originating at the center of the ring. Petrographic examination reveals that the basalt contains an interlocking network of plagioclase laths preferentially aligned perpendicular to the column axes. The circular features have been described previously and attributed to late-stage melt migration driven by a load-induced pressure gradient. The striations were formed from stepwise, downward propagation of the polygonal fracture system, and the plumose structures were formed from tensile stresses during fracture propagation. The small-scale laminations may result from preferential grain alignment of plagioclase laths. 1. Introduction Columnar-jointed basalts, which have been found on all continents, are among the most widely recognizable features of basalt volcanism. Indeed, often they are one of the first igneous features to be correctly identified by students in introductory geology classes. The textbooks used in such classes ascribe the formation of columnar jointing to the volume change (contraction) of flows or shallow intrusions as they cool from the top down (e.g., see [1]). These simplistic explanations are more or less correct at a very basic level, but they do nothing to explain why cooling and the consequent volume changes cause the formation of a regular fracture pattern. In great part, this lack of specificity is derived from the lack of widespread agreement among igneous petrologists themselves on a precise mechanism of formation since their interpretation by Mallet [2]. Subsequent contributions include those of James [3], Tomkeieff [4], Spry [5], Jaeger [6], Peck and Minakami [7], Reiter et al. [8], and Aydin and DeGraff [9]. Goehring et al. [10] used corn-starch analog experiments to derive a scaling law that applies to contraction