A THREE DIMENSIONAL EXPLORATION OF SUBGLACIAL PILLOW ERUPTIONS ON THE SVEIFLUHALS RIDGE IN SOUTHWEST ICELAND

The Reykjanes Peninsula in southwest Iceland contains several NE-SW trending ridges interpreted as having formed during glaciovolcanic eruptions. Located at the northern end of the Sveifluhals ridge, Undirhlíðar is a quarry that exposes a three dimensional view of the internal structure of an early Weischelian subglacial pillow ridge. Pillow basalts are the primary products of subaqueous eruptions in a variety of environments, including effusive glaciovolcanic eruptions (Ollier, 1969; Schminke, 2004), and hyaloclastite commonly forms by spallation of glassy pillow rims (Bourgeois et al., 1998). The exposure in Undirhlíðar allows us to examine intrusions and periods of rest between eruptive events, which are important to understanding the spatial distribution of lava and chemical variations within these subglacial eruptive units.

In July 2010, we mapped and sampled the NW-SE trending walls of Undirhlíðar, which provide a natural cross-section of a subglacial pillow ridge. The walls expose an area ~30 m high by ~120 m wide. The walls are dominated by an interbedded pillow-hyaloclastite unit that consists of pillow basalts ranging from 0.5 m to 3 m in diameter. There are slight variations in the amount and appearance of hyaloclastite surrounding the pillows, but nothing significant enough to separate the unit into smaller eruptive events. The primary pillow unit is separated from a lower pillow unit by a stratified layer of hyaloclastite, consistent with formation during a pause between eruption cycles. Both pillow units are indistinguishable in mineralogy, containing plagioclase crystals up to 5 mm long.

Two dikes, ~2.5 m and ~0.25 m wide, cut the upper and lower pillows on the northeastern part of the wall. Both dikes contain plagioclase and olivine crystals up to 1 cm in size. A third dike, ~3.5 m wide, occurs at the most southeastern part of the wall and clearly feeds an overlying pillow lava flow. The third dike does not contain olivine like the other two dikes, and the center of the third dike has an unusual glassy, pillow-like appearance.

Overall, mapping the NW-SE trending walls of Undirhlíðar documents two eruptive events followed by at least two intrusive events. Further thin section and geochemical analyses will allow us to place additional time and spatial constraints on the construction of subglacial pillow ridges.