The 1961 eruption in Askja, Iceland: morphology, geochemistry, and planetary analogs

I examine the 1961 Askja, Iceland basaltic eruption emplaced from October 26 – December 17. A’a lava was emplaced during the first half of the eruption from Oct. 26 – Nov. 27, the second half emplaced volumetrically minor pahoehoe from Nov. 27 – Nov. 28. Basalt lavas are rich in iron and magnesium and have relatively low silica, making them good analogs to Mars basalts. They typically exhibit two different surface textures called pahoehoe and a’a, which reflect the lava viscosity. Pahoehoe lava is the more fluid end member, while a’a is thicker. The chemical composition of the 1961 lava flow is largely constant, so to understand the transition from thick a’a to fluid pahoehoe, we explore other variables. We observe that the crystal abundance and vesicle abundance are not different between a’a and pahoehoe flows nor do they show significant trends with distance. Lava temperature calculations show that pahoehoe lavas and vent samples are hotter than a’a lavas by 5°C. We interpret that this temperature increase, coupled with the transportation mechanism, promoted the emplacement of pahoehoe. We also observe large convex ridges (5 m high and 10 m long) called festoons that have previously been documented in lavas with a significantly higher silica content. Similar features are found on Venus and Mars but are hypothesized to be more silicic based on Earth analogs. However, the presence of festoons on the basaltic 1961 flow surface indicate that these interpretations should be re-evaluated.