Non-explosive lava–water interaction in Skaelingar, Iceland and the formation of subaerial lava pillars

Abstract

Hollow cylinders of basalt <2.5m tall and generally <1m in diameter were generated by non-explosive lava–water interactions during the emplacement of the Laki lava flow in Iceland during 1783–1784. We know of only one location within the Laki lava flow where these basalt formations occur: a valley called Skaelingar, located at ~64.0°N, 18.5°W, which contains a tributary stream to the Skafta River. Skaelingar was temporarily filled with Laki lava when the main body of the lava flow advancing down the Skafta River valley became blocked, forcing lava to flow upstream into tributary valleys along the north side of the river. After the blockage within the Skafta River valley was removed, the Laki lava mostly drained out of these tributary valleys. We refer to the remaining vertical hollow basalt pipes as lava pillars because they morphologically resemble subaerial lava trees and submarine lava pillars that have been observed at mid-ocean ridges. We propose that the subaerial pillars formed as an inflating lava flow advanced slowly over water-saturated ground, or perhaps into temporarily ponded water, causing heated columns of water to rise between adjacent advancing lava lobes. The subaerial pillars continued to grow in height and diameter as the lava flow inflated. When the lava drained back out of the valley, the lava pillars were left standing. Thus, the Icelandic subaerial pillars represent a non-explosive interaction between lava and water.