The starting phase of the Sandfell eruption was the pillow lava formation, hot magma streamed up to the crusts surface in to a deep melt water lake or a shallow one with a thick glacier on top.
Upphafsfasi í myndun Sandfells var bólstrabergsmyndunin, kvika streymdi upp í djúpt vatn eða í grunnt vatn undir þykkum jökli.