At Festarfjall, a section of a small hyaloclastite hill is exposed in the coastal cliffs. The base is composed of hyaloclastite breccia and tuff overlain by lava sheets. This unit passes undisturbed from the surroundings underneath Festarfjall. The hill itself is composed of different types of rather chaotic hyaloclastite rocks and a few lava sheets at the top with some scoria at the transition. A dyke named Festi (ladder) passes up through the basement and the Festarfjall sequence, branching towards the base of the lavas. The dyke is evidently the feeder for Festarfjall, of which one half may have been eroded away. The shape is that of a tuya, which erupted from a short fissure. Festarfjall overlies an older tuya with the crater emerging from underneath the Festarfjall hyaloclastite to the north. Possibly a thin wedge of this older unit is exposed above the basement lavas in the cliffs.
In 1943, a Czechoslovakian geologist offered Festarfjall as an evidence against a therory, which was popular at the time, that hyaloclastite mountains were formed by faulting. However, he adhered to an equally wrong explanation of them as being inselbergs, i.e., remnants left by erosion. He does not mention the dyke, which might have helped him towards a correct interpretation.
Kristján Sæmundsson, 2010