Askja represents a volcanic system with nested calderas N of Vatnajökull. The largest of those calderas, containing all the others, dates back to prehistoric times whereas the eruption of 1875 did not only lead to the formation of the lake Öskjuvatn, but also had widespread consequences due to ash fall, leading to a wave of emigration from Iceland. The bright rocks seen in this image represent pumice produced in the 1875 eruption. The hole in the foreground might be related to thermokarst.