A little background on the volcano. Askja is a very complex volcano made up of three calderas. The volcano has mostly erupted basaltic material over its recent history, but it has also had a rhyolitic eruption over 10,000 years ago. Now, usually at a basaltic volcano, the calderas are formed by passive sinking of the land surface, much like we see in Hawai’i. However, at Askja, it appears that the calderas are formed more violently due to explosive eruptions out of the ring fractures bounding the calderas. The youngest caldera formed only 137 years ago (in 1875) and the ~4.5 km diameter feature is home to two crater lakes, Öskjuvatn and Víti. The former is the larger lake, over 200 meters deep, while the latter is a very small, warm crater lake (marked in the photo above near the word “ash?”).
The most recent activity at Askja was in 1961 that produced lava flows near Öskjuvatn – a pattern of eruption that was seen in numerous times since the VEI 5 eruption in 1875. That caldera-forming eruption in 1875 was large enough that ash and tephra fell as far away as Norway and Sweden. Much like the 2010 Eyjafjallajökull eruption, the explosivity of the eruption was likely aided by the meltwater that is readily available at Askja. If you go back to the rhyolite eruption in ~8910 B.C., that ash from that caldera-forming event is found over much of Europe.