Or should we ask: when will Katla erupt?All newspapers are stating the same sentence: ”Everytime Eyjafjallajokull has erupted, Katla has followed right after”. But how fast after? Weeks, months, years, decades? Futurity article Will volcano’s big sister erupt next says: ”Every time Eyjafjallajokull as erupted—as it has for the past week—Katla follows behind, always erupting within the same year, sometimes just a few months after its little sister.” Global volcanism program lists all the eruptions. Katla’s eruption in 1823 started on 26.6.1823 and ended 23.7.1823. Eyjafjallajokull’s eruption history says start date 19.12.1821 and end date 1.1.1823. Ok, Katla started half a year after Eyjafjallajokull had stopped and 30 months after the little one started. The other eruptions are not recorded with the same accuracy in terms of start and end dates. Conclusion: we don’t know when. While waiting, have a look at a very nice time lapse video of the Eyjafjallajökull by Jon Mundur.