Iceland’s biggest volcano, Katla, is rumbling. During the last month there have been more than 500 tremours in and around the caldera. Reaching 1512 m a.s.l. the 10 km wide caldera poses a more severe threat than the recent eruptions of Grimsvötn (May 2011) and Eyjafjallajökull (March 2010), where especially the latter caused major disruptions in the air traffic over northern Europe.
The caldera is partly capped by a glacier. If Katla erupts melting glacier ice could cause flash mudflows. Volcanic ash from Katla also have the potential of reaching higher up in the atmosphere, casing devastation not only on Iceland.
In 1783 the Laki fissure eruptions, located within the same volcanic zone as Katla, erupted continuously for 8 months. A minor version of the floodbasalt volcanism typical of large igneous provinces, the Laki fissure released huge amounts of ash, hydroflouric acid and sulfuric acid (Stephenson et al., 2003), causing massive destruction on Iceland, where every fifth islander and half of the livestock were killed.
But the devastation reached farther than that. A sulfuric ash cloud from the eruption was distributed over the northern Hemisphere. Historical records from France and England report severe health issues for both people and animals, as well as misgrowth of crops and fruit. In the aftermath of the atmospheric pollution from the Laki eruptions, colder climate and increased mortality plagued Europe for a decade (Grattan, 2005).
Historical records show that Katla usually erupts every 40-80 years. The last major eruption was in 1918. Volcanologists are worried because they are not sure what to expect. Volcanoes are unpredictable and the impact of an eruption depends largely on its size, volume, ash composition and climatic conditions.