Eyjafjallajökull lies just west of another subglacial volcano, Katla, which is much more active and known for its powerful subglacial eruptions and its large magma chamber.
Each of the eruptions of Eyjafjallajökull in 920, 1612, and 1821-1823 has preceded an eruption of Katla.
Katla has not displayed any unusual activity (such as expansion of the crust or seismic activity) during the 2010 eruptions of Eyjafjallajökull, though geologists have been concerned about the general instability of the larger volcano since 1999.
Some geophysicists in Iceland believe that the Eyjafjallajökull eruption may trigger an eruption of Katla, which would cause major flooding due to melting of glacial ice and send up massive plumes of ash.
On 20 April 2010, Icelandic President Olafur Grimsson called on European and international experts to draw up plans to deal with a possible eruption of Katla.