Since Iceland was settled in the ninth century, Katla has erupted on average every 60 years, but has not had a significant eruption since 1918. Ominously, eruptions of Eyjafjallajökull in 1821-23 and 1612 were followed within months by eruptions of Katla. Judged by the historical calendar, an eruption is overdue.
Last July, a flood of water burst from beneath the ice cap on top of Katla, washing away a bridge. This indicates that an extra pulse of heat reached the base of the ice. Since then, there have been erratic movements of the surface of the volcano, measured by precise GPS instruments, and bursts of high earthquake activity beneath Katla’s caldera. These observations imply that magma has risen to shallower depths.