The crater of the volcano has a diameter of 10 km (6 mi) and the volcano normally erupts every 40–80 years. The last major eruption occurred in 1918, although there may have been a small eruption that did not break the ice cover in 1955. Since 930, 16 eruptions have been documented. The Eldgjá canyon is part of the same volcanic system. It is thought that Katla is the source of the Vedde Ash (more than 6 to 7 cubic kilometers (1.4 to 1.7 cu mi) of tephra dated to 10,600 years BP) found at a number of sites including Norway, Scotland and North Atlantic cores.
Before the Hringvegur (Iceland's Ring Road) was constructed, people feared traversing the plains in front of the volcano because of the frequent glacier outburst floods and the deep river crossings. Especially dangerous was the glacier outburst that followed the eruption of 1918. Katla has been showing signs of unrest since 1999 and geologists have concerns that it might erupt in the near future. Particularly, monitoring has been intensified following the March 2010 eruptions of a smaller neighbouring volcano beneath the Eyjafjallajökull glacier. The eruption of this nearby long-dormant volcano in March and April 2010 prompted fears among some geophysicists that it might trigger an eruption at the larger and more dangerous Katla. In the past 1,000 years, all three known eruptions of Eyjafjallajökull have triggered subsequent Katla eruptions.
At the peak of the 1755 eruption the flood discharge has been estimated at 200,000–400,000 m³/s (7.1-14.1 million cu ft/sec); for comparison, the combined average discharge of the Amazon, Mississippi, Nile, and Yangtze rivers is about 266,000 m³/s (9.4 million cu ft/sec).
- Glaciers of Iceland
- Glacial lake outburst flood
- Iceland plume
- Iceland hotspot
- List of volcanoes in Iceland
- Volcanism in Iceland