Katla is one of Iceland’s largest and most active volcanic systems, located in southern Iceland. Its notoriety stems from its unique location beneath the Mýrdalsjökull ice cap, Iceland’s fourth-largest glacier. This combination of fire and ice creates a dynamic geological entity, making it one of the most closely monitored volcanoes in the world. The ice cover hides the volcano’s summit, but Katla’s periodic eruptions have historically reshaped the surrounding landscape and extended the coastline.
The Geological Classification of Katla
Katla is classified as a Central Volcano, a complex system of interconnected magma chambers and fissures. A central volcano is defined by a deep-seated magma reservoir that feeds eruptions over a wide area. Katla’s system includes a fissure swarm stretching about 70 kilometers northeast of the main crater structure.
The volcano also features a large, elongated depression at its summit, identifying it as a caldera volcano. This caldera measures approximately 10 by 14 kilometers. It formed when the underlying magma chamber emptied during a past eruption, causing the ground above it to collapse. The caldera is currently filled with a layer of ice up to 700 meters thick.
Because the summit is permanently covered by the Mýrdalsjökull ice sheet, Katla is also classified as a subglacial volcano. This ice cover dictates the explosive nature of its eruptions and associated hazards. The interaction between molten rock and glacial ice transforms eruptions into highly explosive events.
Katla’s Eruptive History
Katla is a highly active volcano, with at least 21 major eruptions documented since the settlement of Iceland. Significant eruptions typically occur on average every 40 to 80 years. This regularity provides a baseline for monitoring, though the actual repose period can vary.
The last major confirmed eruption occurred in October 1918 and lasted for approximately 24 days. This event had an estimated Volcanic Explosivity Index (VEI) of 4 or 5, comparable to the 1980 eruption of Mount St. Helens. The resulting flood deposits from the 1918 eruption extended the southern coastline by about five kilometers.
The current period of inactivity, extending over a century, is among the longest in Katla’s recorded history. Minor subglacial events occurred in 1955, 1999, and 2011, but these did not break through the thick ice cover. This extended repose time means the volcano is considered past its typical eruption interval and is under constant surveillance.
The Unique Threat of Glacial Flooding
The primary hazard associated with a Katla eruption is the Jökulhlaup, an Icelandic term for a sudden glacial meltwater flood. This threat is unique to subglacial volcanoes. It arises when rising magma rapidly melts quantities of the overlying ice cap. The volume of water accumulates beneath the ice, creating pressure until it bursts out from under the glacier margin.
These floods are destructive, flowing with discharges comparable to the combined average of the world’s largest rivers, such as the Amazon and Mississippi. The floodwaters are a hyper-concentrated mixture of sediment, volcanic debris, and chunks of ice deposited across the outwash plains. During the 1918 event, the flood was estimated to reach a peak flow of 300,000 cubic meters per second.
The force of a Jökulhlaup can destroy bridges and roads, posing an immediate threat to infrastructure and nearby communities. This rapid, high-volume flooding distinguishes Katla’s hazard from the ash clouds and lava flows typically associated with non-glaciated volcanoes. The sudden release of this water is the most immediate danger, often preceding the full atmospheric impact of the ash plume.