Holocene eruptions within the Katla volcanic system, south Iceland: Characteristics and environmental impact

Abstract

Holocene volcanism within the Katla volcanic system is characterized by: 1) explosive (hydromagmatic) basaltic eruptions along volcanic fissures within the Mýrdalsjökull caldera; 2) explosive silicic eruptions from vents associated with the caldera and 3) predominantly effusive basaltic eruptions involving both the central volcano and the fissure swarm. Typical Katla eruptions are accompanied by basaltic tephra fall, lightning and glacial floods (jökulhlaups) of meltwater, ice and volcanic debris. Twenty eruptions have occurred in the last 11 centuries. The volume of airborne tephra varies by three orders of magnitude, with an estimated volume of 1.5 km³ of freshly fallen tephra from the largest historical Katla eruption. The length of documented eruptions varies from 2 weeks to over 5 months. The average repose period since 1500 AD is 47 years with maximum deviations of 33 and 34 years. All Katla eruptions during the last 400 years have begun in the springfall season. At least 12 silicic Katla eruptions are known from the period ca. 1700 BP and 6600 BP. The silicic magma was most likely erupted by hydromagmatic explosive eruptions. The tephra dispersal axes indicate vent locations within the caldera or along the caldera rim. The volume of airborne silicic tephra varies by orders of magnitude, the largest and most widespread is tephra layer UN with uncompacted tephra volume of 0.3 km³. Intervals between the silicic eruptions have varied from ca. 100 to ca. 1000 14C yrs. Two major “fires” and 5–10 relatively minor, partly effusive eruptions have occurred during the Holocene. The 10th century Eldgjá and the 6800 BP Hólmsá fires are the largest known Holocene eruptions within the Katla system. A ≥75 km long, discontinuous and partly subglacial eruptive fissure was active during the Eldgjá eruption. The opening phase on most fissure segments was explosive, followed by an effusive phase on the subaerial segments. The eruption produced a voluminous basaltic tephra layer with a minute silicic component, two major lava fields and possibly a hyaloclastic flow deposit. Large jökulhlaups accompanied the eruption. The combined volume of erupted material may exceed 19 km³ DRE. Eruptions on the Katla system have caused extensive environmental changes during the past 1100 years. The Eldgjá fires radically changed the landscape, hydrology and utilization potential of large areas in South Iceland. Since then, jökulhlaups accompanying eruptions within the caldera have escaped eastwards, raising the Mýrdalssandur plain and extending its coastline southward.