The Katla volcano S-Iceland: Holocene tephra stratigraphy and eruption frequency

Abstract

— The Katla volcano of the Eastern Volcanic Zone (EVZ) is characterised by subglacial explosive eruptions of Fe-Ti basalts. The Holocene eruption history and magma evolution at Katla volcano is assessed based on the stratigraphy and major-element composition of tephra layers in a composite soil section east of the volcano. A total of 208 tephra layers were found, 18 layers are of historical age and 190 are prehistoric, deposited during the last ∼8400 years. Of the 208 layers, 126 were analyzed for major-element composition. Of those 126 tephra layers, 102 are typical Katla Fe-Ti basalts, another 7 are Fe-Ti basalt layers containing rare grains of silicic composition, and 2 layers are basaltic icelandites with high Fe-Ti content. Ten silicic Katla (SILK) tephra layers were also identified in the composite section. An age model for the section was constructed using soil accumulation rate (SAR) between tephra layers previously dated by the 14 C method. According to the tephrochronology on the east side of the volcano, the prehistoric eruption frequency is higher than that of historical time by a factor of 2. In view of the knowledge that prehistoric Katla tephra layers were also dispersed to the south, west and north of the volcano, the true prehistoric eruption frequency is likely to be as high as 4 events/100 years. Moreover, if prevailing wind directions have not changed much during the Holocene, over 300 explosive basalt eruptions are likely to have occurred at the Katla volcano in the last 8400 years. The highest eruption frequency of basalt is observed between 2.5 to 4.5 ka and between 7.0 to 8.5 ka. Both these periods correspond to main peaks in eruption frequency of the silicic SILK layers. Finally, abrupt shifts in magma compositions divide the ∼8400 years record into eight intervals of 500-1700 year duration, designated with either a constant composition, irregular or systematic variations of major-element composition with time.