Bull Volcanol (1992) 54:238-246 9 Springer-Verlag1992 The 1991 eruption of Hekla, Iceland Agust Gudmundsson 1, Niels Oskarsson 1, Karl Gronvold 1, Kristjan Saemundsson 2, Oddur Sigurdsson 2, Ragnar Stefansson 3, Sigurdur R Gislason4, Pall Einarsson 4, Bryndis Brandsdottir 4, Gudrun Larsen 5, Haukur Johannesson 5, and Thorvaldur Thordarson 6 1 Nordic VolcanologicalInstitute, Universityof Iceland Geoscience Building, 101 Reykjavik, Iceland 2 National Energy Authority,Grensasvegur9, Reykjavik,Iceland 3 Icelandic MeteorologicalOffice, Bustadavegur9, Reykjavik,Iceland a Science Institute, Universityof Iceland, Reykjavik,Iceland 5 Icelandic Museum of Natural History, Laugavegur 105, Reykjavik,Iceland 6 Department of Geology& Geophysics,Universityof Hawaii, Honolulu, USA Received May 21, 1991/Accepted September 25, 1991 Abstract. The eruption that started in the Hekla vol- cano in South Iceland on 17 January 1991, and came to an end on 11 March, produced mainly andesitic lava. This lava covers 23 km 2 and has an estimated volume of 0.15 km 3. This is the third eruption in only 20 years, whereas the average repose period since 1104 is 55 years. Earthquakes, as well as a strain pulse recorded by borehole strainmeters, occurred less than half an hour before the start of the eruption. The initial plinian phase was very short-lived, producing a total of only 0.02km 3 of tephra. The eruption cloud attained 11.5 km in height in only 10min, but it became de- tached from the volcano a few hours later. Several fis- sures were active during the first day of the eruption, including a part of the summit fissure. By the second day, however, the activity was already essentially lim- ited to that segment of the principal fissure where the main crater subsequently formed. The average effusion rate during the first two days of the eruption was about 800 m 3 s-l. After this peak, the effusion rate declined rapidly to 10-20 m 3 s -1, then more slowly to 1 m 3 s -1, and remained at 1-12 m 3 s-1 until the end of the erup- tion. Site observations near the main crater suggest that the intensity of the volcanic tremor varied directly with the force of the eruption. A notable rise in the fluorine concentration of riverwater in the vicinity of the erup- tive fissures occurred on the 5th day of the eruption, but it levelled off on the 6th day and then remained essentially constant. The volume and initial silica con- tent of the lava and tephra, the explosivity and effusion rate during the earliest stage of the eruption, as well as the magnitude attained by the associated earthquakes, support earlier suggestions that these parameters are positively related to the length of the preceeding repose period. The chemical difference between the eruptive material of Hekla itself and the lavas erupted in its vi- cinity can be explained in terms of a density-stratified magma reservoir located at the bottom of the crust. We propose that the shape of this reservoir, its location at the west margin of a propagating rift, and its associa- tion with a crustal weakness, all contribute to the high eruption frequency of Hekla. Introduction The Hekla volcano is the production centre of a 40-km- long and 7-km-wide volcanic system (Jakobsson 1979) of the same name (Fig. 1). Hekla is a volcanic ridge with a conspicuous 5.5-km-long summit fissure, Hek- lugja, striking N 65 ~ E. By contrast, the nearby tectonic fractures and volcanic fissures strike mostly N40- 45 ~ E. The volcano is located where the eastern vol- canic zone, a propagating rift, meets the South Iceland seismic zone, a transform zone where destructive earth- quakes, reaching magnitude 7, have occurred at inter- vals of 45-112 years (Einarsson et al. 1981). Both in terms of eruption frequency and production of lava and tephra, the Hekla system is one of the most active volcanic systems in Iceland. Since the settlement of Iceland, about 1100 years ago, there have been at least 17 eruptions in Hekla itself (including the 1991 eruption), and 5 in its immediate vicinity (Thorarinsson 1967; Thorarinsson and Sigvaldason 1972; Gronvold et al. 1983). During this time the Hekla volcano has pro- duced at least 7 km 3 of eruptive material calculated as dense lava (Jakobsson 1979). The principal aim of this paper is to give a descrip- tion of the 1991 eruption that focuses on the changes in the effusion rate during the course of the eruption, the volume and composition of the lava and tephra erupted, and the earthquakes, strain changes and some secondary effects associated with the eruption. We out- line qualitative explanations for the eruption frequency of Hekla and the chemical difference between the mag- mas of Hekla proper and those erupted in its vicinity. Offprint requests to: A Gudmundsson