Reviewed research article The 1783–1785 A.D. Laki-Grímsvötn eruptions II: Appraisal based on contemporary accounts Thorvaldur Thordarson1,2, Guðrún Larsen2, Sigurður Steinþórsson2 and Stephen Self 3 1) Department of Geology and Geophysics, University of Hawaii at Manoa, Honolulu Hawaii 2) Science Institute, University of Iceland, Reykjavík, Iceland; [email protected], [email protected] 3) Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, UK Abstract – The 1783–1784 Laki eruption along with intermittent explosive eruptions at the subglacial Gríms- vötn central volcano, represents a two-year-long volcano-tectonic episode within the Grímsvötn volcanic sys- tem, from June 1783 to May 1785. The Laki eruption is the second largest basaltic flood lava eruption in historical times (after the 934–940 A.D. Eldgjá event) and its consequences were disastrous for Iceland. De- tailed eyewitness accounts of the Laki-Grímsvötn eruptions exist and are here analysed in order to reconstruct the sequence of events as accurately as possible. This information is catalogued and critical eruption phenom- ena, such as the timing and nature of seismicity, explosive activity and mechanics of lava flow emplacement is evaluated. The results show that the 1783–1785 activity on the Grímsvötn volcanic system included at least 14 eruption episodes. Ten of these episodes were associated with the eight-month-long Laki eruption and linked to rifting and gradual lengthening of the erupting fissure. Each eruption episode began with an earthquake swarm, leading into a vigorous explosive eruption on a new fissure segment that was followed by sudden increases in outflow of lava. Concurrent explosive eruptions were reported at the Grímsvötn volcano during four of these episodes. The remaining four eruption episodes were confined to the Grímsvötn volcano, which remained active until May 1785. Evaluation of descriptions of flow phenomena in conjunction with new field observations of flow structures indicates that endogenous growth (i.e., insulated transport and inflation) was the characteristic mode of flow emplacement. INTRODUCTION and the atmospheric and environmental effects caused by the emissions from Laki were felt over large por- The 1783–1784 A.D. Laki eruption, which took place tions of the Northern Hemisphere (Traumüller, 1885; on a 27 km long volcanic fissure in the Síða high- Thoroddsen, 1914; Thorarinsson, 1979; Sigurdsson, lands of south Iceland (Figure 1), is the second largest 1982; Steinthorsson, 1992; Wood, 1992; Thordarson basaltic flood lava eruption (after the 934–940 Eld- et al., 1993; Fiacco et al., 1994; Thordarson, 1995; gjá event) in historic time (Thordarson and Self, 1993; Grattan and Brayshay, 1995; Grattan, 1998; Thordar- Thordarson et al., 2001). It represents the main vol- son and Self, 2001; 2003). canic event of a two-year-long (May 1783-May 1785) volcano-tectonic episode within the Grímsvötn vol- The events of the Laki-Grímsvötn eruptions are canic system, which featured intense earthquake ac- documented in many contemporary Icelandic chron- tivity and eruptions at the Grímsvötn central volcano icles (e.g. Einarsson et al., 1984; Thordarson, 1990, in addition to those at the Laki fissures. The conse- 1991). This information has been used by volcanolo- quences of this activity were disastrous for Iceland gists to infer the course of eruptive events as well as JÖKULL No. 53, 2003 11.