The Hekla Eruption of 1991

The 1991 eruption began on January 17 and ended on March 11. This photo shows the glow of the eruption on low clouds. All photos by and courtesy of Thor Thordarson.

The onset of activity was marked by an increase in the number of earthquakes and a strain pulse detected by borehole strainmeters. A Plinian phase marked the onset of the eruption. The ash cloud reached a height of 11.5 km in only 10 minutes. This phase was short lived and only 0.02 cubic km of tephra was produced.

During most of the eruption andesitic lava was produced. The flows covered an area of 23 square km and had an estimated volume of 0.15 cubic km.

View of fissure eruptions on Hekla, January 17, 1991.

On the first day of the eruption several fissures were active, including part of Heklugja, the main summit fissure. Lava erupted in January to March, 1991, shown in black. Map of features associated with the 1991 eruption simplified from Gudmundsson and others (1992).

View of the southeast flank of Hekla on January 19, 1991. Note that activity is localized to a single vent.

On the second day of the eruption activity became limited to a single fissure where the main crater began to form. The effusion rate for the first two days was 800 cubic m/s. For most of the remainder of the eruption the effusion rate was about 10 cubic m/s. This photo shows the main vent on January 28, 1991.

View, from the east, across the main vent.

Looking down on one of the main lava rivers, February 16, 1991. The width of the channel is 4 m at the mouth. The effusion rate is about 3 cubic m/s.

View down the middle of three channelized lava flows active on February 16, 1991.

The 1991 eruption fit a pattern defined by Hekla eruptions. The length of the preceding repose period is positively related to the volume and initial silica concentration of the lava and tephra and the explosivity and effusion rate of early phase of the eruption. The previous eruption was in 1981. The 1991 eruption had about 54% SiO2. Graph from Thorarinsson and Sigvaldason (1972).

View of Hekla from the south on February 21, 1991, near the end of the eruption. Note the lava flows draping the slopes of the volcano and compare to the above map.

Gudmundsson and others' (1992) model for the Hekla volcanic system. The magma reservoir is thought to be layered, with more silica-rich magma at the top (andesite) and more silica-poor magma (basalt) at the bottom. Fissures and vents directly on Hekla are fed more silica rich magmas. Eruptions fed by the margins of the magma chamber tend to be lower in silica. From Gudmundsson and others (1992).

One interesting observation was an increase in the concentration of fluorine in river water on the fifth day of the eruption.

This was Hekla's third eruption in 20 years. Since 1104 there have been 55 eruptions, suggesting an average repose period of 55 years.

Chemical composition of Hekla lavas:

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 54.7 2.00 14.41 11.5 0.26 3.10 7.32 4.11 1.23 1.17 FeO = FeO total.

Sources of Information:

Gudmundsson, A., Oskarsson, N., Gronvold, K., Saemundsson, K., Sigurdsson, O., Stefansson, R., Gislason, S.R., Einarsson, P., Brandsdottir, B., Larsen, G., Johannesson, H., and Thordarson, T., 1992, The 1991 eruption of Hekla, Iceland: Bull. Volcanol., v. 54, p. 238-246.

Thorarinsson, S., and Sigvaldason, G.R., 1972, The Hekla eruption of 1970: Bull. Volcanol., v. 36, p. 269-288.

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