The Laki and Grimsvotn Eruptions of 1783-1785

The Laki and Grimsvotn Eruptions of 1783-1785

Last Updated: 16 April 2001

View along part of the Laki fissure. Photograph courtesy of Thor Thordarson.

Map of the Laki fissures and lava flows. The Eldgja fissures and flows are also shown. Map simplified from Thordarson and others.

Lakagigar (also called Skaftar) was the vent for the 1783-1784 eruption of Grimsvotn caldera. It was the second largest basaltic fissure eruption in historic time (after the ~935 eruption of nearby Eldgja) and caused notable atmospheric cooling and effects. Additional vents of Grimsvotn were active from May 1783 to May 1785.

The eruption began on June 8, 1783 and lasted eight months. An estimated 14.731 cubic km of basalt was erupted and covered an area of 565 square km.

Cone of Lakagigar. Photo by B. Edwards.

Careful description of the tephra deposits at Laki have allowed Thordarson and Self (1993) to reconstruct the eruption. Contemporaneous observations by people living near Laki helped in determining the timing and location of major events. Thordarson and Self identified ten eruptive episodes. The onset of each episode was preceded by an earthquake swarm that increased in intensity until a new fissure opened. Once the fissure formed, magma interacted with groundwater to generate short-lived phreatomagmatic activity. As the amount of groundwater decreased, the style of the eruption changed to Strombolian or sub-Plinian and then to Hawaiian-style fire fountains and the effusion of lava. Fire fountains reached heights of 800-1400 m. Thordarson and Self (1993) determined that lava poured from fissures at a rate of about of 8,600 cubic meters per second (the discharge rate of the Amazon River is 10,000 cubic meters per second). The fissures were about 2.5 km long.

Shortly after the eruption began, lava reached the Skafta river gorge and flowed towards the lowlands. It travel 35 km in only four days. The effusion of lava continued to February 7, 1784. Most of the lava was erupted in the first five months.

Fissure of Lakagigar. Photo by B. Edwards.

Ten fissures make up the vent complex. The fissures are arranged in an en echelon pattern that extends for a length of 27 km. Each fissure is covered by a continuous row of scoria cones, spatter cones, and tuff cones. The cones range in heights from 40m to 70 m.

Only 2.6% of the material erupted was tephra but ash fall extended all the way to mainland Europe. Map from Thordarson and Self (1993).

Laki is also known for its atmospheric effects. The convective eruption column of Laki carried gases to altitudes of 15 km. These gases formed aerosols that caused cooling in the Northern Hemisphere, possibly by as much as 1 degree C. This cooling is the largest such volcanic-induced event in historic time. In Iceland, the haze lead to the loss of most of the islands livestock (by eating fluorine contaminated grass), crop failure (by acid rain) and the death of 9,000 people, one-quarter of the human residents (by famine).

Sources of Information:

Thordarson, Th., and Self, S., 1993, The Laki (Skaftar Fires) and Grimsvotn eruptions in 1783-1785: Bulletin of Volcanology, v. 55, p. 233-263.

Thordarson, T., Miller, D.J., Larsen, G., Self., S., and Sigurdsson, H., submitted, New estimates of sulfur degassing and atmospheric mass-loading by the ~935 AD Eldgja eruption, Iceland: Journal of Volcanology and Geothermal Research.

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