THE 1300 AD ERUPTION OF HEKLA VOLCANO, ICELAND

The first eruption of Hekla in historic times took place in 1104 AD. The volcano has erupted 17 times since, most recently in 2000. Hekla is notable for producing a wide range of compositions, from basaltic andesite to rhyolite, even within individual eruptions. Thorarinsson first noted that the longer the repose period between eruptions, the higher the silica content of the first-erupted material and the more explosive the eruption. He also observed that the major eruptions have been compositionally zoned, with eruptive products becoming progressively less siliceous as the eruptions progressed. The 1300 AD eruption is the second largest historical eruption of Hekla. This eruption lasted for one year, produced a volume of 500 million m3 of material, and covered 30,000 km2 in tephra, mostly distributed to the north of the volcano. The lava flow associated with this eruption was one of the longest Hekla flows in historic time, extending 20 km to the southwest.

The 1300 lavas contain 3% olivine, 1% plagioclase, and trace cpx phenocrysts, slightly richer in olivine but otherwise typical of Hekla lavas. Olivine crystals contain abundant apatite and FeTi oxide grains. The SiO2 content varies from 56% in the least evolved lava to 64% in the most evolved tephra. Although this eruption is generally similar to other major Hekla eruptions, the most evolved tephra was collected from the top of the fallout deposit. Field observations indicate that this is a primary deposit, which suggests that either some early-erupted tephra was blown in from another location by syn-eruption wind, or that 1300 AD was not a simple normally-zoned eruption. Variations in the major and trace element contents further suggest that the most evolved tephras are related to the lavas by fractional crystallization of a plagioclase-olivine-pyroxene-titanomagnetite-apatite assemblage. Despite having a wider range of compositions, the 1300 AD rocks are virtually the same as the other 13th and 14th century eruptions at Hekla. All of these events are consistent with Thorarinsson's proposed relationship between increasing initial silica content and repose time.