Abstract

The October 1996 Gjálp eruption beneath Vatnajökull glacier led to one of the largest jökulhlaups (glacial floods) in Iceland in the twentieth century. A catastrophic discharge of meltwater and sediment swept across the Skeidarársandur flood plain to the sea. Tephra from the eruption consists of vesicular sideromelane shards with a basaltic andesite composition (53% SiO2, 3% MgO, 0.8% K2O). After the flood, sediment samples were collected from the flood plain and off the southeast coast of Iceland, where a major sediment plume had been created by the discharge. Compositions of glass shards from flood-plain and seafloor deposits do not match those of the Gjálp magma. Flood-plain samples consist primarily of blocky to poorly vesicular sideromelane clasts with compositions that are characteristic of Grímsvötn volcanic products (∼50% SiO2, 5.5% MgO, 0.4% K2O). Marine samples collected near the jökulhlaup outflow into the sea also consist primarily of blocky to poorly vesicular sideromelane clasts with compositions that are, for the most part, similar to products of the Grímsvötn volcanic center. Distal marine samples have more vesicular sideromelane clasts with compositions that are similar to products of the Katla volcanic center (e.g., 48% SiO2, 4.5% MgO, 0.8% K2O). Significant deposition to the seafloor was apparently limited to an area just offshore of the Skeidarársandur. There is no indication that juvenile volcanic material from the Gjálp eruption was carried by the 1996 jökulhlaup onto the flood plain or into the ocean. Instead, the jökulhlaup carried primarily older volcaniclastic material eroded by the flood.

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