Morphology, surface structures, and emplacement of lavas produced by Laki, A.D. 1783-1784

The A.D. 1783-1784 Laki eruption (Iceland) was one of the largest basaltic eruptions in historical times. In only 8 months, >14 km ³ of lava was erupted, creating an ∼600 km ² lava flow field. The flow field presents a wide range of surface morphologies that can be subdivided into shelly, spiny, slabby, and rubbly pahoehoe types. This report constitutes the first comprehensive description of the Laki lava flow field and links the overall flow field organization seen on aerial photographs with key textural features observed in the field. By examining the transitions between the different surface morphologies, we are able to present a conceptual model for the formation of a rubbly pahoehoe surface during the emplacement of a lava flow. Patterns in the distribution of surface morphologies in the Laki flow field are combined with information from historic accounts to determine the impact of preexisting topography and changes in lava supply rate on flow morphology. The majority of Laki flows were initially emplaced as inflating pahoehoe sheets with hummocky margins. These gradually developed rubbly pahoehoe surfaces through progressive disruption of crust immediately behind the hummocky margin or at the active flow front. This sequence was repeated when fluid lava stored within the interior of the inflating sheet lobes broke through at the flow front to advance the flow. The quasi-absence of aa-type clinkery crusts shows that the lava never reached the point of continuous surface deformation by tearing of core lava at the leading edge of the flow. The Laki lavas have surface characteristics different from pahoehoe and aa, and the term rubbly pāhoehoe is appropriate for the dominant surface texture on the Laki flow field.