Lava penetrating water: the different behaviours of pahoehoe and 'a'a at the Nesjahraun, ?ingvellir, Iceland

The Nesjahraun is a basaltic lava flow erupted from a subaerial fissure, extending NE along the Þingvellir graben from the Hengill central volcano that produced pahoehoe lava followed by 'a'a. The Nesjahraun entered Iceland's largest lake, Þingvallavatn, along its southern shore during both phases of the eruption and exemplifies lava flowing into water in a lacustrine environment in the absence of powerful wave action. This study combines airborne light detection and ranging, sidescan sonar and Chirp seismic data with field observations to investigate the behaviour of the lava as it entered the water. Pahoehoe sheet lava was formed during the early stages of the eruption. Along the shoreline, stacks of thin (5-20 cm thick), vesicular, flows rest upon and surround low (2 m deep, 50 m long, spaced 50 m apart and have sub-horizontal striations on the walls. They likely represent channels or collapsed tubes along which lava was delivered into the water. A circular rootless cone, Eldborg, formed when water infiltrated a lava tube. Offshore from the pahoehoe lavas, the gradient of the flow surface steepens, suggesting a change in flow regime and the development of a talus ramp. Later, the flow was focused into a channel of 'a'a lava, 200-350 m wide. This split into individual flow lobes 20-50 m wide along the shore. 'A'a clinker is exposed on the water's edge, as well as glassy sand and gravel, which has been locally intruded by small (