Widespread inflation and drainage of a pāhoehoe flow field: The Nesjahraun, {Thorn}ingvellir, Iceland

This study describes the emplacement of the Nesjahraun, a basaltic lava flow that entered the lake {Thorn}ingvallavatn, SW Iceland. High-resolution remotely sensed data were combined with fieldwork to map the flow field. Onshore, the Nesjahraun exhibits a variety of textures related to the widespread inflation and collapse of a pāhoehoe flow field. Its emplacement is interpreted as follows: Initially, the eruption produced sheet pāhoehoe. In the central part of the flow field, the lava has a platy-ridged surface, which is similar to some other lava flows in Iceland and on Mars. Here, the texture is interpreted to have formed by unsteady inflation of the brittle crust of stationary sheet pāhoehoe, causing it to break into separate plates. The ridges of broken pāhoehoe slabs formed as the plates of crust moved vertically past each other in a process similar to the formation of shatter rings. Upstream, fresh lava overflowed repeatedly from channels and tubes, covering the surface with shelly pāhoehoe. Formation of a 250-m-wide open channel through the flow field allowed the inflated central part of the flow to drain rapidly. This phase produced 'a'ā lava, which eroded the channel walls, carrying broken pāhoehoe slabs, lava balls and detaching large (>200 m long) rafts of compound shelly pāhoehoe lava. Much of this channelized lava flowed into the lake, leaving a network of drained channels and tubes in the upstream part of the flow. As in other locations, the platy-ridged texture is associated with a low underlying slope and high eruption rate. Here, its formation was possibly enhanced by lateral confinement, hindered entry into the lake and an elevated vent location. We suggest that formation of this type of platy-ridged lava, where the plates are smooth and the ridges are slabs of broken pāhoehoe, can occur without significant horizontal transport, as the surface crust is broken into plates in situ. This reconstruction of the emplacement of the Nesjahraun also demonstrates that high-resolution aerial survey data are extremely useful in the mapping and measurement of lithofacies distributions in large flow fields, but that fieldwork is still necessary to obtain the detailed textural information necessary to interpret them.