Lava penetrating water: the different behaviours of pāhoehoe and ‘a‘ā at the Nesjahraun, Þingvellir, Iceland

Abstract

The Nesjahraun is a basaltic lava flow erupted from a subaerial fissure, extending NE along the Þingvellir graben from the Hengill central volcano. It produced pāhoehoe lava followed by 'a'ā. The Nesjahraun entered Iceland's largest lake, Þing-vallavatn, 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 (LiDAR), sidescan sonar and Chirp seismic data with field observations to investigate the behaviour of the lava as it entered the water. Pāhoehoe 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 (<5 m) piles of coarse, unconsolidated, variably oxidised spatter. Clefts within the lava run inland from the lake. These are 2-5 m wide, >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 pāhoehoe lavas, the gradient of the flow surface steepens, suggesting a change in Editorial responsibility: J.D.L. White flow regime and the development of a talus ramp. Later, the flow was focused into a channel of 'a'ā lava, ∼200-350 m wide. This split into individual flow lobes 20-50 m wide along the shore. 'A'ā clinker is exposed on the water's edge, as well as glassy sand and gravel, which has been locally intruded by small (<1 m), irregularly shaped, lava bodies. The cores of the flow lobes contain coherent, but hackly-fractured lava. Mounds consisting predominantly of scoria lapilli and the large paired half-cone of Grámelur were formed in phreatomagmatic explosions. The 'a'ā flow can be identified underwater over 1 km offshore, and the sidescan data suggest that the flow lobes remained coherent flowing down a gradient of <10 • . The Nesjahraun demonstrates that, even in the absence of ocean waves, phreatomagmatic explosions are ubiquitous and that pāhoehoe flows are much more likely to break up on entering the water than 'a'ā flows, which, with a higher flux and shallow underlying surface gradient, can penetrate water and remain coherent over distances of at least 1 km.

Coastal evolution on volcanic oceanic islands: A complex interplay between volcanism, erosion, sedimentation, sea-level change and biogenic production

“…Lava deltas generated under these conditions tend to be more stable than deltas generated under stable sea level (Lipman and Moore, 1996). However, Gilbert-type lava-fed deltas are frequently very unstable and are prone to slumps that quickly change the shore outline and generate large quantities of volcaniclastic debris The entrance of lava flows (typically 'a'ā or pāhoehoe sheet flows) into water under high effusion rates (generally in excess of 5-10 m 3 /s; Walker et al, 1973;Rowland and Walker, 1990;Griffiths and Fink, 1992;Gregg and Fink, 2000) and over offshore gently-dipping or flat-lying bottoms, in contrast, results in the accumulation/aggradation of submarine sheet flows that may branch out to form dendritic patterns but generally remain coherent subaqueously (Moore and Schilling, 1973;Umino et al, 2006;Mitchell et al, 2008;Ramalho, 2011;Stevenson et al, 2012). Thus, sequences formed under such conditions typically fill existing spaces more by aggradation rather than simple progradation, and are normally composed of an alternation of thick subhorizontal or gently-dipping submarine sheet flows and marine sediments, exhibiting little syn-eruptive hyaloclastitic material (see Fig.…”

Widespread inflation and drainage of a pāhoehoe flow field: the Nesjahraun, Þingvellir, Iceland

“…The initial channel flowed through Grámelur, which was generated by phreatomagmatic explosions. When flow through Grámelur was blocked, the flow direction moved to the west and emplaced most of the subaqueous lava (Stevenson et al 2011). The piling up of large ogives may occur during decelerating flow, while the subdued topography of the channel 'a'ā represents more steady conditions.…”

“…The pāhoehoe sheet flow entered the lake along a length of shoreline west of the Eldborg rootless cone ( Fig. 4; see Stevenson et al 2011). …”

“…The subaqueous extent of the Nesjahraun was mapped beneath the water to 1.5 km from the shore, where it abuts the southern flow front of Þingvallahraun (Thors 1992;Bull et al 2003). The lava-water interaction as the Nesjahraun entered the lake was investigated by Stevenson et al (2011), who found that phreatomagmatic explosions were common, generating the rootless cones of Eldborg and Grámelur (Fig. 1) and that the bulk of the lava beneath the lake was emplaced during the 'a'ā phase of the eruption.…”

Radar Albedo Feature

“…Rootless cones are commonly associated with lava fields composed of tube-fed lavas entering into wet environment where active lava interact with the external water producing explosive phreatomagmatic eruptions (Hamilton et al 2010;Sheth et al 2004;Stevenson et al 2012). In this respect, the identification of rootless cones have been used to demonstrate the presence of water, water-saturated sediment, or ice (e.g., permafrost mounds or ▶ pingos) in extraterrestrial bodies (Bruno et al 2004;Christensen et al 2003;Hamilton et al 2011;Lanagan et al 2001;Martinez-Alonso et al 2011;Page and Murray 2006).…”