LARGE CHANNEL-FED ROOTLESS VOLCANIC CONE COMPLEX, LAKI, ICELAND

Here we describe a large rootless volcanic cone complex located between the Hnuta-Hrossatungur and Leidolfsfell rootless cone groups —part of the 1783–84 Laki volcanic flow field. This part of the flow field was fed by segments 1–4 of the Laki cone row and was active between 8 June & 12 July 1783; ultimately forming the Eldraun (western) branch of the flow field. The volcanic field includes numerous geologically features (i.e., lava channels and volcanic rootless cone complexes), which are of particular relevance to understanding similar features in the Tharsis and Elysium Volcanic Provinces of Mars.

We combined field observations, mapping, and drone remote sensing. The rootless cone complex includes a series of large channel-fed tephra cones with a paired half-cone morphology along the main lava channel and numerous associated smaller sheet-fed and tube-fed cone complexes. The feature extends ~1 km E–W by 1.5 km N–S (1.5 km²). At the highest point the main paired half-cone complex (elev. 486 m) is 28 m above the lowest observable part of the associated lava channel. The main lava channel is between 35 and 140 m wide and the preserved surface is at an elev. of 470 m at the upper (NE) end & 448 m at the lower (SW) end of the complex—a drop of 22 m over 1000 m. The narrowest part of the channel is associated with the steepest gradient and highest tephra accumulation. Lateral and transverse ridges comprising rafted material and broken plates, occur within the channel.

The complex formed when the lava interacted with water in a low lying part of the original valley. Smaller rootless cone fields formed early and created barriers which partially blocked the lava resulting in lava lakes (elev. 474 m) upstream of the cones. Continued lava flow partially bypassed the complex but the main flow became concentrated through the center. Over time a series of smaller channel-fed rootless paired half-cones were built downstream of the main cone as the lava progressively interacted with water. This indicates a waning of the water–lava interaction over time. Tephra formation eventually stopped and the lava channel narrowed as the flow rate decreased and lateral ridges comprising plates and rafted tephra formed along the edges of the channel. Finally sometime after July 12, 1783 the active flows shifted to the NE where they fed the Brunahraun branch of the flow field.