Rootless Cones and Tree-Root Lava: Recognizing Lava-Water Interactions Remotely

Lava­­-water interactions (LWIs) are rarely considered in lava flow hazard assessments or emergency planning scenarios, though they can generate a range of secondary hazards, including tephra blasts, rootless eruptions, disruption to water supplies, and flooding. These hazards may endanger life, damage property, and hinder evacuation or rescue efforts, so identifying the signs of LWI in the products of past eruptions may help emergency planners identify potential hazards for future eruptions. The physical products of LWI, such as abundant hyaloclastite, high proportions of fine ash, lava pillows, and irregular columnar jointing, have long been recognized in the field. However, remote sensing offers the opportunity to assess whole lava fields relatively quickly and cheaply, and allows investigation of inaccessible lava fields and planetary volcanism. In addition, the large-scale view can reveal features that are not immediately visible in the field, and tools like LiDAR can be used to strip away vegetation to show hidden morphology and structure. We present features indicative of LWI that can be identified by remote sensing techniques and discuss what they can and can’t tell us about LWI in past eruptions. We illustrate these with data from the well-documented 1783-84 Laki fissure eruption, supplemented with other case studies from Iceland, Hawai’i and the Pacific NW. In particular, the size, type and spacing of rootless cones can tell us about the availability of water and intensity of rootless eruptions. When examined in conjunction with lava flow morphology and local topography, we can learn about the local lava flux and the likely water sources and pre-eruptive landscape. In the absence of rootless cones, dendritic textures on the lava flow surface may indicate passive LWI. These textures are found across the Laki lava field, commonly in areas where the lava encountered rivers or floods, and match those at other lava basaltic flows known to have interacted with water, including the 2018 eruption of Kilauea. Together, these features are useful indicators for identifying and interpreting past LWIs, both as a complement to field observations and when field studies are not feasible.