COMPARISON OF ICELANDIC ROOTLESS CONES AND EXPERIMENTAL LAVA FEATURES

Rootless cones (hornitos) in the post-glacial Younger Laxárhraun basalt flow in northeastern Iceland formed from the interaction between basalt lava and marshy ground. The flow’s vesicular porphyritic basalt is characterized by poikilitic plagioclase phenocrysts (~2-10 mm across, ~An₆₀), in a dominantly intergranular groundmass of plagioclase, clinopyroxene, minor olivine, and ilmenite. On a broad (kilometer) scale, the rootless cones are clustered, especially in areas of low topography. There are three basic cluster types: small, tightly clustered spatter cones; large, widely spaced cones with craters; and craterless cinder cones. Within clusters, patterns of spatial distribution may indicate competition for water, providing insight into the dynamics of rootless cone formation (Hamilton et al., 2010). The distribution of cones within two clusters was documented through reconnaissance mapping in summer, 2014. These maps will serve as a basis for further analysis to provide a better understanding of the processes impacting rootless cone formation. Another way to understand the formation process of pseudocraters is through experimental analogues. An apparatus well suited for experiments with lava and water is the Syracuse University Lava Project, where a natural-gas-fired furnace can melt up to 800 pounds of basalt. The resulting lava is poured onto sand, the structure of which is tailored to the experiment. This project required two pours. The first entailed pouring across different ratios of sand to water in hopes of creating a range of lava behavior. This flow was viscous, thick, and frothy, and the features produced did not seem to vary based on water-to-sand ratios. The second flow was poured across a 20 cm wide strip of nearly saturated wet sand. The resulting lava was glassier, thinner, and less viscous than the first flow, and it produced clustered glassy bubbles (limu) more analogous to the Icelandic spatter cones than were the larger, thicker air bubbles produced by the first flow. A better understanding of how rootless cones form and the relation of their distribution pattern to variations in lava-water interactions could help to identify areas vulnerable to phreatic eruptions. It could also provide more positive remote interpretation of lava-water features on Mars and other planetary bodies.