Detailed topographical, distributional, and material analyses of rootless cones in Myvatn, Iceland

• We surveyed rootless cones in Myvatn, Iceland in point of volcanological interesting and comparison with Martian cones.
• In Myvatn, many rootless cones are large and complex-shaped relative to the other fields ones.
• We posit that they depend on the high explosivity under suitable water to lava ratio during the rootless eruption.
• The characteristic structure of DCs strongly indicates that they were formed by eruptions of different styles.
• Eruptions delayed by the slow water percolation through lacustrine sediments are responsible for the formation of DCs.

Unique types of rootless cones have been found both on Earth and on Mars. They have inner cones inside their summit craters, and we call them double cones (DCs) and multiple cones. Though previous studies have mentioned their existence, there are no quantitative data on their topography, distribution, or constituent materials. Having this information available as a comparative reference could promote our understanding of Martian cones. For this study, we conducted a field campaign on terrestrial cones in Myvatn, Iceland. To provide high-quality reference data about the morphology of rootless cones, we performed kinematic Global Positioning System (GPS) measurements. In the study area DCs and multiple cones are located in the vicinity of the lake (Lake Myvatn) within 15 km of a fissure vent. There are no such cones on the paleo-riverbed. In crosscut outcrops of DCs on Geitey Island, a clearly recognizable structural boundary between the inner and outer cones shows that the inner cone was formed by a separate explosion episode. We propose a probable formation scenario for DCs here. We posit that the inner cones of DCs are formed by delayed explosions due to percolation of water through the lacustrine sediments. These unique characteristics of rootless cones are useful in the interpretation of Martian cones.