The Fold Illusion: The Origins and Implications of Ogives on Silicic Lavas

Folds on the surfaces of mafic lavas are among the most readily recognized geological structures and are used as first-order criteria for identifying ancient lavas on Earth and other planetary bodies. However, the presence of surface-folds on the surface of silicic lavas is contested in this study and we challenge the widely accepted interpretation that silicic lava surfaces contain folds using examples from the western United States and Sardinia, Italy. We interpret the ridges and troughs on their upper surfaces, typically referred to as ‘ogives’ or ‘pressure ridges’, as fracture-bound structures rather than folds. We report on the absence of large-scale, buckle-style folds and note instead the ubiquitous presence of multiple generations and scales of tensile fractures comparable to crevasses in glaciers and formed in ways similar to already recognized crease structures. We report viscosity data and results of stress analyses that preclude folding (ductile deformation in compression) of the upper surface of silicic lavas at timescales of emplacement (weeks to months). Therefore, analysis of fold geometry (wavelength, amplitude, etc.) is erroneous, and instead the signal produced reflects the strength and thickness of the brittle upper surface stretching over a ductile interior. The presence of ogives on the surfaces of lavas on other planetary bodies may help to elucidate their rheological properties and crustal thicknesses, but relating to their tensile strength, not viscosity

Investigating mercury in road sediment in Michigan City, Indiana: A new type of environmental pollution record

Mercury (Hg) is well recognized as a toxic heavy metal known to detrimentally impact the health of humans and wildlife. A common source of Hg pollution is coal combustion emissions associated with energy generation. This study takes the novel approach of using road sediment to assess the presence, distribution, and concentrations of Hg in the environment. Here, the spatial distribution of Hg in road sediment is also evaluated relative to the Northern Indiana Public Service Company (NIPSCO) Generating Station in Michigan City, Indiana, U.S.—a coal-burning power plant. Given that NIPSCO plans to decommission the plant by 2028, this data provides a baseline for evaluating Hg concentrations in the local environment and for future assessment of environmental recovery from legacy pollution and/or impacts of pollution redistribution associated with site redevelopment. Our data reveal an average concentration of 6.8 (µg/kg) of Hg and a range from 1.5 µg/kg to 28.5 µg/kg in road sediment samples (n = 42). Across the ∼53 km2 study area, the overall distribution of Hg is patchy and irregular spatially, however, higher concentrations are more proximal to the coal plant and are generally consistent with prevailing wind directions. Other significant Hg inputs are present in the region although, specifically two major areas of steel manufacturing located to the west of Michigan City and broadening the scope of sampling may be an essential next step. However, this investigation demonstrates that road sediment may be an effective medium for investigating Hg pollution in the environment globally