While the final manifestations of caldera-forming eruptions can be easily observed on the Earth’s surface and are thus relatively well studied, the physical conditions and causes of caldera collapse and the mechanisms of brittle failure of the underlying rocks are still a mystery. The aim of a long-standing project by a joint team from the Faculty of Science of the UK and the Czech Technical University in Prague was to understand under what conditions the gravitational collapse of calderas occurs and what effect the shape of the underlying magma mantle, its size and depth in the Earth’s crust has. New research has shown, using a combination of geological data and mathematical modelling, that magma pressure oscillations lead to intense brittle failure in places, and therefore to a reduction in the strength of the magma crust ceiling, but that collapse and caldera formation can only occur under specific conditions.
Somr M., Žák J., Kabele P., Tomek F. (2023): Analysis of fracturing processes leading to caldera collapse. Earth-Science Reviews241, 104413 (DOI)
Evaporation from rocks is a poorly understood, yet important process. In the recent study, an evaporation rate from 10 lithologies, including sedimentary, igneous, and metamorphic granular rocks, was measured and calculated. The measured evaporation rate varied over four orders of magnitude (0.4–2447 mm/year), and the calculations reasonably followed the measured values. Within the range of observed values, the evaporation rate was mostly influenced by the vaporization plane depth below the rock surface (by up to 2.2 orders of magnitude), which was followed by: lithology (up to 1.1 order of magnitude), local climate (up to 1.0 order of magnitude), and climate seasonality (up to 0.8 order of magnitude). Thus, our study shows the key role of the vaporization plane depth in the evaporation rate. This approach can find employment in a large number of investigations such as in the evaporation estimates and hydrologic balance in rock landforms and rocky slopes, hydrologic processes in the shallow rock subsurface, living conditions of endolithic and epilithic organisms, weathering processes, and in the protection of carved or rock constructed cultural heritage.
Slavík M., Bruthans J., Schweigstillová J. (2018): Evaporation rate from surfaces of various granular rocks: Comparison of measured and calculated values. Science of the Total Environment856, 159114. (DOI)
Diachronous opening of the Rheic Ocean and separation of Avalonian–Cadomian terranes from Gondwana was a major, but still poorly understood paleogeographic event in the late Ediacaran to early Cambrian. A recent study from the Příbram–Jince basin in the Bohemian Massif revealed a significant provenance and paleocurrent changes in response to dextral transtension that enlarged the basin into a pull-apart structure, suggesting that strike-slip movements along the former Avalonian–Cadomian belt controlled the diachronous opening of the Rheic Ocean. Putting this piece of information into a plate-tectonic picture, it seems that an inherited suture in the Avalonian ribbon terrane facilitated complete rifting and rift–drift transition while the Cadomian terranes, including those now forming the Bohemian Massif, remained attached to Gondwana during this large-scale rifting event.
Syahputra R., Žák J., Nance R.D. (2021): Cambrian sedimentary basins of northern Gondwana as geodynamic markers of incipient opening of the Rheic Ocean. Gondwana Research 105, 492-513.(DOI)
Cobalt is one of the most important critical metals which could be potentially extracted from the old metallurgical slags in the Zambian Copperbelt. The slags from Luanshya, the oldest mining and smelting site in the Copperbelt, contain up to 5990 ppm Co (median: 2370 ppm). The detailed mineralogical investigation combined with the sulfuric acid leaching simulating hydrometallurgical recovery indicated that up to 67% of Co can be extracted from slag in a short period of time (24 h). However, despite the dramatic increase of Co prices on the global market, its recovery from the Luanshya slags appears to be non-economical due to the high costs of the mechanical and chemical processing of the slag materials. The paper is freely available via open access:
Ettler V., Mihaljevič M., Drahota P., Kříbek B., Nyambe I., Vaněk A., Penížek V., Sracek O., Natherová V. (2022): Cobalt-bearing copper slags from Luanshya (Zambian Copperbelt): Mineralogy, geochemistry, and potential recovery of critical metals. Journal of Geochemical Exploration237, 106987. (DOI)
We compared analogous healthy topsoils and ones burned by fires of different temperatures for their elemental and isotopic Pb content. The fires only affected the top 5 cm of the soil. We found that soils affected by low-temperature fires had increased Pb compared to healthy soils. The accumulation of Pb was not observed in the soils exposed to higher temperatures. The same results were found in ash. Lead isotopic ratios were affected by the fires. These increased in soils affected by higher temperatures but didn’t change with the low temperatures. We proposed that the temperature dependence is due to anthropogenic Pb (206Pb/207Pb < 1.16) volatilizing at lower temperatures than geogenic Pb (206Pb/207Pb > 1.19). This work suggests that it may be possible to determine the temperature of a forest fire from the Pb isotopic signatures of the burned materials.
Baieta R., Vieira A.M.D, Vaňková M., Mihaljevič M. (2022). Effects of forest fires on soil lead elemental contents and isotopic ratios. Geoderma414, 115760. (DOI)
New paper reports groundbreaking findings on the distribution of non-carotenoid pigments in gypsum endoliths. In the study, Raman microspectrometry was used for analysis of UV-protective pigments of dark-pigmented endolithic colonizations in gypsum outcrops from Sicily, Poland and Israel. Colonized samples were investigated using 445-, 532- and 780-nm excitation lasers. Scytonemin and gloeocapsin were detected at all studied sites. Major Raman signatures of scytonemin were detected at 1593, 1552, 1438 and 1173 cm−1. Gloeocapsin shows characteristic Raman bands similar to anthraquinone-based parietin of lichens: at 1665, 1575, 1378, 1310 and 465 cm−1. Scytonemin and gloeocapsin are highly specific for cyanobacteria and can be thus used as biomarkers for certain taxa of cyanobacteria in geobiological and astrobiological studies. Detection of such pigments by Raman spectroscopy combining three excitation wavelengths allows gathering more information about the composition of endolithic consortia in their natural habitats using fast and non-invasive methods.
Němečková K., Culka A., Němec I., Edwards H. G. M., Mareš J., Jehlička J. (2021): Raman spectroscopic search for scytonemin and gloeocapsin in endolithic colonizations in large gypsum crystals. Journal of Raman Spectroscopy52, 2633. (DOI)
The article presents a new look on sandstone weathering/recession characterized by rapid disintegration of portions of the rock massif which are no longer physically connected with the main rock mass, though still in situ. A set of field and laboratory measurements testing mechanical and hydraulic properties of the rock were applied to compare the disconnected portions with those of the surrounding rock mass. Also, physical weathering experiments were performed to characterize the effect of confinement on the breakdown rate of several sandstone samples. The surfaces of disconnection follow bedding planes, planar elements of other sedimentary structures, subhorizontal fractures and stress shadows on subvertical cliff faces. Weathered surfaces of the disconnected portions show noticeably impaired mechanical properties, a faster capillary water absorption and a higher surface moisture compared to the much less weathered surfaces of the surrounding rock mass. Physical experiments demonstrated that a confinement by the surrounding rock mass may considerably delay the loosening of rock during weathering. The much faster disintegration rates of the disconnected portions of rock compared to their surroundings are explained by the fact that they are not confined and have a larger surface area.The recession rates of sandstone surfaces with disconnected portions are highly variable both in space and time and their genesis is demonstrated by two suggested conceptual models.
Filippi M., Slavík M., Bruthans J., Weiss T., Řihošek J. (2021): Accelerated disintegration of in situ disconnected portions of sandstone outcrops. Geomorphology391, 107897. (DOI)
New paper recently published in Precambrian Research examines an illustrative example of a Late Archean granitic pluton emplaced along a major tectonic boundary in the Superior Province, northeastern Québec, Canada. To characterize the syn-magmatic strain patterns, we integrate structural and kinematic analysis with anisotropy of magnetic susceptibility (AMS) and numerical modeling of fabric development in response to strain increments. This dataset allowed us to set-up a general geological model that discusses the significance of pluton fabrics, interfering intrusive and tectonic strains, and finally also on evaluating various scenarios for the Late Archean assembly of the Superior Province.
Žák J., Tomek F., Svojtka M., Vacek F., Kachlík V., Ackerman L., Ježek J., Petronis, M.S. (2021): Distributed crustal shortening followed by transpressional shearing in the Superior Province, northeastern Canada: a Late Archean analogy to modern accretionary plate margins? Precambrian Research362, 106322. (DOI)
The temperature-dependent releases of metal(loid)s (As, Cd, Cu, Pb, Zn) from biomass-rich savanna soils collected near a Cu smelter in Namibia have been studied under simulated wildfire conditions. For this purpose, new wildfire-simulating setups were introduced. Laboratory single-step combustion experiments (250–850 °C) and experiments with a continuous temperature increase and online ICP-OES detection (25–750 °C) were coupled with mineralogical investigations of the soils, ashes, and aerosols. The results indicate that metals are dominantly concentrating in the ash residue, and part of As is remobilized depending on temperature. Therefore, the active and abandoned mining and smelting sites, especially those highly enriched in As, should be protected against wildfires, which can be responsible for substantial As re-emissions.
Tuhý M., Ettler V., Rohovec J., Matoušková Š., Mihaljevič M., Kříbek B., Mapani B. (2021): Metal(loid)s remobilization and mineralogical transformations in smelter-polluted savanna soils under simulated wildfire conditions. Journal of Environmental Management 293, 112899. (DOI)
We used pine tree rings and soil geochemistry to expose the activity of a Pb-Zn smelter in Kabwe, Zambia. We found that local soils are extremely contaminated (up to 16000 mg/kg Pb; 14000 mg/kg Zn; 600 mg/kg Cu in the topsoil). Also, metal bearing particles share their Pb isotopic signature with smelting by-products, and more concerning, metals in tree biomass. We concluded that these metals enter the tree though the bark and leaves, not the roots, implying airborne contamination. Using Pb and C isotopes we mapped the dendrochronology of the tree and verified its susceptibility to changes in smelter production throughout the late 20th century.
Baieta R., Mihaljevič M., Ettler V., Vaněk A., Penížek V., Trubač J., Kříbek B., Ježek J., Svoboda M., Sracek O., Nyambe I. (2021): Depicting the historical pollution in a Pb–Zn mining/smelting site in Kabwe (Zambia) using tree rings. Journal of African Earth Sciences181, 104246. (DOI)