Abstract
Active anhydrite hydration during weathering leading to crystallisation of secondary gypsum and significant volume expansion was investigated and documented by the authors at three sites: the environs of Walkenried (Germany), Dingwall (Canada), and Pisky (Ukraine). As a result of these processes, peculiar landscape forms were created: hydration domes and ridges with empty internal chambers, some of them large enough to be called hydration caves (German: Quellungshöhlen). Currently, there are only four recognised sites on Earth featuring such a unique landscape and with a large group of hydration caves in one place (the fourth site is in the Alebastrovyye Islands, Russia). These sites constitute a particularly valuable geological and geomorphological heritage, including potential geosites and geomorphosites which require special protection. Actively growing hydration domes and caves change shape and size within a short time span, on the scale of months, years, or decades. Their study and proper protection require these changes to be monitored. Several different methods of documentation were applied in the field in order to document continuing morphological changes. The practical aspects of the use of each of these methods were assessed, demonstrating that the photogrammetric methods offer the greatest utility; not only are they the most efficient (fast and sufficiently precise) but also, compared with other methods, they yielded the most complete results. The key documentation of outcrops in Canada and Ukraine was executed with the application of terrestrial photogrammetry at Pisky (GoPro camera) and aerial photogrammetry at Dingwall (unmanned aerial vehicle). Application of these methods enabled the recording of the morphology associated with the hydration process in the form of 2.5D and 3D models as well as of orthophotomaps. The maps and the models were created using the Photoscan programme. The authors demonstrate that the photogrammetric models can be used for spatial morphological analysis of hydration forms in the ArcGIS programme. Repetition of this documentation in future will enable analysis of the morphological changes expected to occur during the progressive expansive hydration of anhydrite.
Highlights
Hydration of the mineral anhydrite is a common process observed in many places on Earth
It is known that this reaction and the growth of gypsum crystals can lead to significant volume expansion: theoretically, up to 62.6% in an open system (Zanbak and Arthur 1986; Reimann 1991)
The first four methods, terrestrial photogrammetry, and modelling methods were used at all three studied sites (Pisky, Dingwall, Walkenried), the application of photogrammetry at Walkenried was rather limited since the relief there is hidden by forest; 3D laser scanning and scanning with LED structured light methods were used only at Pisky, aerial photogrammetry only at Dingwall
Summary
Hydration of the mineral anhydrite (chemical formula: CaSO4) is a common process observed in many places on Earth. Most often, exposed anhydrite rocks are subjected to hydration in the weathering zone under the influence of meteoric waters. As a result of this process, a secondary mineral, gypsum (CaSO4·2H2O), crystallises according to the reaction CaSO4 + 2H2O → CaSO4·2H2O. It is known that this reaction and the growth of gypsum crystals can lead to significant volume expansion: theoretically, up to 62.6% in an open system (Zanbak and Arthur 1986; Reimann 1991). In many cases, no volume expansion is observed or expansion is too small or too slow to be detected. There are relatively rare examples where volume expansion is more rapid and results in dramatic deformational structures and forms
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