Hydrothermal activity is common at active volcanoes. Volcanic gasses rise and form strong acids that lead to fluid-rock interactions affecting a rock’s mineral assemblage by dissolution and remineralization, eventually influencing essential rock parameters like strength and permeability. Despite the far-reaching consequences for the stability of a volcanic edifice, our understanding of extent and variability of hydrothermal alteration is often limited. Within Multi-Marex we aim to better understand the causes and effects of hydrothermal alteration and volcano stability on land and underwater. By close-range remote sensing, we analyze hydrothermal alteration, aiming to describe the morphology (shapes) and optical appearance of hydrothermally active sites over scales and to reveal the general pattern of alteration and its regional variability. We give an overview of optical methods for tracing hydrothermal alteration, compare patterns observed at different systems, and expand our view to the submarine regime. In particular, we compare the alteration pattern at Nisyros, a hydrothermally active volcano in the Aegean Arc with hydrothermal-dominated or magmatic-hydrothermal systems at locations elsewhere. The approaches and the pattern of hydrothermal alterations observed vary, but all systems have in common that there are patterns that can be detected and that indicate variability of gas flux and alteration and therefore zones of contrast considering materials, permeabilities, strength, or other physicochemical properties. Revealing these patterns is beneficial for detailed and focused further investigations and may be particularly useful for monitoring and future risk assessment studies.

The Delitzsch carbonatite complex, located 25 km NW of Leipzig, constitutes a late Cretaceous ultramafic lamprophyre (UML)-carbonatite occurrence covered by Cenozoic sediments [1,2]. We have analysed UML samples from drillcores for their mineral chemistries by electron-probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to better understand their mantle source and magmatic history.

Based on their mineralogy, the rocks can be classified as alnöite, olivine-alnöite, phlogopite-olivine alnöite, and damtjernite [3]. Compositionally, the rocks lie between UMLs and kimberlites with Mg# [= Mg/(Mg+Fe)] ranging from 0.72 to 0.79, and are likely derived from partial melting of carbonated peridotite at pressures of around 5 GPa [4]. Their deep origin is further corroborated by a garnet-peridotite mantle xenolith which equilibrated at 1350 °C and 5.8 GPa.

Olivine is the most common mineral in the studied rocks and exhibits complex zonation. The core compositions show different groups that we attribute to i) mantle xenocrysts (high Mg# and NiO), ii) antecrysts which have crystallized at different levels in the lithosphere (variable Mg# and NiO), and iii) carbonatitic olivines (extremely low NiO). The rims show distinct differentiation trends converging to Mg# of about 0.87 with decreasing NiO contents.

Our results demonstrate the complex magmatic history of the Delitzsch UMLs and provide evidence for a formerly thick lithospheric mantle beneath Central Europe.

[1] Seifert et al. (2000) Lithos 53: 81-100
[2] Krüger et al. (2013) Chem Geol 353: 140-150
[3] Tappe et al. (2005) JPet 46: 1893-1900
[4] Gudfinnsson & Presnall (2005) JPet 8: 1645-1659

Research on hydrothermal alteration investigates the impacts of hot, corrosive fluids circulating within a volcano, which are crucial for comprehending volcanic risks, slope instability, and steam-driven eruptions. Observable phenomena like fumaroles and mineral deposits at the surface offer direct evidence of subterranean hydrothermal systems or volcanic unrest, detectable through remote sensing techniques. In this study, we introduce a novel Hydrothermal Alteration Index (HAI) derived from Ultra Blue, Red, SWIR 1, and SWIR 2 bands of multispectral satellite imagery, facilitated by Google Earth Engine (GEE), to monitor hydrothermal changes. Identifying three primary alteration zones covering a total area of 600,000 m² at Lastarria Volcano, our findings are corroborated by field surveys, affirming the utility of HAI. Through temporal analysis, we pinpoint three distinct events indicating expansion, contraction of alteration zones, and the emergence of new sulfur flows. By aligning spatiotemporal patterns detected by HAI with independent monitoring data, we infer heightened hydrothermal activity. Lastly, we offer fresh insights into the progression of surface hydrothermal phenomena, starting from the summit crater and extending towards the flank region.

North Atlantic rifting during the Palaeocene-Eocene was accompanied by explosive volcanic eruptions. These led to distribution of about 200 ash layers of mainly basaltic composition covering wide areas of NW and Central Europe, also reaching the Tethys realm (Obst et al. 2015).

The ash layers, which are often interbedded in clayish successions, are known from offshore and onshore drillings but also from surface exposures, e.g., cliff sections or clay pits. In part, the pyroclastic material is well preserved in eogenetically carbonate cemented concretions, which occur in northern Germany and Denmark in glacially dislocated rafts of Eocene sediments or as isolated glacial erratic boulders named cement stones (“Zementsteine”).

Petrographic and sedimentological investigations of numerous cemented ashes from several locations in northern Germany (Fehmarn, Klütz Höved, Groß Roge, Grimmen, Wobbanz/Rügen and Greifswalder Oie) allow to distinguish different types of preservation. Single and rarely double ash layers up to 15 cm in thickness may either be preserved undisturbed, intensively bioturbated or reworked. Especially in shallow marine environments, the ashes can partly be eroded by currents or waves, and the basaltic glass particles may be redistributed.

In detail, variations in thickness and grain size as well as varying glass composition and alteration can be used to characterize distinct layers and will help to correlate ashes of the same volcanic event between different occurrences. Furthermore, changes of the sedimentation environment are documented in a NW–SE transect reflecting still water conditions in the central part of the North Sea Basin and near-shore environments at the eastern basin margins.

Volcanic flank instability poses a significant multi-hazard risk, encompassing caldera collapses, landslides, rock avalanches, and potential tsunami generation in active and dormant volcanoes. The mechanical strength and regions of hydrothermal alteration may play a fundamental role in locating and scaling volcano instability. Therefore, investigating hydrothermal alteration, which consequently alters the physicochemical properties of volcanic rocks, is crucial to better understanding the processes that lead to volcanic flank instability and collapse.

Here, we use the southernmost exposure of the Aeolian volcanic archipelago, La Fossa of Vulcano Island (Italy), as our focus site. La Fossa's history of mass wasting, regions of hydrothermal alteration, and episodic fumarole activity make it an ideal natural laboratory for our investigation. Here, we used high-resolution drone remote sensing techniques coupled with in-situ uni-axial compressive strength measurements to identify regions of hydrothermal alteration and assess their associated compressive strength properties. In summary, our results show (1) a heterogeneous distribution of alteration types and intensities, (2) a relationship between increasing alteration intensity and decreasing rock strength, (3) a correlation between regions with the weakest rock strength and the most intensely altered areas, and finally (4) a spatial association of alteration and deep scars resulting from erosion and landslides. Our combined approach allows us to explore the association between rock strength and hydrothermal alteration, enabling us to understand volcanic flank instability better and help us improve future hazard assessment.