Anja Moebis

Inferring the Effects of Compositional Boundary Layers on Crystal Nucleation, Growth Textures, and Mineral Chemistry in Natural Volcanic Tephras through Submicron-Resolution Imaging

Crystal nucleation and growth are first order processes captured in volcanic rocks and record important information about the rates of magmatic processes and chemical evolution of magmas during their ascent and eruption. We have studied glass-rich andesitic tephras from the Central Plateau of the Southern Taupo Volcanic Zone by electron- and ion-microbeam imaging techniques to investigate down to sub-micrometre scale the potential effects of compositional boundary layers (CBLs) of melt around crystals on the nucleation and growth of mineral phases and the chemistry of crystal growth zones. We find that CBLs may influence the types of mineral phases nucleating and growing, and growth textures such as the development of swallowtails. The chemistry of the CBLs also has the capacity to trigger intermittent overgrowths of nanometre-scale bands of different phases in rapidly growing crystals, resulting in what we refer to as cryptic phase zoning. The existence of cryptic phase zoning has implications for the interpretation of microprobe compositional data, and the resulting inferences made on the conditions of magmatic evolution. Identification of cryptic phase zoning may in future lead to more accurate thermobarometric estimates and thus geospeedometric constraints. In future, a more quantitative characterization of CBL formation and its effects on crystal nucleation and growth may contribute to a better understanding of melt rheology and magma ascent processes at the onset of explosive volcanic eruptions, and will likely be of benefit to hazard mitigation efforts.

Die Erde knallt! Vulkane in Sachsen; Begleitband zur Sonderausstellung

The German book Die Erde knallt! Vulkane in Sachsen is accompanying material to an exhibition in a museum in Kamenz/Westlausitz (Saxony), an area in the southeast of Germany on the borders of Poland in the east and Czech Republic to the south. The book stands completely on its own, without the reader having to see the exhibition, and contains two main sections: Volcanism in the Rotliegend (Permian) and Volcanism in the Tertiary of Saxony, each comprising several articles written by different authors. The book title translates into The earth bangs! Volcanoes in Saxony [sic] and is, from my perspective, unimaginative and partly misleading, creating the impression that the volcanoes in Saxony are still active. At first sight, the book seems very old fashioned with bright-colored backgrounds and framed textboxes; but at a closer look, the individual articles are well written and informative about volcanological processes of Permian to Cenozoic volcanism in Saxony and the history of the region’s mining, and use of its volcanic eruption products. The first section of the book: Volcanism in the Rotliegend (Permian) contains six articles, three introducing the reader to specific regions where volcanism at that time occurred and the other three explaining the processes and deposits of the volcanism of this time. The Permian volcanism taking place at the end of the Variscan Orogeny along fault systems generated rhyolitic magmas which either stagnated within the crust to produce large areas of plutonic rocks such as granites and granodiorites or were erupted during large caldera-forming explosions to form thick ignimbrites. The individual chapters explain with beautiful sketches how the eruptions happened and illustrate the appearance of their product with images of thin and magnified sections of rock samples. The most interesting parts for me, having grown up in Saxony, were the mining history of the thick and extensive ignimbritic rocks and how these rocks and their weathering products were used over centuries. For instance, the white clay kaolin is a weathering product of volcanic rocks and was used to develop and produce world famous porcelain from a factory in Meißen. Another chapter is dedicated to descriptions of diverse life forms of a Permian forest in the area of present-day Chemnitz covered by an ignimbrite and thereby superbly preserved. The second section: Volcanism in the Tertiary of Saxony consists also of six articles with the first explaining the origin Editorial responsibility: K. Németh