The ~30 Ma Stolpen Volcano near Dresden (Saxony) is situated at the western margin of the Lausitz Volcanic Field. It forms a small isolated basaltic hill, the famous Stolpen Castle Hill, penetrating the granodioritic basement of the Lausitz Block and is worldwide the type locality for the term 'basalt', as coined by Agricola (1546). The volcano has always been interpreted as subvolcanic crypto-or lava dome. New geological mapping, dip measurements of the basaltic columnar jointing and the first evidence of scoria for the Stolpen Volcano allow for a new interpretation of the volcanic edifices. On this basis the structure is best described as a maar crater volcano filled by a 110 m thick basanitic lava lake. According to genetic classification, it is a complex monogenetic volcano formed in three phases: (1) a phreatomagmatic diatreme phase with a maar crater, (2) a scoria cone phase with the final basanitic lava lake filling, and (3) a post-volcanic phase with neotectonic uplift, denudation and exposure of a basaltic hill since c. 1.3 Ma. The volcano reconstruction indicates an ongoing change in the eruption style from phreatomagmatic (phase 1) to eruptive and, finally, effusive processes (phase 2) without significant gaps between the eruption episodes. The only difference appears to be caused by a variable water supply during the magma ascent and volcanic eruption. New mineralogical (QAPF) and geochemical (TAS) investigations of the Stolpen lava samples reveal that the Stolpen Hill is not built by a basalt, but basanite trending toward a nephelinite, following the current nomenclature. Microscopic analyses also provide evidence of magma mingling, such as nephelinite melt enclaves, and show local lava contamination by the country-rock granodiorite, also supported by the presence of quartz xenocrysts. The combined field observations, petrography and geochemical data indicate overall heterogeneity of the Stolpen lava. Therefore, the Stolpen Castle Hill is not-in a scientific sense-a suitable type locality for either basalt or basanite. However, the outcropping volcanic rocks as well as their scientific and historical importance undoubtedly give relevance to Stolpen as a type locality for basaltic volcanic rocks in a broader sense.
Sunburn texture, a white-grey mottling of dark grey basalts, is frequently found in outcrops around the world. The sunburn basalt is more fractured and less resistant than the fresh one, and so its identification is of interest in all types of basalt quarries. Three localities were chosen with different types and shapes of volcanic bodies in the Lausitz Volcanic field in the border area of the Czech Republic and Germany (Borská Skalka, Mittelherwigsdorf, Wittgendorf) in order to characterize sunburn lavas and study the petrographic and magnetic changes between fresh massive and sunburn basalts. The sunburn basalts have cracked, disrupted outcrop surfaces where white-grey spots are observed. As seen in thin section, the spots contain mainly analcite, the mineral that is responsible for the sunburn effect. The appearance of analcite is indicated by the enhanced amount of Al and Na. The whole rock is affected by many cracks that are filled with limonite. Limonite also infiltrates the sunburn spots, which are rich in microcracks. In the field were measured thirteen profiles perpendicular to the volcanic bodies in order to describe the magnetic properties in situ (magnetic field intensity, inclination and declination values). The intensity of magnetic field and its azimuth change with the distance from the rock. A major change occurs c. 130 cm away from the wall, which implies that the major cause is at some distance. The changes in magnetic inclination were negligible with the exception of the Borská Skalka site, where standard deviation of the magnetic field was 29.3 µT, and the standard deviation of the azimuth 78.7° for the sunburn part. In the laboratory were measured natural magnetic resistivity (NMR) and isothermal remanent magnetization (IRM). The calculated REM (remanent magnetization) parameter averaged 2.0 % for the sunburn parts and 0.8 % for the massive basalt. The sunburn basalts are affected mainly by weathering and hydration that causes changes in petrophysical and magnetic properties. The most prominent changes in all the variables could be seen at Borská Skalka, whose top is exposed to lightnings. Because of the increased conductivity, the sunburn parts become preferred lightning paths and the initial magnetic properties of the basalt can be overprinted by the electrical activity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.