Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Lucie Nováková, Petr Schnabl, Joerg Büchner

The characterization of sunburn basalts and their magnetic and petrographic properties

Journal of Geosciences, volume 63 (2018), issue 4, 333 - 344

DOI: http://doi.org/10.3190/jgeosci.274

Sunburn texture, a white-grey mottling of dark grey basalts, is frequently found in outcrops around the world. Sunburn basalt is more fractured and less resistant than fresh basalt, 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.