Andesite - Rock of the Year 2020


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First described by the German geologist Christian Leopold Freiherr von Buch (1774-1853), the location of the rock in the Andean mountain range in South America gave the Andesite its name. An alternative, somewhat rarer name for the rock is Islandite - named after the deposits of the rock on the northern European island of Iceland. The term Porphyrite was also used until the last century as a synonym for a volcanic rock of mostly andesitic composition.

The volcanic rock, which is not only virtually common, shapes the rather explosive volcanism at the earth's plate boundaries through relatively viscous magma. This creates mountain ranges, such as the Andes, or island arcs (such as Japan or the Philippines). Among other structures, layered and stratovolcanoes are formed - beautiful, but dangerous. The rock thus plays a very decisive role for the "Ring of Fire" along the plate boundaries around the Pacific and is distributed widely on site.

The intermediate vulcanite, which consists of approx. 57-63 percent by weight of SiO2, often has a porphyric structure with a fine crystalline matrix. Fascinating are the extensive voids, which can be filled with e.g. zeolite. The magmatic rock of extrusive origin is found as solidified rock at or near the surface of lava flows, lava domes or corridors.

International examples of andesitic lava generating volcanoes are the Merapi in Java, the Fuji in Japan or the Hekla in Iceland.

On a national level, an equally widespread occurrence can be found in the Rotliegend deposits of the Saar-Nahe depression in Rhineland-Palatinate, Eifel, Vorerzgebirgssenke (Saxony and Thuringia), near Neustadt am Rennsteig (Thuringia), NW Saxony, Doehlen depression (Saxony), Hunsrueck, Thuringian Forest, Halle, and Flechtinger Hoehenzug (Saxony-Anhalt).

The fine- to medium-grained, dark rock is a mass raw material in Germany. Due to its hardness and toughness combined with extraordinary weather resistance, the stone is important for many areas of the construction industry, road and path construction as well as concrete and asphalt production.

For wall and floor coverings, statues and monuments the "beautiful" andesites were used for a considerable time. In representative buildings, such as the Westminster Cathedral in London, andesite is embedded in the mosaic floors.

Nowadays, andesite is used almost exclusively in the construction industry, where it is processed into crushed products, e.g. track ballast, road gravel, hydraulic construction materials and high-grade chippings. There are currently 17 andesite quarries in operation in Germany, in which the rock is mined through multiple-row blasting.

A regional case study of andesite deposits is the Wildfrauhaus near Ulrichstein-Wohnfeld (R: 35 09 550, H: 56 03 555, also ref. Geotope-Flyer Ulrichstein) - it is assumed that the rock cliff of tholeiitic andesite served the Celts as a form of sacrificial site.

The rock formations on a hilltop protrude up to about 8 m from the surroundings, the light grey rock is light-brownish on the surface due to weathering and therefore already striking in comparison to the mostly dark grey-black volcanite in the Vogelsberg.

The basaltic andesite (Schottler 1924: acidic Trapp basalts) has extruded into the basanite as a vein. The total of three rock formations suggest that the corridor ascended along a roughly north-south directed fissure. Thus the location with a well opened tholeiitic passage is a special feature.

The completely crystallized rock at the site is to be understood as the residue of a melt, it consists mainly of intermediate feldspars (plagioclase), as well as ortho- and clinopyroxene, ore, olivine, quartz, amphibole and biotite.


Sources:

Berufsverband Deutscher Geowissenschaftler e.V. (BDG)

Hessisches Landesamt für Umwelt und Geologie (2009): Der Vogelsberg - Geotope im größten Vulkangebiet Mitteleuropas. Wiesbaden.

Lapp, M. und P. Müller: Andesit - Gestein des Jahres 2020, in GMIT (2019), Nr. 78, S. 30-31.