The three main rock groups

Intrusive rocks and metamorphic rocks are also grouped under the term “crystalline rocks”. Crystalline rocks are commonly referred to within Austria as “Urgestein” (primary rock). Describing these rocks as “Urgestein” is frowned upon from a geological point of view because although it reflects the extreme age of these rocks, it neglects their diverse properties – such as their chemical and mineralogical composition and fabric – all of which have an impact on viticulture.

Igneous rocks are those that have solidified from a molten mass. Solidification of intrusive rocks occurs underground, while extrusive rocks solidify on the Earth’s surface, either on land or in the sea. The chemical and mineralogical composition of intrusive and extrusive rocks can differ greatly – the former tend to be more coarse-grained, e.g. granite, while the latter are more finely grained (and, in exceptional cases, glassy), e.g. basalt.
 

Sedimentary rocks are formed on the Earth’s surface as a result of the erosion of other rocks (clastic sediments) or as sedimentary deposits from water, helped along by living organisms (chemical and biogenic sediments). Clastic sediments are differentiated according to grain size and classified as boulders, stones, coarse rock debris, gravel, sand, silt or clay. The consolidation process is called diagenesis and results in the formation of breccia, conglomerate, sandstone, siltstone or claystone. Limestone is formed from the skeletons of organisms. During consolidation, in the presence of magnesium, limestone can transform into dolomite.

Metamorphic rocks are formed when rocks in a solid state are subjected to a change in temperature and pressure. This process is usually associated with some form of deformation, e.g. foliation, and a material transformation, related to changes in the water content of the rock, which may result in the formation or transformation of minerals. Granite transforms into orthogneiss, limestone into marble, and quartz sandstone into quartzite. Basalt can transform into greenschist, amphibolite or, when subject to extreme pressure, eclogite. Clayey and sandy sediments transform into (in order of the extent of metamorphism) slate, phyllite, mica schist, paragneiss and, at temperatures above 700°C, migmatite.

Rocks are part of a perpetual cycle. A topographical relief may be formed as a result of orogeny, volcanic activity or basin subsidence. Rock material is then eroded from the relief and deposited elsewhere as sediment. A subsequent orogenic process causes the sediments and their substratum to subside; they become transformed and partially molten. Tectonic uplift causes the formation of another relief and the cycle repeats itself again.