From a geological point of view, there are three main rock groups: igneous rocks (magmatic rocks), which are either intrusive (plutonic rocks) or extrusive (volcanic rocks), sedimentary rocks; and metamorphic rocks.
Intrusive rocks and Igneous rocks
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.
The types of rock and their composition, age, formation, fabric and stratigraphic characteristics allow geologists to draw extensive conclusions about the geological development of the Earth, as well as about the significance of rocks in relation to mankind, economic and cultural development and sustainability. Besides the difference between consolidated and unconsolidated rocks, two other properties have the most significant impact on viticulture:
- the rock’s fabric; and
- its mineralogical and chemical composition
The description of a consolidated rock’s fabric specifies whether it is coarse-grained or fine-grained, whether it is massively, coarsely or finely bedded, whether it is layered, foliated, fissured or fractured, and the degree of weathering. In the case of unconsolidated rocks, the fabric describes the grain size distribution and particle shape, the degree of rounding and any evidence of cementing. Another very important factor to consider is the pore content and the presence of clay minerals – the minute rock particles (< 0.002mm) with a large internal surface.
The individual grain size fractions
|boulders||> 20 cm|
|sand||0,063 – 2 mm|
|stones (pebbles)||> 63 mm|
|silt||0,002 – 0,063 mm|
|gravel (angular: grit)||2 – 63 mm|
|clay||< 0,002 mm|
In nature, different grain-size fractions are usually mixed together, such as in clayey silt or sandy gravel. There is rarely only one grain size present. The fabric of a rock has an impact on several factors, including the way the rock weathers, the capacity of a soil to warm up, the rooting depth, the balance between air, temperature and water, and the transfer of nutrients (which is linked to the content of clay minerals).