The three major rock groups

There are three superordinate groups of rock differentiated by geology: solidification rocks or igneous rocks occur as intrusive rocks (plutonic rocks) or as extrusive rocks (volcanic rocks), deposits or sedimentary rocks, alteration or metamorphic rocks.

A picture shows Volcanic tuffs in Kapfenstein
Volcanic tuffs in Kapfenstein (Vulkanland Steiermark), © M. Heinrich

Intrusive rocks and Igneous rocks

Intrusive rocks and metamorphic rocks are also grouped under the term crystalline rocks. Crystalline rocks are commonly summarised within the wine community under the term ‘Urgestein(= primordial rock). Urgestein is a term that alludes to the great age of rocks. The term is not precise from a geological point of view and does not do justice to the diversity of the rocks with reference to their distinct properties – such as chemistry, mineral composition and texture – that are relevant to viticulture. Igneous rocks are rocks that have solidified from a melt. Solidification of intrusive rocks occurs within the Earth while that of the extrusive rocks takes place on the Earth’s surface, whether on land or in the sea. The chemical and mineralogical composition of each may be very different: intrusive rocks are rather coarse-grained, for instance granite; extrusive rocks are rather finely grained, rarely glassy, one example being basalt.

Conglomerate of the Zöbing Formation
Conglomerate from the Zöbinger-Formation with characteristic red feldspars and clasts at Heiligenstein (Kamptal), © M. Heinrich

Sedimentary rocks

Sedimentary rocks occur on the Earth’s surface as erosion products of other rocks (clastic sediments) or as precipitates from water or by organisms (chemical and biogenic sediments). Clastic sediments are differentiated according to grain size in boulders, stones, coarse rock debris and gravel, sand, silt and clay. The process of consolidation is called diagenesis: then one refers to breccia, conglomerate, sandstone, siltstone and mudstone. Limestones are formed from the frameworks and skeletons of organisms. Upon consolidation, limetsone can be converted to dolomite from input of magnesium.

A picture shows Crystalline gneiss with amphibolite blocks from Achleiten (Wachau)
Crystalline gneiss with amphibolite blocks from Achleiten (Wachau), © M. Heinrich

Metamorphic rocks

Metamorphic rocks are formed when rocks in a solid state are subjected to variations in temperature and pressure conditions. This process is usually associated with some form of deformation, such as foliation or alteration in texture, related to changes in the water content of the rocks. The latter may result in alteration or formation of minerals. Granite is altered to orthogneiss, limestone to marble, quartz sandstone to quartzite. Basalt may be altered to greenschist, amphibolite and, at very high pressure eclogite. Clayey and sandy sediments with increasing degrees of metamorphism are altered to slate, phyllite, mica schist and paragneiss and, at temperatures above 700°C, socalled migmatites.

Rocks are part of a cycle that repeats itself over and over: a topographical relief may be formed due to processes such as orogenies, volcanism or basin subsidence. Then rock material is eroded from the relief. This is deposited elsewhere as sediment. In the course of a subsequent orogeny sediments and their basement subside, become metamorphosed and undergo partial melting. As a result of uplifting processes a relief is again formed and the cycle repeats itself.

Important rock properties for winegrowing

Type and composition, age, formation, structure and bedding allow geologists to draw far-reaching conclusions not only regarding the geological development, but also in relation to the significance of rocks for mankind, economic and cultural development and sustainable public services. In addition to the distinction between consolidated and unconsolidated rocks from among the variety of parameters used to determine rock properties the following are the most important for viticulture:

  • structure or texture
  • mineralogical and chemical composition

In describing the structure of a rock, specification is given of whether a consolidated rock is coarse-grained or fine-grained, whether it is massively, coarsely or finely bedded, layered, foliated, jointed, fractured, weathered or dehydrated. In unconsolidated rocks it is important to pay attention to the grain size distribution and the particle shape, the degree of rounding and possible compaction. A very important factor to consider is the content of pore space and of the smallest rock particles (< 0.002 mm) with a large inner surface, the so-called clay minerals.

The individual grain size fractions

boulders> 20 cm
Sand0,063 – 2 mm
stones (pebbles)> 63 mm
silt0,002 – 0,063 mm
gravel (angular: grit)2 – 63 mm
clay< 0,002 mm

In nature, various grain-size fractions are usually mixed together, such as in clayish silt or sandy gravel. Structure has an impact on the weathering behaviour of the rocks, the heating and root penetration capacity of the soil, the air, temperature and water balance, and through the content of clay minerals, on the transfer of nutrients.