Volcanic rocks
Their origin is connected with volcanic activity. They originate by crystallization from lava on Earth´s surface or from magma very close to the surface.
Volcanic rocks are named after Volcano – the Roman god of fire. Their mineral composition is very difficult to estimate by naked eyes. They are typical of aphanitic texture and their classification is somewhat more complex than that of plutonic rocks. If their mineral composition is known, the modal QAPF classification for volcanic rocks is used. If the mineral composition is impossible to determine, the TAS diagram based on chemical composition must be employed. Many specific types of volcanic rocks have their individual classification schemes, which are always discussed together with particular rocks included in this electronic Atlas.
Volcanic rocks are enriched in volatiles (CO2, H2O, SO2, F and Cl), which are liberated when pressure drops. This process is accompanied by the formation of typical porous and vesicular texture. Secondary infilling of the open cavities gives rise to almond-like (amygdaloid), and/or miarolytic textures. Volcanic rocks formed at surface exhibit fluvial texture reflecting the direction of lava flow. Spherolitic texture is diagnostic of more acidic effusive rocks. According to volumetric phase ratio between crystallized minerals and volcanic glass, one can distinguish hemicrystalline and glassy (hyaline = vitritic) structure, where the glass predominates. Volcanic rocks often contain larger porphyric crystals – phenocrysts embedded in finely grained matrix. This unequally grained – porphyric structure results from gradual crystallization of the ascending volcanic rock from depth and its rapid crystallization on surface. The rapid cooling on the surface may result in the matrix composed merely of glass. In this case, the rock acquires a vitrophyric structure. Additional specific types of structures in volcanic rocks are as follows: ophitic, pilotaxic, tholeiitic, and hyalopilitic.
Main and accessory minerals in common types of volcanic rocks are essentially the same as in the plutonic rocks. In contrast to them, however, the volcanic rocks contain also high-temperature feldspars, as for instance sanidine, accompanied by volcanic glass which is absent in plutonic rocks. Besides olivine, pyroxene and perovskite, ultramafic volcanic rocks contain also melilite. The only know volcanic carbonatite – natrocarbonatite – has specific composition. It consists of gregoryite and nyerereite phenocrysts, which occur together with fluorite and sylvite also in matrix. Apart from common accessory minerals (perovskite, aragonite, brucite, titanite, pyrochlore, periclase, kaersutite, spurite), natrocarbonatites contain also some exotic accessory phases, such as alabandite, galena, Mn-sphalerite, cuspidite-niocalite and Mn-monticellite. Mineral composition of alkalic volcanic and hypabbysal rocks (lamproites, lamprophyre, kimberlite) is very specific and variegated, and it is described in detail for individual rocks included in this Atlas.
Volcanic rocks can be subdivided in 6 major groups:
1. Volcanic rocks oversaturated with quartz: alkali feldspar rhyolite, rhyolite, dacite, quartz alkali feldspar trachyte, quartz trachyte, quartz latite, alkali feldspar trachyte, trachyte, latite, andesite and basalt. All these types are classified in upper triangle of the QAPF diagram for volcanic rocks and some also in the TAS diagram, such as basaltic andesite and picrobasalt.
2. Volcanic rocks undersaturated with quartz: foid-bearing alkali feldspar trachyte, foid-bearing trachyte, foid-bearing latite, phonolite, tephritic phonolite, phonolitic basanite, phonolitic tephrite, basanite, tephrite, phonolitic foidite, tephritic foidite and foidite. All these rock-types are classified in lower triangle of the QAPF diagram for volcanic rocks and some also in the TAS diagram, such as trachybasalt, basaltic trachyandesite, trachyandesite and trachydacite.
3. High-magnesium volcanic rocks: boninite, picrite, komatiite and meimechite. They are classified in the TAS diagram in combination with special diagram for high-Mg volcanic rocks.
4. Volcanic rocks with more than 50 vol. % of carbonates: natrocarbonatite.
5. Volcanic ultramafic rocks with more than 10 vol. % of melilite and foids: melilitite, olivine melilitite. If modal composition is known, they are classified in the modal classification for melilite-bearing volcanic rocks. According to the modal composition, they can be classified also in QAPF diagram for volcanic rocks, where they project in the foidite field. If this classification is used, they are named according to the dominant foid with the adjective melilitic, for instance melilitic nephelinite is the foidite with melilite, where the dominant foid is melilite. If the modal classification cannot be used, bulk rock chemical analysis projected in the TAS diagram must correspond to foidite.
6. Alkaline volcanic and hypoabysal rocks with specific composition and origin. They have individually tailored classification schemes based on mineral and chemical compositions, and/or textural signs:
- a. lamproite,
- b. calc-alkaline lamprophyres: minette, vogesite, kersantite, spessartite,
- c. alkaline lamprophyre: sannaite, camptonite, monchiquite,
- d. kimberlite.