klasifikasi batuan vulkanik

Volcanic rocks 

Volcanic rocks have, by definition, an aphanitic groundmass but their classification is more complicated than that of plutonic rocks. As we have seen, the latter are classified according to the minerals present. This is often diffcult or impossible to determine for volcanic rocks. When it is possible a QAPF double triangle is used, but in many cases (e.g. when a rock is extremely fine grained) a chemical analysis is required and the so-called TAS classification is used (TA = total alkalies Na2O + K2O; S = silica). Many volcanic rocks are formed by explosive activity, resulting in the formation of fragmentary rocks – the so-called pyroclastic rocks (pyro = heat; clast = fragment). This is also taken into consideration in the naming of volcanic rocks.
Ultramafic volcanic rocks are extremely uncommon. Ultramafic plutonic rocks are formed by the separation of mafic minerals (commonly olivine and/or pyroxenes) from magmas. The magma itself is not ultramafic.

QAPF double triangle for volcanic rocks
This classification can only be used if the mineral mode can be estimated. In many cases it is based exclusively on the phenocryst phases present. The numbers of the fields are the same as those for the plutonic rock classification 

 
 

1. There are no volcanic rocks in this field.
   
2. The term alkali feldspar rhyolite corresponds to alkali feldspar granite.
   
3. Rhyolite is the volcanic equivalent of granite.
   
4. & 5. Fields 4 and 5 are covered by dacite which is the volcanic equivalent of both
   granodiorite and tonalite.
   

6. Alkali feldspar trachyte. With minor (5–20%) quartz the term quartz alkali feldspar trachyte
   is used; with minor foid (0–10%) the term e.g. nepheline-bearing alkali feldspar trachyte is
   used.
   
7. Trachyte is the volcanic equivalent of syenite. See point 6 for quartz- and foid-bearing varieties.
8. Latite is the volcanic equivalent of monzonite. See point 6 for subtypes.
   
   9. & 10. These fields contain the large majority of volcanic rocks – basalts and andesites. These are broadly the volcanic equivalents of gabbros and diorites respectively. Distinction can be made using the colour index (basalt has M > 35; andesite M < 35). The plagioclase composition is difficult to apply because andesites commonly contain phenocrysts of calcic plagioclase (labradorite or bytownite) and compositional zoning is commonly developed in the plagioclase in volcanic rocks. The TAS classification is very widely used for dark volcanic rocks (which includes basalts and andesites). 
    
   
   11. Phonolites are the volcanic equivalents of foid syenites e.g. nepheline syenites. In volcanic rocks leucite (K[AlSi2O6]) may be an important foid mineral.
       
   12. Tephritic phonolites are relatively rare.
       
   13. Phonolitic basanites and phonolitic tephrites are relatively rare. For distinction between
       them see point 14.
       
   14. Basanites and tephrites belong to this eld. ey are separated by the amount of olivine
       present. Basanite has > 10% olivine; tephrite has < 10% olivine. Because these rocks are very fine grained it is not possible to determine the amount of olivine optically (even with a microscope) and the amount of olivine is estimated on the basis of the chemical composition of the rock which is recalculated in terms of simple mineral proportions – this is called the normative composition of a rock (hypothetical mineral composition of a rock calculated from its chemical composition) and is not to be confused with its modal composition (volume % minerals actually present).
       
   15. This field is subdivided into three sub fields:
       15a: Phonolitic foidite e.g. phonolitic nephelinite 15b: Tephritic foidite e.g. tephritic leucitite
       15c: Foidite e.g. nephelinite
       

   
   Volcanic rocks commonly have holes formed by escaping expanding gas during solidification. These are called vesicles and the rock is vesicular. Vesicles are commonly called by later material; they are then referred to as amygdales and the rock is amygdaloidal. 
   
   
   

   Some special terms are used for glassy rocks:

   
   
   

   
   

   OBSIDIAN Massive glass. Commonly black. The most common type of obsidian is rhyolitic. 
   PERLITE Pearly grey glass with concentric curved fractures called a perlitic texture. 
   PUMICE Lightweight, highly vesicular, frothy, glassy rock. The most common types of pumice are rhyolitic or phonolitic. Pumice can usually float on water.

   
   
   
   Rocks intermediate between plutonic and volcanic
   The main subdivision of igneous rocks is into those which have crystallized slowly at depth (plutonic) and those which cooled rapidly at the surface (volcanic). Lava flows are, of course, volcanic, but the centre of a thick lava flow may cool so slowly that a phaneritic rock results. A hand specimen of the centre of a thick basaltic lava flow may therefore be identified as a gabbro. Some magmas cool at shallow depths and form rocks which can be considered as texturally intermediate between plutonic and volcanic. For example, feeders to lava flows may have glassy margins (called a chilled margin because it is formed by rapid cooling against cold wall rocks) but phaneritic centres. 
   
   
   
   Dykes and sills (these are dealt with later) are examples of minor intrusions that cooled near the surface of the Earth. The igneous rock types in minor intrusions are compositionally the same as those that cooled more slowly at depth, but will typically be relatively fine grained (e.g. 0.3–0.8 mm). This can be expressed by using the prefix “micro-” which refers to grain sizes intermediate between those of typical plutonic and volcanic rocks. Examples are microgabbro and microgranite. 
   
   There are some terms that are specifically used for rocks that are texturally intermediate between plutonic and volcanic equivalents. They typically occur in minor intrusions (most commonly in dykes and sills), near the margins of larger intrusions (which cooled relatively quickly) and near the centres of thick lava flows (which cooled relatively slowly). Some of these terms are:
   
   
   
   
   dolerite and diabase synonyms (originally English and American respectively) for microgabbro. These terms are very commonly used for dykes and sills of appropriate composition
   
   
   
   
   granophyre a granitic rock in which the groundmass quartz and alkali feldspar occur in a micrographic intergrowth. 
   
   
   
   aplite fine grained granitic rocks which typically occur as dykes, veins or patches in or associated with granitic intrusions. 
   
   
   
   
   

   lamprophyre this porphyritic, usually melanocratic, rock type usually occurs in minor intrusions. Feldspar/feldspathoids only occur in the groundmass; biotite ± amphibole are common; hydrothermal alteration of ma c minerals is widespread; calcite is a common primary mineral. Lamprophyres are geochemically distinct.
   
   
   
   
   

   lamproite similar to lamprophyre but with more extreme chemical composition. 
   
   
   

   
   

   kimberlite ultramafic minor intrusions which typically occur as pipes. Important minerals are olivine (largely serpentinised), Mg-rich mica (phlogopite), pyroxenes, garnet and carbonates. Kimberlites are formed at very high pressure and are the main source of diamonds.