Volcanic rock

The volcanic rocks are magmatic rocks, resulting from the fast cooling of a Lave, magma arrived at surface, from where their other names of extrusives rocks or effusive rocks .

Classification

When one hears volcanic rock one thinks, in general, with a dark rock as one meets some in Auvergne or in many areas of the world. These dark rocks are called " Basalte" in the broad sense. The volcanic rocks recover in fact a much larger variety, from where need for classifying them. Thus basalts train in fact a family of rocks to the varied geochemical characteristics and many volcanic dark rocks are not therefore basalts. It should be added that many volcanic rocks are clear or take varied colors, far from the idea that one is done some at the beginning.

Vis-a-vis several samples the rocks can be classified according to various criteria: chemical composition, mineralogical, mode of installation, texture, type of deposit… Classifications the most used today are the mineralogical classification and the chemical classification . The latter is very practical because it makes it possible to treat all the magmatic rocks (plutonic or volcanic) in the same way. The passage with mineralogical classification is easy thanks to the standard (indeed, contrary to the plutonic rocks, the mineralogy of the volcanic rocks does not allow an direct access with mineralogical classification because of the vitreous phase; it is thus necessary to use a skew called “calculation of the standard”) described further. One being the reflection of the other, they are thus equivalent in their employment. The tables presented often use mineralogical classification because it is more expressive than the chemical one and makes it possible to make big classes of rocks, but it is necessary to be conscious that to each mineralogical border corresponds a chemical border. It should be noted that mineralogical or chemical classification makes it possible to distinguish the origins and major evolutions from the magma, independently of its mode of installation. On the contrary, the classification based on the volcanic facies will inform us about his mode of installation on the surface. Thus a rhyolithe will be able to be presented in the form of a viscous lava flow, a deposit of welded pumiceous pumice tuff (ignimbrite), or of an obsidian casting without any minerals (glass volcanic). All that results from process of different surface installation but the magma of origin is of the same chemical composition each time, therefore a mode of in-depth formation similar.

Other types

Obsidian
Pounce
Ignimbrite
Domite

…

Classification of the UISG according to the principles of Streckeisen (mineralogical and chemical)

To classify the volcanic rocks or effusive (microlithic structure) one can use the same principle as that of the magmatic rocks of depth (known as plutonic) based on the presence or not of certain major minerals which reflects the chimism of the rock (silica saturation, alkalinity…). But contrary to these last the speed of cooling allows often only to crystallize small crystals, invisible with the naked eye, to see only volcanic glass. It is the mésostase. The mésostase is often accompanied by phenocrysts, but those represent only one small portion of the rock. To determine the rock precisely it is necessary to call upon a chemical analysis (one will explain all the same later on how to approach the type of rock by looking at phenocrysts when they are visible). The rock will be placed then in its petrographic field thanks to the calculation of the standard. Indeed the classification of the magmatic rocks being based on the presence and the exclusion of major minerals, it is necessary to reconstitute the mineralogy of the rock as if it had entirely crystallized. For example if we take a basalt tholeitic, which belongs to the rocks supersaturated out of silica, we never find quartz phenocrysts in the rock because silica with tendency to remain in the mésostase, it is only with the calculation of the standard which reveals virtual minerals that we can correctly determine the rock. The principal minerals met in the volcanic rocks and which are used to establish classification are the following:

quartz (SiO2);
Feldspar S alkaline, Orthoclase mainly: (If 3Al) 08 K
Plagioclase S ranging between the pole White feldspar (sodic): (If 3Al) 08 Na and the calcic pole Anorthite: (If 2Al 2) 08 Ca
for the Plagioclase S, An<50 mean that the rock is more sodic than calcic (i.e. is that plagioclases contained in the rock have a composition closer to the Albite), An>50 the reverse (i.e. a composition closer to the calcic pole, the Anorthite);
Feldspathoïde S (under-saturated rocks);
Olivine S, Pyroxene S and Amphibole S constitutes the ferro-magnesian minerals which bring their color sinks with the roche.

It should be noted that 3 large major fields are distinguished, which originate in the degree of silica saturation of the rock. Thus a rock known as will be supersaturated out of silica if it expresses quartz (in the form of phenocrysts as in the rhyolithe or only in virtual or normative form as in basalts tholeitic). A rock known as under-will be saturated if it contains Feldspathoïde S extremely overdrawn minerals out of silica. Between the two the rocks known as saturated range which will contain neither quartz nor feldspathoides (within the limit of a tolerance of 10%) but only of mineral silica feldspars much less overdrawn than the feldspathoïdes.
It should be noted that no rock can contain at the same time feldspathoïdes and quartz because it would react to give feldspars, it is what makes the effectiveness of this classification thanks to the mutual exclusion from these two minéraux.
One should not either confuse silica saturation and silica content. Thus a rock with silica 60% (average rate for a rock) will be able to belong to the three categories of enumerated rocks. Indeed the term of saturation expresses the high content in silica compared to the alkaline ones and not its absolute content. A rock very rich in alkaline will be able not to express quartz (thus not to be supersaturated) while having much silica, on the other hand if the rock is very low in alkaline quartz can appear as of low levels in silice.
These three enumerated fields are then recut according to their degree of differentiation which is parallel to the content of ferro-magnesian. This will be detailed in the chapter devoted to the series volcaniques.
The classification of the rocks with vitreous structure or cryptocristalline can be done only starting from the study of their chemical composition.

The first 2 lines of the table above correspond to rocks Leucocrate S (clear, white rocks with clear gray), 3rd with leucocratic rocks or Mésocrate S (mediosilic rocks, gray), 4th and 5th with rocks Mélanocrate S (dark rocks, dark gray with black).

Basalts and the andesites constitute more than 95% of the volcanic rocks.

Chemical classification alkaline/silica

One finds the same rocks naturally as previously, except that the fields are characterized by their chemical composition and not by the presence or not from virtual normative minerals. By directly pointing the composition out of silica and alkaline, one will find directly the field corresponding to the rock analyzed without the calculation of the standard. It is also this kind of diagram which makes it possible to see the evolution of the volcanic series.

Classification according to the mode of installation

The installation of the volcanic rocks can be done in a way varied according to the eruptive dynamics under which they are installation. One will distinguish mainly the lava flows which form massive rocks which will be distinguished according to their texture. On the other hand, at the time of the explosive phenomena, the installation is done at the solid state forming of will téphras which will be classified rather according to their granulometry.

Texture lava

It generally microlitic, is characterized by the presence of microlites, fine microscopic rods generally made up of plagioclases. The rock bottom consists of an amorphous paste (volcanic glass or mésostase generally structure cryptocristalline).

Phenocrysts, crystals visible with the naked eye, are more or less frequent according to the composition of the rock.

Certain rocks, like the obsidian, are entirely vitreous.

Classification of Téphras

August 1st

Principal volcanic rocks

Some examples of volcanic rocks:

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