La Chaîne des Puys

... face à cette fontaine de pierres incandescentes qui déchire les ténèbres, tout le groupe comprend qu'il vit là des instants inoubliables..

De l'Eden à l'Enfer BD

Volcanic rocks

Sommet Lasssolas

The "Puy de la Vache" and its "lava Gorilla"


Some notions : elements of the Earth crust, the rocks

Oxygen, silicon, aluminiun, and so on...

Eight elements constitute to 99% the earth's crust. It make major the part of the rocks in the form of silicates. Chemical arrangements of these huits elements, of which will depend their mode on crystallization, will establish an infinite pallet of rocks. The basalts family accounts for 95% of the crust, of which they ensure the bases.

Abundance of the elements of earth's crust GIF

Eléments de la croûte terrestre


The three principal classes of the rocks

A rock is a mineral agglomeration formed by crystallization during their solidification. Petrography (Science of the rocks) class rocks in 3 families dependent on their process of formation: the petrogenrating. We distinguishe :

- igneous or magmatic rocks resulting from the Mantle. We find there naturaly all the volcanic rocks called eruptive rocks, and also the intrusive rocks born from coolings in-depth, without appearance on the surface, of the magmatic increase (plutonism). The granite forms part of this last category

- the sedimentary rocks: they result from degradation, transport, the deposition, all materials of surface of the lithosphere, follow-up of a physicochemical transformation which will lead to a more or less thorough agglomeration of the components.

Two under families define the many recognized formations. It is the detrital rocks among which are classified the likings, clays, and the chemical rocks coming from chemical modifications in solution (water), or of biological processes (living organisms). In this last category, one meets there for example limestones, coals, oils...

- the metamorphic rocks: in fact there are all the rocks (eruptive, sedimentary or even already metamorphic) which have undergoes a modification of structure and texture by increase of pressure and temperature related on the tectonic movements of the Earth crust or the contacts with igneous rocks. The schists, the marbles are metamorphic rocks.

If the metamorphic and sedimentary rocks constitute the majority of the surface of the continents, the igneous rocks, by their volume, constitute the bases of them.

Cycle des roches


Silica: principal element

From the basic elements seen previously the rocks will be consisted of following chemical associations : SiO2, ALÒ3, FeiOi, MgO, CaO, NaÒ, MnO, KÒ, etc. These oxides will join between them during the cooling of the solution to form while crystallizing, the minerals of the rocks. The average proportions (in %) of oxides of all the known rocks are given by the following table:

















SiO2, the silica, most abundant, will be the basic element around whose will develop the silicated minerals. The latter account for 90% of minerals, the remainder being metallic oxides, the sulphates, the chlorides, the carbonates, etc...

Sable volcanique

A siliceous element in metamorphic phase: sand, here a volcanic sand (Stromboli Island)


Les 3 grandes familles de roches

3 principal rocks classes samples

From Magma to Lava - The different volcanic rocks

The solidification of the magma: the crystallization of minerals

We call magma materials depths of the Earth crust and the Mantle. In these zones, the temperatures (of 650 to 950°C) the high pressures (several kilobars) contribute directly to the fusion of materials. The melting point also depends on the composition of the magmas, in particular the presence of volatile element such as the gases (dioxide carbon, hydrogen, chlorine, fluore, sulphur).

In the increase and the lying of the magmas, the drop temperatures and pressures, the liberation of gases, cause the solidification of materials, giving rise to the lava. During these transfers, the minerals are formed, crystallized and agglomerated. The rock, thus formed, will present a more or less grained aspect, (proportion of phenocrysts and microlites), more or less clear (proportion of silica).

It is by the variation of following minerals and their chemical composition that is described a volcanic rock:

- the mineralogical composition:

l'olivine, le quartz, les feldspaths (alcalins et plagioclases), les feldspathoides, les pyroxènes, les amphiboles, les micas.

- the chimical composition:

SiO2, AL2O3, Fe2O3, FeO, MgO, CaO, Na2O, K2O, TiO2 , MnO, P2O5, H2O

- the classification of the volcanic rocks: it is the proportion of silica contained in all the minerals which differentiates names. This proportion east, let us point out, very significant the bus if it determines, in addition to the aspect of the rock, but also the behavior of the volcano (explosive or effusive), dependent on the fluidity of the magma. The following table is used as reference:

Classification des roches volcaniques


Les roches volcaniques

Some volcanic rocks samples according to the classification

(C) Copyright June 2001 Photos et Tableaux: Bernard Dichamp