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... facing this
fountain of burning stones that tears through the darkness,
the whole group realizes that they are experiencing unforgettable
moments.. .
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Volcanic rocks
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Stacks of lava flows
in Iceland (Dettifoss)
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Some concepts about rocks
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Oxygen, silicon, aluminum, etc.
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 Eight elements make up
99% of the Earth's crust and mantle. They form
the majority of rocks in the form of silicates. The chemical
arrangements of these eight elements, also called major
elements, on which their mode of crystallization
will depend, create an infinite palette of rocks. But these
assemblies will differ depending on the nature of the crust
in which they are found and the underlying mantle. Typically,
the continental crust
is rich in Si, Al, K, and Na
and poor in Fe and Mg, compared to the oceanic
crust, which is primarily enriched in Si,
Mg, Fe, and Ca. The mantle,
in turn, is characterized by a high
proportion of Si, Mg,
and Fe.
Abundance of elements in the
Earth crust
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The 3 main types of rocks
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A rock is an aggregation
of minerals formed by crystallization
during their solidification after fusion, metamorphism, or diagenesis.
Petrography (the study of rocks) classifies rocks into 3 families
depending on their formation process: petrogenesis. These are
distinguished as follows:
- Igneous
or magmatic rocks originating from the mantle. This
includes, of course, all surface volcanic rocks, called eruptive
rocks, but also intrusive
rocks formed from magma rising and cooling deep within
the crust without appearing at the surface (plutonism). Granite
belongs to this latter family.
- Sedimentary
rocks: they result from the breakdown, transport,
and deposition of surface materials from the lithosphere. They
are clastic rocks, which
include sandstones, clays, etc. Some sedimentary rocks also
come from biological or organic
processes (living organisms). In this latter category, you find
limestones, coal, petroleum... A physico-chemical transformation
(diagenesis and lithification)
will lead to a more or less advanced aggregation of the components.
- Metamorphic
rocks: these are all rocks (igneous, sedimentary,
or even already metamorphic) that have undergone physicochemical
changes, and therefore changes in structure and texture,
due to increased pressure and temperature related to tectonic
movements (burial, overthrusting, folding, exhumation) of the
Earth's crust or contact with igneous rocks. Schists, gneisses,
and marbles are metamorphic rocks.
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Based on the basic elements seen previously, rocks
are made up of the following chemical
combinations: SiO2, Al2O3, FeO, MgO, CaO, Na2O, MnO,
K2O, Na2O, etc. In magmatism, these oxides combine with each
other during the cooling of the solution to form, upon crystallization,
the minerals of the rocks. The average proportions (in %) of
the oxides of all known rocks are given in the table below:
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SiO2
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AL2O3
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FeiOi
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MgO
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CaO
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Na2O
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MnO
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K2O
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55,2
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15,3
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8,6
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5,2
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8,8
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2,9
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0,2
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1,9
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A siliceous element in the sedimentary (clastic)
phase: sand, here a volcanic sand (Stromboli Island)
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SiO2, silica,
the most abundant, is the basic element around which silicate
minerals develop (a kind
of polymer where elements other than silicon share oxygen atoms
with it, building geometric or crystalline networks).
Silicates account for 90% of minerals, the rest being metallic
oxides, sulfates, chlorides, carbonates, etc.
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The three main type of rocks illustration
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From Magma to Lava - The Different Volcanic
Rocks
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The solidification of magma: the crystallization
of minerals
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In the page dedicated to the formation of volcanoes,
we saw the processes of magma generation. During their ascent,
the lowering of temperatures and pressures, and the release
of gases, give rise to lava. During these transfers, minerals
form, crystallize, and clump together.
The rock, thus formed, will exhibit specific physico-chemical
characteristics in terms of density, grainy or smooth texture,
and lighter or darker color, depending on the range of minerals.
It is around the variation of minerals and their subsequent
chemical composition that a volcanic rock is described:
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- Mineral composition:
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Olivine, pyroxenes, amphiboles,
micas (biotite), feldspars (alkali and plagioclase), quartz,
feldspathoids, metallic oxides, etc.
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- Chemical composition:
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SiO2, AL2O3, Fe2O3, FeO, MgO, CaO, Na2O, K2O, TiO2 , MnO, P2O5, H2O
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The order
of mineral formation: the elements dissolved in the
liquid phase react with
each other during cooling.
These are chemical reactions
that build minerals in crystalline form. As the temperature
drops, the most refractory crystals (such as olivine, composed
of magnesium and iron) appear first, followed by progressively
less refractory ones like muscovite, whose potassium provides
weak bonds. Norman Levi Bowen,
the first scientist to lay the foundations of experimental
petrology in the 20th century, determined the sequence
of magma solidification reactions and the appearance of minerals
according to temperature.
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In Bowen's series, it
can be seen that olivine crystallizes
(solidifies) first,
followed by pyroxenes, amphibole, etc. At
the same time, the continuous series of plagioclases
begins, first calcium-rich, then calcium-sodium and
sodium-rich. Sodium-potassium feldspars appear last.
Around 800°C, everything is
crystallized. If silica remains, quartz will
appear. If it is deficient, then feldspathoids will
replace the feldspars.
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Molecular
structure of olivine: here forsterite
with a magnesium site. The magnesium positions can be
shared with iron. The same structure can also be found
with iron. In this case, the mineral becomes
fayalite (iron site).
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In the chapter dedicated to
the formation of the Chaîne des Puys, we will show
how essential the Bowen series is for understanding the general
mechanisms of magmatic differentiation
that occur in planetary volcanism and, in particular, those
behind the remarkable evolution
of the lavas of the chain, a true reference in volcanology,
from the source basalts to the trachytes. The eruptive dynamics,
the shapes of the volcanoes, and therefore the landscapes,
are the result of this.
The classification of volcanic rocks:
The mineralogical
or modal classification, which is based—after measuring
it (physical measurements)—on the quantity of main minerals
(QAPF or Streckeisen
diagram: Quartz, Alkali
Feldspars, Plagioclase,
Feldspathoids). Example:
a rock containing 50% quartz and 35% alkali feldspars is a rhyolite.
A rock containing 0% quartz and feldspathoids, and 80% alkali
feldspars is a basalt or andesite. To distinguish between them,
it is necessary to use the color index and the mass percentage
of silica. It can be seen that for basic (or mafic) eruptive
rocks, the Streckeisen diagram must be supplemented with other
criteria.
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QAPF modal classification
for volcanic rocks
(according to Streckeisen, 1978). The vertices of the
double triangle are: Q = quartz, A = alkali feldspars,
P = plagioclases, and F = feldspathoids.
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Modal QAPF classification
of plutonic rocks
(according to Streckeisen, 1976). The vertices of the
double triangle are Q = quartz, A = alkali feldspars,
P = plagioclases, and F = feldspathoids
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Chemical
classification: it is based on the quantity of chemical
elements contained in all the minerals of the rock, generally
in the form of oxides. Representative diagrams of one or several
chemical elements can then be drawn relative to another to
perform classifications or observe the evolution of a process.
For example, K2O versus SiO2
for calc-alkaline lavas, MgO vs SiO2
and CaO vs SiO2 to study differentiation. The most
common is the TAS diagram (Total Alkali
Silica, that is Na2O+K2O vs SiO2). Rocks are characterized
by specific zones on this diagram. (Example: a rock containing
6% alkalis and 49% silica is a trachy-basalt. More precisely,
if it contains 5% Na2O, it will be a hawaiite).
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Classification géochimique
des roches volcaniques
TAS (d’après Le Bas et al., 1986).
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It is possible to show in
the diagram the notable evolutionary
series of lavas during their stay in magma
chambers. For simplicity, this evolution is linked to
the nature of the initial magma,
which depends on the geodynamic context, time,
and the conditions of 'storage'
in magma chambers
or reservoirs. In
the diagram, as an example,
we have included the alkaline
and moderately alkaline series of intraplate
or hotspot volcanism, the typical calc-alkaline
series of subduction zones, and the tholeiitic
series of mid-ocean ridges.
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Classification
by mineral proportion: when one knows the quantity
of minerals that make up the rock through modal or chemical
analysis using the CIPW standard.
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Exemple d'une classification
minérale d'une série calco-alcaline typique
d'un volcanisme de subduction.
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Another simplified classification
of volcanic rocks: :
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On this page, we discussed the major
chemical elements that make up and characterize primarily
volcanic rocks. However, and this could be the subject of another
page on this site, geologists also use:
- isotopes:
they are used to characterize the nature of the source mantle,
to assess the rate of crustal contamination, etc., but also, with
radiogenic isotopes,
to date the rock.
- trace
elements: although in small quantities, from a few
parts per billion to a few parts per thousand, they are useful
for measuring melting rates, evaluating conditions, identifying
the geodynamic context, understanding the nature of the mantle,
etc. Among others, one can mention lead Pb, strontium Sr,
niobium Nb, uranium U,
or the rare earth elements
including ytterbium Yb, neodymium
Nd, etc.
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Some samples of volcanic
rocks according to the classification
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