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On these cursed
slopes, which the ancients populated with spirits and
malevolent ghosts, we believe we can hear the howling
of the damned...
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Volcanoes building
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Mount Shasta (California-USA)
: an almost perfect strato-cone
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Volcanism and tectonics: an intimate
marriage
Geodynamic contexts
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 In the chapter on tectonics,
we saw that there is a close connection between the structure,
the dynamics of the plates, and volcanoes.
It is observed that the major volcanic chains on Earth are located
at plate boundaries, in areas of accretion and subduction. However,
plates can also be "punctured" in their
middle. The following volcanic families or geodynamic
contexts can then be distinguished :
- The
accretionary volcanism of oceanic ridges is submarine
and very subtle, as the water pressure limits the gas release
from the magmas. However, it can be above water, as in Iceland
or in the East African Rift.
- Subduction
or convergent plate volcanism, often explosive
and dangerous. This characteristic is linked to the
complex composition of the magmas (elements from the subducted
oceanic plate, water).
chargés
en gaz.
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Carte des Volcans
(GIF 26 Ko)
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-Intra-plate
volcanism, also called hotspot volcanism : it occurs
in the middle of a plate,
sometimes forming chains of volcanoes like in Hawaii. Intra-plate
oceanic volcanism is distinguished from intra-plate
continental volcanism.
The following
diagram summarizes the different contexts, also called
geodynamic contexts, in
which volcanoes form:
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The mantle's materials, actually rocks called peridotite
or lherzolite, remain solid despite the high temperature
due to pressure. Indeed, geotherms,
or curves that represent the variation of temperature with depth
inside the Earth, do not exceed the melting range. However,
under certain conditions, a drop in
pressure (adiabatic decompression) or an
increase in temperature,
as well as mantle hydration, can trigger melting
reactions among the constituent minerals of peridotite. It is
important to note that the melting
remains partial, with only 5 to 20% of the material
actually melting. (See below for the melting conditions of peridotite
in temperature/pressure diagrams
depending on the geodynamic context).
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Melting of mantle peridotite
by decompression in
an accretionary ridge or rift context (thinning of the
lithosphere and upwelling of the oceanic geotherm). Case
of a decompression from 35 kbars to 8 kbars. This situation
is also possible in the case of continental rifts (Limagnes
for example).
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Melting of mantle peridotite
due to increased temperature
(hotspot) in an intraplate continental or oceanic
context. Case of a temperature increase of 400°C under
an oceanic plate and 600°C under a continental plate.
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Melting of mantle peridotite
by hydration (water
supplied by the subducting plate) in a subduction context
(shift of the partial melting zone along the continental
geotherm). Case of the burial of an oceanic plate with
the onset of melting at around 100 km depth.
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Mécanisme de remontée des
magmas
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 The rise occurs due to density
differences. Magma reservoirs
form along the faults of the Earth's crust. Magma near the surface
can, under the pressure of the gases it contains, erupt—that's
an eruption—and give rise
to volcanoes. The lava, degassed magma,
then spreads in the form of pyroclastic
materials such as ash and blocks, and flows.
But magma can also spread within
the crust and cool without ever reaching the surface. In this
latter case, it cools to form granite
foundations, which will later be exposed by the erosion
of the overlying rocks, called plutons, which will later be
exposed by the erosion of
the overlying rocks, or remobilized
and transformed (metamorphism) by tectonics..
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Les différents dynamismes volcaniques
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 The caracteristics of volcanoes depends on the
characteristics of the magmas that reach the Earth's surface.
The classification of eruptive dynamics
refers to volcanoes that have been studied based on the materials
they emit. It takes into account the quantity of
gas contained in magmas and their temperature.
The nature of magmas, depending on their origin and their transformation
(differentiation) in crustal
magma reservoirs, determines their dynamism. Fluid magmas, poor
in gas and very hot, generate effusive
volcanoes with abundant lava emission (red volcanoes).
On the other
hand, viscous magmas that
are heavily charged with gas produce
explosive volcanoes (gray volcanoes).
If an eruption builds only a single
structure over a short period of time (a few months or years)
and then stops, it is referred to as a monogenetic
volcano. This is the case with the volcanoes of the
Chaîne des Puys. In the
next chapter, we will describe the structure of Strombolian
and Pelean dynamics, as the other types are not represented
in this formation.
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If eruptions occur repeatedly in the same location
over several hundreds of thousands,
or even millions of years, magma accumulates and
builds large polygenetic
structures called stratovolcanoes.
In this context, we distinguish between shield
volcanoes, typical of effusive emissions from hotspots,
made up of stacked lava flows and pyroclastic projections (Hawaii,
Réunion, etc.), and stratovolcanoes
or volcanic complexes, consisting of juxtaposed and
superimposed structures ranging from effusive to explosive in
nature (Popocatépetl, Sinabung, Mount St. Helens, Cantal, etc.)
as well as pyroclastic flows and breccias. These large structures
are often the site of cataclysmic phenomena
such as the collapse of magma chambers forming calderas
(Crater Lake, Mont-Dore, Santorini, ...) and destabilizations
leading to debris avalanches
(Mount Shasta, Cantal, Nevado de Toluca, ...). It should be
noted that for many stratovolcanoes, the initial phase may have
seen the formation of monogenetic volcanoes.. It should be noted
that for many stratovolcanoes, the initial phase may have seen
the establishment of monogenetic volcanoes.
Crater Lake caldera,
Oregon, USA, Image © Bernard Dichamp
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