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Puy : Crater or volcanic promontory
in Auvergne.
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Volcanic structures - Various dynamisms
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The puy des Goules:
a perfect strombolian cone
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Dynamims types of eruptive structures seen
in the "Chaîne"
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  Eruptive
dynamism refers to the behavior of an eruption that
determines the morphology of the outpourings
of products or effects on the surface. An effusive
dynamism will produce accumulations and flows (cones
and lava flows) different from an explosive
dynamism that will shape the landscape with craters
or other excavations (maar craters,
for example, blocks and pyroclastic deposits, pumice),
as well as dynamisms with viscous and
cool lavas that will spread with difficulty (domes).
The shape of volcanoes results directly from this.
The Chaîne des Puys offers a particularly
interesting range of volcanoes almost
monogenic, to study. While the classic cone
shape is well represented, the less common domes
sometimes form complex structures. The explosion maars,
born from the meeting of magma and water, complete the collection.
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The most widespread dynamism: Strombolian
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 From the name of the Sicilian
volcano which is characterized by this behavior (and
not by morphology, Stromboli being a stratovolcano). In this
type of dynamism, the liquid lava is
fragmented and propelled by the expansion of the
gases from the magma, through the feeding chimney, in incandescent
showers. These fragments, slag,
but also called clasts,
fall around the exit point and build a cone of pyroclasts.
 At
the base of the cone, completely degassed, the liquid lava will
produce flows. Very fluid
due to the high temperature (around 1000°C), they spread into
the surrounding area, creating outpourings
several kilometers long and a few tens of meters thick, before
cooling, often into prismatic columns
called 'orgues.' Today,
the old flows give rise on the surface to these chaotic, wooded
landscapes called 'cheires.'
Clasts consist of volcanic
bombs for the largest ones (a few tons), down to
lapilli and ash (a few millimeters)
for the smallest (see classification). A sort of 'basalt foam,'
they have a spongy appearance and are exploited under the name
pozzolana. The term 'volcanic
bombs' comes from the fact that, shaped by the air while still
pasty during their ejection, they display remarkable aerodynamic
forms. The extraction of pozzolanas, which is highly
regulated, allows, by exposing the successive layers of deposits,
to trace the history of the volcano's eruption (Lemptegy).
 After
the eruption, a cone with a summit
or off-center crater remains, referred to as "drunken".
The crater can be very pronounced, completely
opening the structure, as at the Puys of La Vache
and Lassolas, or the Puy de Louchadière. A resumption of activity
can generate a double cone
(Puy de Côme).
The emitted products range from
basalts and basanites
to trachy-andesites, including
trachy-basalts and basaltic
trachy-andesites, with colors from dark to light
gray for the flows. The scoriae are of the same composition,
but the iron they contain oxidizes during aerial projection
while still at a high temperature, giving the Strombolian deposits
their red color.
Twenty
per cent the Chain's buildings are 'Strombolian' cones
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Classification of pyroclasts according
to their size:
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A dynamism with surprising buildings:
sub-Plinian to Plinian
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The sub-Plinian
to Plinian dynamism formerly
called Péléan (from the
name of Montagne Pelée in Martinique, but the analogy stops
there because the dome of Montagne Pelée is an andesitic dome
at the top of a strato-volcano) builds volcanoes without
lava flow. The nature of the lava, very viscous
and cold, is responsible. The high
gas content of the magmas propels the mass of lava
into the conduit. It accumulates above the latter.
The eruption often begins with an
initial explosive phase,
creating a crater in the
substrate that will contain all the emitted materials.
During this phase and during the
growth of the domes, the gases contained in the lava are released
violently and generate airborne plumes
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which fall back and cover the surroundings
with ashes and pumice, and
with flows of blocks and ashes
that incinerate everything in their path. Sometimes the
building dome, unstable, collapses and releases gases
that trigger pyroclastic flows.
The presence of these flows is limited to the immediate vicinity
of the volcano (a few kilometers) on the Chaîne des Puys formations,
unlike what can be observed on the large stratovolcanoes of
the planet.
The materials, trachytes,
that are emitted are light-colored
lavas, whitish, sometimes friable, very siliceous.
At the
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base, they find the debris of
construction and the tephras
left by pyroclastic deposits. At the top, there is no crater,
but a striking rounded shape, sometimes bristling with lava
spines.
Within this
dynamism, one can distinguish between true
domes and protrusions.
The dome
is the perfect result of continuous dynamism. It is represented
in the Chain by the Grand Sarcouy
and the Clierzou, with remarkably
regular shapes, like an inverted cauldron. The protruding
needle, typical of Mount Pelée in Martinique (which
serves as a model for this type of volcano), is present here
in the Puy Chopine, a true
lava piston that has pierced the base to rise more than 180
meters above.
Puy de
Dôme volcano is a component of both features, dome
and needle. It is one of the tallest volcanoes of this type,
thus constituting a unique volcanic
formation. (The needle of Mount Pelée had collapsed
shortly after its formation).
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An explosive dynamism: the phreatomagmatic
eruption
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 The encounter of very hot magma with the water
contained in groundwater aquifers causes amazing
explosions. The energy released shatters
the crystalline basement, lifts it,
and hurls materials to the
periphery of the emission vent, which will form a ring of deposits.
The supply can be continuous or periodic.
In this way, a crater of remarkable dimensions is created in
the basement. The volcanic structure is often complemented by
a small Strombolian cone, a witness of a final phase without
water. The ring of pyroclastic deposits
consists of volcanic bombs, lapilli, and ash, but also elements
of the crystalline basement (gneiss and granite).
Most of the time, these materials are closely bound together.
This type of activity is called a "maar."
 After the explosion, the base collapsed directly
below the feeding chimney. Gradually, this depression would
host a lake and give rise
to some of the most beautiful crater
lakes in Auvergne, such as Lake
Pavin or Gourt de Tazenat.
Over time, sedimentary deposits
gradually fill in the lake, creating a flat, marshy bottom.
This is how the Maar of Beaunit
and the Narse of Espinasse
have evolved. It is also worth noting
that Clermont-Ferrand is built on one of the largest maars in
the region.
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 Hydrovolcanism
is a general term that applies to the meeting between water
and magma from the depths of the mantle to the surface
of the Earth's crust. In the field of volcanology, hydromagmatism
encompasses the explosive eruptive dynamics that result from
the encounter of high-temperature magma with water. The nature
of the dynamic depends on the quantities of each component (mass
ratio), which defines the explosive energy (see adjacent graph)
and thus the clearly defined volcano morphologies. The following
are distinguished:
- Phreatomagmatic
eruptions (in red on the chart) occur when ascending
magma comes into contact with an aquifer (groundwater). The
expansion of the steam generated by this contact results in
a series of very violent explosions that
carve out the substrate, creating craters called maars.
The ejecta accumulate around the crater, generally in the form
of a ring of pyroclastic tuff. The
maars of the Chaîne des Puys follow this pattern.
- Surtseyan
activity (in green on the graph) can be observed
when magma erupts into a shallow
lake or marine water body. The arrival of the magma at high
temperature causes the water to vaporize violently, breaking
the lava into small fragments. The abundance of water rapidly
cools the fragments (ash, lapilli) and hardens them. This is
how glassy tuffs—hyaloclastites—are
formed, which accumulate with blocks in the form of breccias,
lining the crater walls and building a tuff
cone. Surtseyan eruptions are characterized by spectacular
jets of pyroclastic material and steam shaped like cypresses
(cypressoid eruptions).
This type of activity is not seen in the Chaîne des Puys.
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Lidar Image of
the maar la Narse d'Espinasse (Source Géoportail)
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- Finally, in deep bodies of water,
there is no aerial development of the
eruption due to the high hydrostatic
pressure. The lavas produced cool in a characteristic
pillow shape, known as pillow lava (shown in orange on the diagram).
- In certain circumstances, a
local increase in temperature caused by the presence of a magma
chamber near a groundwater reservoir can lead to the vaporization
of water in the subsurface. Under the expansion of water vapor,
violent explosions occur without direct involvement of the magma.
These so-called phreatic eruptions
create a crater similar to a maar. In any case, if volcanic
activity continues, as soon as the magma is no longer in contact
with water, the dynamics change and shift toward activity
related to the magma's composition (shown in blue
on the diagram).
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