: cratère ou promontoire volcanique en Auvergne.
buildings - Various dynamisms
The Puy des Goules: a perfect stromlian
Dynamims types of eruptive
buildings seen in the "Chaîne des Puys"
The Chain of Puys
offers a particularly interesting range of volcanos.
If the traditional shape in cone is well represented,
the domes, fewer, constitute surprising buildings.
The maars of explosion, born from a cataclysm,
supplement the collection
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The most widespread dynamism: Strombolian
In this type of dynamism, the magma, propelled
by the relaxation of gases which it contains, is emitted by the chimney of supply
in sheaves of incandescent slags and
lava flow. The violent gas, continuous or periodic relaxations propel
materials to several hundred meters height, which fall down by building a characteristic
cone slowly. In same time, at the base runs out by gravity starting
from the mouth, the degassed lava. Very fluid
by its high temperature (about 1000°C),
it invaded vicinity, generating lava flows
of several kilometers length and a few tens of meters thickness, cooling out of
basalt. Today old castings, give on the surface these chaotic and wooded landscapes
which one calls "Cheires".
of volcanic bombs for largest (a few tons),
to the lapilli and ashes (a few millimetres)
for smallest. Left " basalt foam ", they have a spongy aspect and are exploited
under the pozzolana name. Name " volcanic
bombs ", comes owing to the fact that, modelled by the air whereas they were still
pasty during their projection, they present remarkable
aerodynamic forms. The exploitation of pozzolanas, very regulated,
allows, by putting at the air the successive layers of the deposits, to recall
the history of the eruption of the volcano.
After the extinction of the
volcano, there remains a cone with summit crater.
This last can be very marked, breaking the building completely,
as in "Puy de Come" and "Lassolas", or "Puy de Louchadière". A renewal
of activity can generate a double cone (Puy
Ninty percent of the buildings
of the chain are Strombolian cones
A dynamism at the daring buildings: Pelean
dynamism builds volcanos without cone and without lava
flow. The nature of the lava, very viscous
and cold, of it is responsible. In addition the strong content
gas of the magmas slowly propels the mass of lava out of the conduit.
It accumulates with the top of this last in the shape of dome.
Often the eruption starts with an initial explosive phase thus creating a crater
which will contain all emitted materials. In this dynamism we will distinguish
the cumulo-domes and the
from protrusion. With the base of the building in formation, the gases
release themselves violently in the form
clouds which calcine all on their passage. Pyroclastic projections
are volcanic bombs called "in bread crust",
elements of the bases crystalline lens torn off at the time of the rise of the
lava and the blocks of lava of the building.
The emitted materials are
lava clear, whitely, friable, very siliceous.
At the base, we find, the falls of construction, and the lying deposits left by
the volcanic clouds are. At the node, not of crater,
but an astonishing round form
is the perfect
resultant of a continuous dynamism
without incident. It is represented in the Chain by "Grand
Sarcouy" and "Clierzou" with the
remarkable regular forms, out of reversed cauldron. The needle
of protrusion, typical of the Mountain Peeled in Martinique (which
is used as model with this type of volcano), is present here by "Puy
Chopine", genuine sword having crossed the basic base to rise with
more than 180 m to the top.
"Puy de Dôme"
is a component of the two facies, dome and needle. It is one of the highest volcano
of this type, thus constituting a volcanic, single formation.
(the needle of the Peeled Mountain had collapsed little time after its construction).
An explosive dynamism: the eruption phréatomagmatic
The meeting of the magma, very hot, and of the
water contained in the ground water causes formidable
explosions. Released energy splits up
the crystal base, raises it and projects
with the periphery of the mouth of emission of the materials which will constitute
a ring of deposits. The feeding can be continuous or
periodic. Thus creates for itself a crater with dimensions relatively
more significant than the Strombolians craters. The volcanic apparatus is often
supplemented small Strombolian cone, pilot of a calmer final phase. The
pyroclastic ring of deposit is composed of volcanic bombs, of lapilli
and ashes, but also of elements of the crystal base
(gneiss and granite). Most of the time these materials are closely dependent.
We call " maar " this type of édification.
After the explosion the base is ploughed up with
the balance of the chimney of feeding. Slowly, this depression will shelter a
lake and will give the most beautiful lakes
of crater of Auvergne such as the "Pavin lake"
and "Gourt de Tazenat". In time, the sedimentary
deposits fill slowly the lake to establish a marshy flat bottom there.
Thus evolved "Maar de Beaunit" and "Narse d' Espinasse".
Let us note moreover than Clermont Fd is built on one
of largest the maars of the area.