Puy : cratère ou promontoire volcanique en Auvergne.

Dictionnaire Larousse

Volcanic buildings - Various dynamisms

The Puy des Goules: a perfect stromlian cone

 

 

Dynamims types of eruptive buildings seen in the "Chaîne des Puys"

All the forms...

 

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

La Chaîne coté Nord
   

 

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),

Puy de Côme

Stromboli

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".

Slags consist 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 de Côme).

Ninty percent of the buildings of the chain are Strombolian cones


A dynamism at the daring buildings: Pelean

The 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 needles

Puy de Dôme Grotte du Clerziou

Puy de Dôme Vue Sud

from protrusion. With the base of the building in formation, the gases release themselves violently in the form

of volcanic 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

The cumulo-dome 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).

Type Péléen

Péléen.gif (13 Ko)


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.

Type Phréato

phreato.gif 26Ko

Maar de BeaunitAfter 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.


© Copyright Dec 2000 Photos et Tableaux: Bernard Dichamp