THE VOLCANISM OF THE ESTEREL

(Excerpt from the Guide des lacs du Dramont- Ch. Bonnet and G. Olive, 1994)

The constituent rocks of the Estérel come from three volcanic phases very different in nature.

The first thrust occurs at early Permian (primary era) between -290/-280 and -250 million years (Ma).

Thanks to faults, a rhyolitic lava which puts in place the characteristic red rocks of the Esterel.

• 290/-280 Ma: distension of the European continental plate;

• 280 Ma: Collapses in the north of the Estérel massif. Some big breaks (faults) weaken the massive ; they face West- East ;

• The opening of these faults delivers the transition to various lavas, including the most common is rhyolite red-brown;

• Increasingly abundant injection of magma. Then, by overflow, magma and ignimbrites will come out of the cracks and flow on large surfaces.

• The considerable weight of castings resting on the vacuum formed under the Estérel causes a sinking of the central part of the depression (subsidence). A volcanism weakly active persists. The second push, to the limit between the primary era and the secondary era (-250/-225 Ma), sees the setting place of a “classic” volcanism. Few traces remain:

• Building a real volcano like that of Maurevieille, which alternately expels lavas, ashes, and, thanks to explosions, fiery clouds. These materials are deposited in strata well-typed, distinguishable. It is why this volcano is called "stratovolcano".

• Classic diagram of volcanism: the volcanic chimney closes, secondary volcanic devices appear on the sides or on the outskirts of the volcano. They will lead outside vertically (necks) or horizontally (sill). Important masses of magma remain trapped below the ground surface (lacolith, domes), from which emerge some times peaks due to erosions posterior (dykes).

• The Volcano explodes: ashes and debris thrown onto large surfaces will settle and create thus a "benchmark level" of cinerites. These first two phases of volcanism of the Estérel will have lasted from -280 to -225 million years ago, i.e.… 55 million years, i.e. another 27,500 times the elapsed time since the beginning of the Christian era. The third push, at the end of the tertiary era (-31 to -28 Ma), in place the esterellite in which are dug lakes. In this phase, the lava does not reach the surface, but remains trapped in the rocks volcanics installed during the Permian (first phase). We are talking then of intrusive volcanism. Esterellite will appear on the surface as a result of the erosion that affects the site since that time.

• Magma rises to the surface. The rocks in contact with it, under the influence of pressure and temperature, transform (metamorphization).

• The magma cools slowly, which allows the formation of crystals ranging in size from a few millimeters to several centimeters. Due to these peculiarities local, this variety of granite takes, in the Estérel, the name of esterellite.

• In our time, erosion has exposed bare this esterellite and its halo of metamorphosis. The Quaternary era sees the last stage of our history: the profile current of the North Coast of the Mediterranean gets ready. The middle part of the Pyrénéo- Alpine, who trained at the end of Secondary, switches to the Mediterranean and sinks several thousands of meters, forming the Gulf of Lion. The southeastern part of the chain rotates 90° and slowly moves away from the Continent thus forming and isolating Corsica and Sardinia. By backlash, the Estérel recovers (this rise still occurs at the rate of 0.6 mm per year).

THE ESTERELLITE

Although a phenomenon volcanic here is 28 to 31 million years, the establishment of esterellite differs significantly from red rocks of the Estérel which date from the Permian (-230/-280 Ma). Here the magma does not borrow a chimney (extrusive volcanism) but slowly injects itself in favor of elders “accidents” (faults, cracks) dating from the Permian. This volcanism called "intrusive" has two consequences important:

The magma cools a lot more slowly than on the surface, allowing the formation of crystals clearly visible and sometimes very large crystals (phenocrystals of several centimeters). Pressure exerted by the lava on the nearby rocks (“country rocks”) and the ambient temperature induce the physico-chemical transformation of these rocks. We then speak of contact metamorphism.

The main esterellite intrusion set end following a strip of 6 km long, 3 km wide, oriented N-W/S-E, from “les Cous” to north of Boulouris, to Dramont. She is accompanied by three small satellite intrusions, occasionally, in Boulouris (field of Creps), Castellas and Dramont (Prola career).

Petrography (branch of geology that studies the composition mineralogical and chemical rocks), tells us that esterellite (“porphyry” blue from Estérel) is a quartz microdiorite (from micro: small and grainy texture; diorite: fully crystallized rock, and quartzic: which contains quartz), light to dark gray in color, whitish or bluish, rarely green, and which contains crystals visible to the naked eye. She presents a micro-grainy texture.

ESSENTIAL MINERALS ESTERELLITE

• Plagioclases: feldpaths of whitish color. The size of crystals is variable. They are all bigger than the lava slowly cooled.

• Quartz: always present, it is present in the form of crystals dodecahedral automorphs, bipyramidal, of variable size, from a few millimeters to one centimeter.

• Ferromagnesians: these are amphiboles (silicate of iron and magnesium)

• Biotite (black mica). She crystallizes in the form of small black chopsticks. Accessory minerals present in the form of veins insinuating itself into faults and host rock fissures. We finds oxides and sulphides, calcite, garnets, malachite.

THE TRANSFORMATION ESTERELLITE:

The intrusion of esterellite causes the metamorphosis of rocks "collecting". Depending on whether these are on the periphery of the intrusion and the surrounding rock wall at minerals: muscovite (white mica) and cordierite (dark, prismatic stocky) on the one hand, green biotite and quartz on the other hand.

In some cases, as in the north du Dramont, the surrounding land seem to have been affected by hydrothermal activity (circulation and concentration of "juice" in the faults). We find then "bubbles" lined with centimetric quartz, chlorite (which sometimes gives a green tint with esterellite), oligist rosettes (very pure iron ore) and epidote.

enclaves of surrounding rocks, subjected to endometamorphism, are of various kinds: granite, gneiss (flaky rock), or minerals such as epidote in Dramont.

THE INDUSTRIAL REVOLUTION

FROM THE END OF THE 18TH CENTURY, THEN AT THE BEGINNING OF THE 19TH IN FRANCE, BELGIUM, SWITZERLAND, GERMANY AND THE UNITED STATES, ECONOMIC ACTIVITY IS CHANGING BY NATURE IN A FEW DECADES, GOING FROM A SOCIETY TO DOMINANT AGRICULTURAL AND CRAFTSMANSHIP TO LARGE MANUFACTURED GOODS PRODUCTION LADDER. THE FIRST INDUSTRIAL REVOLUTION IS ON.

These great technological innovations change the lives of the French:

• Machine development steam which allows the industrialization from the country ;

• The invention of the internal combustion engine;

• The development of the first automobiles;

• The first communications radio;

• Locomotive development;

• The invention of photography;

• The Bessemer process facilitating the steel production.

All these innovations allow increase growth, productivity and mechanization. We therefore witnessed the rural exodus towards cities as they grow transport and telecommunications. At the same time emerging the bourgeois class and the class factory Girl. It is then the creation of Trade unionism and socialism.

These phenomena generate considerable demands: routes of communication (roads, paths iron, canals, etc.) but also housing, factories, construction works civil engineering. These new needs require many materials of construction.

In 1864, the geological composition basements in the Var region is rich and meets the needs of the growing industry. We extract hard coal and lignite at Callian, Saint-Zacharie, La Londe, Fréjus; copper in Pradet, zinc and lead in the Massif des Maures, bauxite at Brignoles.

In Saint-Raphaël, requests farms are addressed for fossil coal in the forest domain of the Terres Gastes, for an iron mine in 1849 in the district Petits Caous or Agay. It is in this context that appear the first farms of porphyry in the territory of Saint-Raphaël who will contribute to pave the world. At Dramont, the exploitation of esterellite will be activity for nearly a century.