Volcano : The Gravenne of Thueyts

Volcano, natural and volcanic heritage in Thueyts

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Strombolian volcano (Gravenne), attached to the young volcanoes of Ardèche, the Gravenne de Thueyts is a Strombolian type volcano erupting crater culminating at 600 m. altitude.

The Gravenne volcano experienced a very brief activity in the form of two manifestations (explosive then effusive) which largely impacted the morphology of the current valley. During the explosive phase, many bombs and slag accumulated near the crater, building a vast cone of slag, or volcanic cone. It seems that other explosive phases allowed to deposit slag on the flow visible at the football stadium and the chapel St Roch.

The initial cone was largely overflowing into the bed of the Ardèche. Indeed we observe that during the effusive phase (projection of fluid lava), the effusion of the flow downstream, is blocked by the slag previously projected. The flow then widened by flowing about two kilometers north in the bed of the Ardèche, thus blocking the small stream, the Merdaric. It also seems that, still because of the cone, there may have been a lake on the Ardèche, as evidenced by the sediment visible under the flow at the start on the left bank of the via ferrata. Subsequently, the Merdaric and Ardèche recreated their beds revealing the basalt prisms located at the base of the flow. If the Merdaric cut the flow almost symmetrically in its center, the Ardèche, deflected by the flow of lava, cut its new bed between the flow and the granite rocks. The opposite between these two types of rock formations (volcanic and metamorphic) is visible today from the site of the Devil’s Bridge. Today its bed is located at a level much lower than at this period.