Languedoc Volcanics

Whilst at this year's Symposium someone said to me "its all limestone down there isn't it?". I don't know if this is a commonly held opinion but it isn't all limestone. In addition to the complex metamorphism and granite towards the Pyrenees, it doesn't seem to be too well known that we have our own chain of volcanoes. These stretch for about 100 kms from the "Causse d'Aubrac" in the north, right down and into the Mediterranean sea at Cap d'Agde. They may not be as spectacular as those in the Auvergne that are well known to many OUGS members but nevertheless they have several interesting features.

The ages of the Languedoc basalts range from 14 Ma in the northern parts at Séverac-le-Chateau to 0.7 Ma at Cap d'Agde in the south, i.e. from Plio-Villafranchian to Pleistocene, and many volcanic features can be seen in the area. Layered volcanic ash and cinder deposits are well displayed in the cliffs at la Conque in Cap d'Agde, unfortunately many of the other features of the Agde volcanic complex cannot now be seen due to the rapid development of this holiday town. Inland there are numerous small Strombolian cones with scoriaceous lava and "bombs". There are several eruptions of lava flows in the area with columnar jointing; these are well displayed in the quarries south of St Thibéry just by exit 34 on the A9 . Around the lac de Salagou the Permian pelites know locally as the "ruffes" have been heavily eroded leaving the hills around the lake topped with basalt, these were once valleys fossilised by the lava flows, and the terrain around the lake is punctuated with "chimneys" and plenty of examples of dykes either isolated or associated with the chimneys.

In most cases volcanism began with the deposition of tuffs filling in the valleys and lakes and then subsequently the building up of cones by the piling up of scoriaceous blocks, spindle bombs etc. The issue of lava is normally a late phase; the lava being very fluid flowed into the valleys fossilising them, some flows such as that from the Baumes volcano north of Pézenas reached 10 Kms. The thickness of the lava varies between 2 and 10 metres but is generally in the order of 4-5 metres, this often results in an inversion of relief where the valleys originally fossilised by the lava flows have been left as an elevated plateau by the erosion of the surrounding area. The basalt is normally compact, black or greenish grey with columnar jointing in the upper parts and bedded at the base. The composition is on the whole very homogeneous with under saturated alkali basalts with microlites of augite, feldspars, titanium-magnetite, analcite and leucite at the heart of a brownish glass. There are numerous phenocrysts of olivine, amphibole and pyroxene. Some of the flows are characterised by large nodules of olivine. For the technically minded the groundmass is characteristic of analcite-basanite (alkali feldspars, biotite, analcite and nepheline as an accessory mineral). The principal variations are in the degree of crystallisation which (from 20-60%); this indicates a slight evolution between black labradorites and ankaramites. There are enclaves (inclusions) of Kaersutite or high titanium augite representing an early crystallisation stage, with classic nodules of peridotites, most often lherzolites but some are more rich in olivine (dunite) or orthopyroxene. Some basement enclaves of blocks of ultra-metamorphic minerals show the presence of a deep ultra-metamorphic basement that could be part of the charnockite basement of the Pyrenees that outcrops in the Algy massif.

The basalt is actively quarried in several places where the columnar jointing is often well displayed in particular near St Thibéry. The basalt was used for road cobbles but now is used in large blocks to prevent erosion at cuttings or as coastal protection, and crushed for rail ballast and roadworks. Some of the volcanic deposits are used to a limited extent in cement due its pozzalanic properties.

The technical data has been taken from the data booklets supplied with the geological maps. The maps used are 1015 (Pézenas), 988 (Bédarieux), 989 (Lodève) in the Carte Géologique de la France series, 1/50 000.