Agde volcanic complex
The volcanic complex of Agde in southern France is located at the end of a north-south aligned series of volcanic centers that reach the Mediterranean from the Cézallier via the Cantal , the Aubrac , the Grands Causses and the Escandorgue in the Hérault department near Agde . The eruptions in Agde are geologically young and occurred in the Quaternary .
description
The volcanic complex is wedged between the Hérault in the west, the Canal du Midi in the north and the Mediterranean in the south and east; in total it covers an area of 15 to 20 square kilometers. It consists of three strombolian volcanic cones, the current 113 meter high Mont Saint-Loup , the only 35 meter high Pioc'h and the 55 meter high Mont Saint-Martin . The Pioc'h , which originally culminated at 80 meters, has lost a lot of height because its ejecta was mined for the extraction of pozzolans . This former quarry now serves as a garbage dump. The Mont Saint-Martin has been with increasing urbanization of the resort of Cap d'Agde increasingly integrated into the current cityscape. Numerous lava flows cover parts of the municipality of Agde and continue out into the sea.
introduction
The Agde volcanic complex is the most important of its kind in Languedoc . Its complicated structure has given rise to several interpretations, in particular the precise storage conditions between tuffs and lava flows have not been clearly clarified. Usually a single extraction center south of Mont Saint-Loup is assumed, occasionally an eruption in the sea not far from the coast is also used. However, it is also possible for several centers, which are located separately from one another, to follow one another in time. This would most easily explain the varying angles of incidence of the pyroclastic deposits, their reprocessing and small faults .
Nevertheless, there is much in favor of a single emission point south of Mont Saint-Loup . This is interpreted as a maar crater with a diameter of 1500 meters, on the edges of which the chimneys of Mont Saint-Loup , Petit Pioc'h and Mont Saint-Martin protrude secondary. The summit to the north-east of Saint-Martin-les-Vignes , culminating at 38 meters , probably represents an independent central cone from which up to two meters long, spindle-shaped bombs were ejected and also packets of lava flowed out.
stratigraphy
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The lying of the volcanic deposits of Agde is built up by Hawaiian lava flows that have emerged from several emission centers, which unite to form three smaller basalt plateaus (10 square kilometers large and up to 40 meters thick western section near Agde , northern section near Baldy-Batipaume and southeast section near La Clape ). The Basalt Current from La Clape poured at least a kilometer into the sea. In the sea, the small offshore island of Brescou is also built from basalt. Incidentally, basalts could still be detected geophysically up to 40 kilometers from the coast.
The basalts overlie turn clays and red sands of the continental and marine Astiums , sometimes even gravel layers of Villafranchiums in the meter range, such as the basalt flow of Agde .
The basalt plateaus are separated from the ejection flapilli of the Strombolian cones by a tuff horizon in which clays and sands can be interposed. This tuff horizon consists of yellow tuff at its base , which in turn is covered by gray tuff .
The yellow tuff comes from a Surtseyan breakthrough tube ( diatreme ) and was deposited in shallow water, probably in a lagoon , as a hot base surge . Its hue is due to its clay content. It shows alternating layers with thin layers of volcanic glasses ( palagonites ) and occasionally also contains juvenile impact bombs made of basalt and even parts of basalt that have slipped in. In Baldry-Batipaume it is covered by a basalt current that has changed it metamorphically in contact. At Plage de la Conque it is intruded by the steep passage of the Deux Frères , but otherwise not further deformed. However, at the opposite northern end of the bay, it is dragged up by a basaltic storage corridor, whereby even the overlying gray tuffs are still deformed.
The gray tuffs also emerged from Base Surges; they show partly chaotic storage conditions, sloping layers and anti- dunes . After initially stratified storage, they can develop a completely unregulated and unstratified horizon, and then go back to stratification.
In the hanging wall of the volcanic rocks, the Notre-Dame limestone , a tufa layer of the neighboring Vias volcano and a layer of rubble follow .
development
After the basalts had initially flowed out, the first eruptions were hydromagmatic. They had a Surtseyan character and were created in the sea (lagoon area) - these include the yellow tuffs in the cliff of La Grande Conque beach . The later eruptions then formed the central crater on Mont Saint-Loup with a classic Strombolian character. A lava lake was created in the central maar and smaller lava flows were sent out. The lagoon gradually silted up, recognizable by the change from pyroclastics to gray tuffs, which, unlike yellow tuffs, no longer contain hyaloclastics and were deposited dry. Then, 750,000 years ago, basalt intrusions with corridors and storage corridors followed. After the volcanic activity was over, the marine erosion forces took over and prepared the current course of the coast. The black beach of La Grande Conque - the only one of its nature in France - owes its horseshoe-shaped shape to two resistant, gang-like intrusions protruding into the sea . The pyroclastic deposits in between, falling slightly inland, could be eroded away much more easily by the sea. One of these gait intrusions now forms the Deux Frères rock protruding into the sea .
Age
The volcanic activity began about a million years ago BP at the end of the Old Pleistocene ( Villafranchium ) and lasted about 250,000 years until the beginning of the Middle Pleistocene . The following radiometric age information is available:
- 1.0 ± 0.2 million years BP
- 0.85 ± 0.1 million years BP
- 0.74 ± 0.07 million years BP
- 0.73 million years BP
According to this, volcanism began in the Günz Ice Age towards the end of the Villa Franchium and ended at the transition to the Mindel Ice Age when the volcano of Saint-Thibéry , the volcano of Vias and Roque Haute became active.
Petrology
The relatively homogeneous lavas are holocrystalline basalts with 50 to 52 percent by weight SiO 2 (with a low of 47 percent by weight). Normatively , they are characterized by 2 to 5% nepheline and are therefore alkaline rocks that are undersaturated with silicon . Mineralogically they contain phenocrysts of olivine (absorbed in part and at the edges iddingsitiert ) and green, rosette-forming augite as well as in their base composition microlites of olivine, augite, magnetite and plagioclase ( Labradorit ), which be zoned (can of An 60 -An 30 ). Peridotite nodules carried along as xenolites can also occur in the basalts .
Individual evidence
- ↑ geze, B .: Languedoc Méditerranéen Montagne Noire . In: Guides Géologiques Régionaux . Masson, Paris 1979, ISBN 2-225-64120-X .