[hal-01084610] Discovery of “basalt columns” in the molten rocks (paralavas) of the Saint-Pierre coal heap (La Ricamarie, Loire, France)

Starting in the thirties, the building of the Saint-Pierre coal heap lasted until 1983, from residues (shales and sandstones) of coal mining activity in the Loire basin, especially in the Pigeot pit (La Ricamarie). The remains of coal rock associated with the heap burnt, resulting in a sharp increase in its temperature, reaching or exceeding 1300° C (Guy et al, 2001). In some places the material melted. The resulting rocks, which resemble basalts, are called paralavas. Scoriaceous surfaces, fluidal textures, cracks, vacuolar facies, are observed, all characters very similar to those encountered on a volcanic flow. Microscopic observations confirm the field data: clastic rock textures can no longer be seen, but typical textures of rocks having melted: automorphic high temperature minerals (orthopyroxene, spinel, cordierite) in a matrix made of alkali feldspar. In a sector of the paralavas as revealed by the digging of the heap (for obtaining aggregates), similar columns as those observed in basalts were observed. These have a polygonal cross-section (diameter of prisms of about thirty centimeters), and their visible lengths is of a few meters. The outcrop disclosed by quarrying, twenty meters of horizontal amplitude, continues at depth and laterally; further digging work will help discuss its size. There are two main hypotheses to explain columnar jointing: - H1: thermal contraction of a cooling solid mass that makes fractures develop (solidification mechanism is not studied); - H2: fingering of the solid growing within the liquid during solidification; in this case the solid mass is organized and its possible subsequent contraction upon cooling preferentially opens joints formed previously (Guy, 2010). The original observations on the heap may contribute to this discussion. In particular, contrary to what is found on natural geological bodies, prisms of the heap reveal between them large irregular vertical or horizontal voids; in the case of the vertical voids, the concavities of the adjacent faces do not match and the prisms do not perfectly fit into each other. The surfaces open to the voids are not smooth, but have a "lumpy" nature, as if a magma had frozen. Explanation: the fusion affected unconsolidated blocks leaving between them a high porosity; we can imagine, first occasional emptying of liquid through the porous parts (not subject to combustion and fusion) underlying the melting zone and/or, second, the collapse in the course of the melting / solidification phenomenon, revealing voids that do not necessarily fill by the descent of the overlying parts. The prisms can then appear not welded together, fixed in an unfinished solidification stage of the liquid between them that escaped. This interpretation (TBC) is not consistent with H1 and favors H2. Other factors must be taken into account to understand the formation of these prisms and their characters: - the liquid has not been brought in from an external source (it did not flow) and has been created on the spot, the heat source energy being present in the rock itself; - the redox regime is original: the melting is accompanied by oxidation under the conditions of the earth surface (carbon combustion), and the phenomenon evolves toward a reduction (increase in CO2 , decrease in O2 partial pressures); the columns are red, and their border is sometimes gray / black; this is the inverse of the standard situation where the magma is brought in a reduced state from the earth crust depths: gray / black rocks can oxidize at their periphery due to weathering in surface conditions (possible red peripheries). The black rim of the prisms observed in the heap is another argument for early structure before complete solidification (H2) in as much as late weathering could not result in a black border (if we had H1). In the literature on paralavas, the columnar jointing phenomenon (whatever its interpretation) does not seem to have been observed; here it shows a remarkable extent and is probably one of the few global occurrences (the only one?). Thank you to those who have contributed, directly or indirectly, to this work. Guy B., Garcia D., Le Coze J. & Theisse V. (2001) Les cahiers de la Rotonde, Publications de l’Université de Saint-Etienne, 61-69. Guy B. (2010) Journal of Volcanology and Geothermal Research, 194, 1-3, 69-73.



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