Discussion: Comments on “Basalt columns: Large scale constitutional supercooling? by John Gilman (JVGR, 2009) and presentation of some new data [J. Volcanol. Geotherm. Res. 184 (2009), 347–350]
Résumé
Some comments are offered on the paper by John J. Gilman (J. Volc. Geoth. Res., 184, 2009, 347–350), proposing that large scale constitutional supercooling may explain the formation of basalt columns. This hypothesis was presented twenty years ago (Guy, B. and Le Coze, J., C. R. Acad. Sc. Paris, 311, II, 1990, 943–949). However, in contrast to what is postulated by Gilman, I proposed that the chemical variations that may (optionally) show in the prisms do not relate to the major elements (Al, Si, ...) but rather to H2O and other volatiles. Thermal contraction, which is the more generally accepted hypothesis, also plays a role, particularly in the central “entablature” of the flows. The fingers formed by constitutional supercooling show instead in the colonnades. Original observations are presented that support the hypothesis of supercooling, in particular the existence of circular and radiating structures within the columns, that cannot be explained by meteoric weathering nor by the influence of the fractures delimiting the prisms. Various directions for research are proposed.
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