Jo De Waele, Italian Institute of Speleology, Department of Biological, Geological and Environmental Sciences, University of Bologna, Italy
Maria Teresa Melis Department of Chemical and Geological Sciences, University of Cagliari Cagliari, Italy
The Azrou plateau is composed of a basaltic plain, dotted with volcanic cones and lava flows, covering Jurassic limestones. Satellite investigation (Landsat and Sentinel-2), together with Google Earth images, have allowed the mapping of over 300 basalt collapses basalts ranging in diameter from a few tens to over 200 and up to 65 metres deep. These sinkholes are sometimes aligned and resemble lava tube collapses as those known from Mars and the Moon. We surveyed almost 150 of these depressions measuring minor and major diameters and depth, and making geomorphological observations. Most collapses are entirely hosted in basalts, with only two of the field-surveyed sinkholes showing some limestone at their bottom. Their genesis is mostly due to collapse by failure of the basaltic roof over an underground void. These voids are related to the underground removal of material through unknown karst conduits and not to collapse of a lava tunnel roof. This is confirmed by the position of most of the sinkholes, as they are close to the boundary with adjacent limestones. The exploration of four small caves entirely hosted in the basalt has confirmed the presence of semicircular voids formed between thicker and more solid lava beds. Underground removal of weathering products of these volcanic rocks through the underlying karst aquifer allows for a stepwise sagging and collapse and the formation, over time, of these sinkholes.
Photos by Sergio Passanante, Università di Bologna
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