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Geothermal Flux and Phreatic Speleogenesis in Gypsum, Halite, and Quartzite Rocks Giovanni Badino* Department of Physics, University of Torino, Via P

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Geothermal Flux and Phreatic Speleogenesis in Gypsum, Halite, and Quartzite Rocks Giovanni Badino* Department of Physics, University of Torino, Via P International Journal of Speleology 47 (1) 1-11 Tampa, FL (USA) January 2018 Available online at scholarcommons.usf.edu/ijs International Journal of Speleology Off icial Journal of Union Internationale de Spéléologie Geothermal flux and phreatic speleogenesis in gypsum, halite, and quartzite rocks Giovanni Badino* Department of Physics, University of Torino, Via P. Giuria 1, 10125 Torino, Italy & Associazione La Venta Abstract: The first layers of rock underground are in thermal contact with the external atmosphere mainly through infiltrating meteoric water. This relatively cool zone absorbs rising geothermal energy, which heats the water. If the aquifer consists of gypsum, halite or quartzite, the water at those depths is usually salt-saturated, so the increase in temperature renders the water aggressive again. This in turn leads to rock dissolution and formation of phreatic conduits. This way, the geothermal flow creates caves that do not necessarily reach the surface. This paper analyzes the speed of the excavation, which, in different types of rocks, depends only slightly on temperature and meteoric precipitation. The time scale of this speleogenesis appears to be similar to that of other known cave systems. These processes are probably able to greatly increase the permeability around underground radioactive waste storage in halite. Keywords: geothermal flux, phreatic speleogenesis, hypogenic caves, evaporite, quartzite, radioactive waste storage Received 4 December 2016; Revised 19 July 2017; Accepted 24 October 2017 Citation: Badino G., 2018. Geothermal flux and phreatic speleogenesis in gypsum, halite, and quartzite rocks. International Journal of Speleology, 47 (1), 1-11. Tampa, FL (USA) ISSN 0392-6672 https://doi.org/10.5038/1827-806X.47.1.2098 INTRODUCTION 1.8×1017 W, which makes the geothermal contribution about 4000 times smaller. Rocks below the Earth’s surface have temperatures Underground temperature begins its geothermal that increase with depth. The geothermal energy heating well below the surface (hundreds or flux in the upper few dozens of kilometers of the thousands of meters, as shown below), because in Earth’s crust is described by the usual thermal the upper layers the infiltration of meteoric water conduction equations. The resulting temperature forces the rock to assume its average temperature, gradient depends on the local rock conductivity, but which is essentially the average local yearly surface it has an average value around 25-30°C/km. This temperature (Badino, 1995). The consequence is energy flux is small, and plays no role in heating the obvious: geothermal energy does not flow through Earth’s atmosphere. At first, its effect in caves these upper layers, as deep-flowing water completely appears to be negligible because, unlike mines, these intercepts the meteoric infiltration and carries it away environments are usually quite cold, essentially at at the base of the infiltration zone (Mathey, 1974). the external mean-annual temperature. This work The thickness of this shielded stratum depends attempts to show that geothermal energy can heat on the local rock permeability. In a non-karstic deep water, increasing the solubility of gypsum, environment, it is usually around 50-100 meters, but halite, and quartzite, and thus forming caves. it can be much greater along major rock discontinuities that are able to drain water, as observed during the The geothermic intensity flux excavation of the Mt. Blanc tunnel (Guichonnet, The geothermal flow through oceanic crust is 1967). In deep karst, this “cold” layer usually includes not relevant here. However, the global average of the entire underground system (Badino, 2005), which continental heat flow is (Davies, 2010): extends at least 1-2 km below the surface (Sendra, 2012). F = 0.07 Wm-2 gt Near-surface ground temperature This corresponds to a total flux of 4.6×1013 W for It is possible to distinguish a top layer affected by the whole planet. The solar power received on Earth is daily temperature fluctuations, the “heterothermic *deceased August 8, 2017 The author’s rights are protected under a Creative Commons Attribution- NonCommercial 4.0 International (CC BY-NC 4.0) license. 2 Badino daily layer”, which in compact rock has a typical establishing the rock temperature. This means up thickness of less than a meter. Seasonal temperature to the level where rock permeability is sufficiently variations can penetrate 15-20 times deeper (the high to permit a significant flux of external water conductive penetration length of sinusoidal thermal to the springs, in the lower parts of the phreatic alterations depends on the square root of the drainage system. This permeability horizon is the period; Isachenko, 1969), and this depth defines the boundary between the homothermic and geothermal “heterothermic seasonal layer” (Fig. 1). This layer is layer (HGB). usually a few meters deep; over this distance, seasonal Below this level, rock and interstitial water (almost temperature variation decreases to zero, a condition motionless here) are afflfected by the geothermal flux, which defines its lower boundary (heterothermic- and its temperature regularly increases by conduction, homotermic boundary, HHB) (Luetscher, 2004). as shown above. In the absence of a general term, it is possible to call this zone the “geothermic layer”; it extends downward for kilometers, i.e., indefinitely for the present discussion. Physics of the homothermic layer The homothermic layer is thus enclosed between two surfaces, the boundary layer (HHB), where the yearly temperature variation is very close to zero, and the HGB, where the rock temperature starts to increase because of the local geothermal lapse rate. As noted above, HGB is essentially the level at which the rock permeability becomes too small to allow flow of water sufficient to subdue the geothermal flux. It is usually the lower part of the drainage zone, in other words a relatively thick horizon of fissures or, in karst, the floor surface of draining conduits. In this layer, flowing water subdues the rising geothermal flux, preventing it from reaching the upper rocks, while at lower levels the flow is so slow that water reaches thermal equilibrium with the surrounding hot rock. An analysis of thermal exchange by dynamic Fig. 1. Geothermal profile: Heterothermic layers, with daily and similarity (Ishachenko, 1969) makes it possible seasonal temperature ranges, Homothermic Layer at the average to estimate that the temperature differences in temperature of infiltrating water and a deep, undisturbed layer in this region have a scale dimension of 1-10 mK thermal contact with deep rocks. (millikelvins), which is experimentally undetectable. In permafrost studies, these two upper layers are Therefore, sudden temperature changes through the called the “active layer” because here ice can be HGB are not expected. formed and melted, if weather conditions allow it It is now possible to estimate the thickness of the (Shiklomanov, 2013). homothermic layer, where flowing water dominates Beneath the HHB, the rock assumes the average the thermal exchange. In sufficiently homogeneous local temperature of infiltrating waters (hereafter underground environments (e.g., in poorly permeable indicated as Taw). Infiltrating waters are usually a material) most water infiltrates and flows below significant fraction of precipitation (Celico, 1986), the surface for just a few dozen meters (Luetscher, which in temperate regions has a scale dimension 2004). In karst environments, however, this is not the of 1000 kg/m2/year. This means that on average case, because water can descend for great distances the water that penetrates underground each year through the vadose zone. The homothermic layer has the same thermal capacity as a one-meter- may potentially comprise an entire mountain system; thick rock layer. Hence, on a geological time scale, the deepest caves in the World (in Abkhazia) cross flowing water forces a rock layer, no matter how vertically through more than 2,000 meters of rock large, to thermal equilibrium. (Klimchouk, 2013), with a temperature increase of less The water temperature depends on the seasonal than 5°C along the entire depth (Provalov, pers. comm.). precipitation, i.e., on the regional climate, but it As noted above, infiltrating water in the is usually close to the external yearly local average homothermic layer has an essentially constant temperature Tave (Badino, 1995). In karst studies, temperature Taw. Its temperature is not independent the rock layer (and its caves) beneath HHB, which of depth. In deep karst, air flow can also play a major is at constant temperature Taw, has different names: role. Intersecting fluids undergo complex energy “neutral zone” (Dublijanski, 1977), (Tikhomirov, exchanges, increasing temperature along their decent 2016), “isothermal zone” (Crestani, 1939), and others. through the homothermic layer at a typical “karstic” In this paper the term “homothermic zone” is used (as rate of 3-4°C/km (Badino, 2010). This temperature in Luetscher, 2004). This layer extends down in the increase with depth is much lower than the expected water table up to the level where the flowing external value in the case of heat exchanges between air and water thermally prevails over the geothermal flux in rock (around 5-6°C/km). It shows the dominant role of International Journal of Speleology, 47 (1), 1-11. Tampa, FL (USA) January 2018 Geothermic speleogenesis 3 infiltration water in establishing the rock temperature
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