鍾乳洞の気象と鍾乳石の成長

Abstract

Speleothem, which is mainly built of CaCO₃, is formed from water dripped from ceiling of limestone cave: Ca²⁺ + 2HCO₃⁻ → CaCO₃↓ + H₂O + CO₂. The reaction is accelerated by CO₂ degassing from the drip water in the cave and the driving force of this reaction is difference between the equilibrium CO₂ partial pressure of the drip water (pCO₂, w) and the partial pressure of CO₂ in the cave air (pCO₂, a). In the winter season, the speleothem will be well grown because of pCO₂, a < pCO₂, w condition, whereas it is not expected to grow because pCO₂, a does not get smaller than the pCO₂, w in the summer season. The low pCO₂, a condition in the winter season is resulted in ventilation of the cave air by the atmospheric air having very low pCO₂ caused by difference of air temperatures in and out of the cave.