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Saline Lakes
Tufa tower formation is entirely dependent on water. Beneath the surface of ancient salt lakes, these towers were formed incredibly slowly through the precipitation of minerals out of saturated solutions. As these lakes drained or dried, they have begun to expose the formations beneath the surface. Unlike other tufa deposits, the salinity of the lakes prevent tufa towers from forming around macrophytic organisms.
Soda Springs
As the soluble minerals precipitate, they do so in regular patterns around crystalline structures already present in the solution. This process occurs at the site where calcium-rich sodium springs enter the lake. These soda springs are rich in dissolved carbon dioxide, creating an acidic environment that keeps the alkaline minerals in solution. Upon entering the saline lake, the dissolved carbon dioxide degases due to a lower atmospheric partial pressure. This raises the pH of the solution, causing the minerals to arrange themselves instead on the seed crystals nearby, fueling their continued growth.
Waterfalls
Tufa towers can also form alongside waterfalls, where mineral-rich waters release dissolved carbon dioxide through mechanical mixing with atmospheric gases. Evaporation of spray contributes to the formation of tufa towers, as does the presence of photosynthetic organisms. Photosynthetic algal membranes on the surface of tufa structures make use of dissolved carbon dioxide, as well, increasing the rate of mineral deposition.
Other Biological Factors
Several microorganisms produce biofilms that dramatically increase the speed of tufa formation, causing dissolved minerals to crystallize directly on their surface through an unknown mechanism, according to sedimentology author Martyn Pedley. Cells trapped by these crystals die. Their bodies are buried inside the formations, creating the characteristic porosity of tufa towers. The colors of the tufa towers are caused both by non-calcareous mineral impurities and staining from biological pigments.