When carbon dioxide-rich water percolates through rocks in limestone areas, the water dissolves the limestone and becomes saturated. Also, heated hot spring waters are saturated with silica that was picked up at depth. Any change in the environmental conditions (esp. pressure, temperature, photosynthesis of biotic material) causes the water to rapidly precipitate calcium carbonate or silica as coatings or thick layers, often encrusting small debris or biotic material such as moss or algea.