Groundwater and spring deposits
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.
Stalagmites
Domed calcite formations built up from mineral-rich water that deposits the dissolved mineral on the floor of caves
Size: 21cm
Stalagtite
Calcium carbonate-rich percolating groundwater precipitate as icicle-like forms of calcite from the roof of cave
Size: 20cm
Inside a Stalagmite
The evaporation of lime-bearing waters dripping into caves produces stalactites, which grow downward from the roof of a cave, and stalagmites, which grow upward from the floor. Both build up a layer at a time, and resemble tree rings in a cross section. A difference in mineral content on deposition causes color banding.
Size: 70cm
Travertine
Compact calcium carbonate deposit precipitated from warm water springs in hydrothermal systems
Size: 10cm
Tufa
Highly porous CaCO3 deposit with a spongy texture, precipitated from cool fresh spring water
Size: 10cm
Siliceous Sinter
Deposit of silica through precipitation from hot spring and geysers
Size: 10cm
Tufa
Calcareous sinter formed from precipitating water that traps in organic matter, such as leaves, moss and algae
Size: 15cm