About 12 km north-east from the town of Analavory in Madagascar are located four unusual geysers. There is no volcanic activity underneath Analavory that superheats underground water and forces them up as steam. The water gushing out from the mouth of these limestone mounds is therefore not scalding hot, but warm to the touch. In fact, the Analavory geysers aren’t natural at all.
The “geysers” are located in an area close to some aragonite mines. Excess water from the mines are removed by metal pipes that terminate at the bank of Mazy River. The water is warm and rich with carbonic acid that easily dissolves large amounts of lime along its way. Further the water goes through iron pipelines, and carbonic acid dissolves iron as well. The carbon dioxide rich water rushes along the pipe under pressure and when it emerges at the end of the pipeline, the sudden decrease in pressure causes the dissolved carbon dioxide gas to erupt in bubbles producing a geyser like phenomenon. This is similar to opening an aggravated bottle of soda.
Over time, the dissolved lime and iron precipitates into large mounds of travertine, a type of limestone, around the mouth of the outlet. The rusty orange color comes from the iron. The mounds are more than 4 meters high and will continue to grow.
For most time the carbonated water spouts 20 to 30 cm, but sometimes when the vents are blocked by the precipitated lime, the build up of pressure produces spouting to several meters once uncovered.
Geysers such as those at Analavory are called cold water geysers, and there are only a handful of natural cold water geysers on earth. The best known examples are Crystal Geyser, in Utah, the Wallender Born and Andernach Geyser in Germany, and one in Slovakia, Herľany.
A geyser very similar in appearance to Analavory Geyser is the Fly Geyser in Nevada, which is also an artificial geyser.
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