Tufa mounds are actually more like jacuzzis made of mineral deposits.
These structures sit on hot springs, which are basically locations in or near geologically active areas where water is superheated underground. The water that seeps out of the ground is really hot and, just like dissolving the sugar in your coffee, it dissolves different types of minerals as it makes its way through the cracks in the earth.
When this water reaches the surface, it is cooled rapidly and can’t hold those minerals anymore. These dissolved minerals immediately start to crystallize and precipitate, attaching themselves to the first best thing. So first, little thimbles form, then these continue to grow as the water repeats this cycle of spouting out and cooling. Multiply this process by 85 thousand years and those thimbles grow to epic proportions, like almost 90ft / 27m tall and 250ft / 74m across!
What you end up getting is a house of cards type structure of stacked bathtubs full of hot water and made of crystallized minerals. These delicate structures are very well protected because they are extremely fragile and they have remained intact partly because of how remote their location is.
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