"Mammoth Hot Springs, located about 8 km inside the
north entrance to Yellowstone National Park, consists of
nearly 100 hot springs scattered over a score of steplike
travertine terraces. The travertine deposits range in age
from late Pleistocene to the present. Sporadic records of hot-spring activity suggest that most of the current major
springs have been intermittently active since at least 1871.
Water moving along the Norris-Mammoth fault zone is
heated by partly molten magma and enriched in calcium
and bicarbonate. Upon reaching Mammoth this thermal
water (temperature about 73°C) moves up through the old
terrace deposits along preexisting vertical linear planes of
weakness. As the water reaches the surface, pressure is
released, carbon dioxide escapes as a gas, and bicarbonatein the water is partitioned into more carbon dioxide and
carbonate; the carbonate then combines with calcium to
precipitate calcium carbonate, forming travertine." US Geological Survey 1978