Yellowstone is situated on an active volcano! Three major eruptions have occurred in the last 2.1 million years, the most recent only 650,000 years ago. Magma only 3-8 miles down provides the heat for the thermal features in Yellowstone. The Upper Geyser Basin, home of old faithful, is one of 3 large geyser basins along the Firehole River. We were there on a sunny day and we treated to eruptions from 3 other geysers in addition to Old Faithful. Along the many miles of boardwalk we passed numerous hot springs, mud pots, and fumaroles (steam vents). At the north end of the park, the boiling water and steam travel to the surface through limestone, which dissolves to form a weak carbonic acid. When the hot water reaches the surface, calcium carbonate is deposited to form the
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