Though hot springs are the most common type of hydrothermal feature in Yellowstone, those found at Mammoth Hot Springs are vastly different from the deep pools of water throughout the rest of the park. Here, the water flows down a large terraced hillside looking like a gentle cascade.
The springs here are shaped uniquely because the hot water flows through limestone instead of volcanic rhyolite rock. The limestone is from an ancient inland sea that once covered the park. As the hot water and gases travel through cracks and fissures on their way to the surface, dissolved limestone is carried out and deposited as white travertine—or calcium carbonate, like in some antacid tablets.
The water and minerals sculpt intricate patterns and designs and the overall look is that of a staircase. Where the water flows over flat terrain, the terraces form steps holding pools of water with rounded overhanging rims. Where water flows more steeply downhill, it cascades over tiny scallops. In places, bacteria and algae thrive in the hot water and color the springs yellow, orange and brown.
The terraces are growing daily where the water flows over them, depositing minerals as it goes. But due to the complex network of underground channels, the water often changes course. One day the water could be flowing here, and the next day this could be dry and the water could be flowing over there.
New channels form as minerals dissolve and other channels become blocked with mineral deposition. Earthquakes can open or close cracks in the system as well.
Though dry springs eventually turn gray and brittle, dormant springs have been known to reawaken. Surprisingly the overall amount of water doesn’t change all that much, but where it flows out does—sometimes overnight. It’s geology on a fast track and a work in progress. Be prepared to see something different every time you come.