The Mammoth Hot Springs terraces make me think of ancient ruins dreamt up by H.P. Lovecraft. I half expect some alien creature to open an unseen door and step out of the terrace in front of me. On close inspection I realize I'm not that far off.
The terrace is formed by water heated from the hydrothermal features in the park interacting with limestone. As the hot water travels the Morris-Mammoth Fault it dissolves the limestone rock. As the water reaches the surface it begins to cool, releasing carbon dioxide and depositing the remaining calcium carbonate as travertine. Anything in the way becomes coated and eventually is consumed and integrated into the framework of the terrace.
So what are the alien creatures I alluded to earlier? Thermophiles - millions of them. These heat-loving microorganisms thrive in the hydrothermal features in Yellowstone National Park. Archaea live in the hottest waters, some plate-like, others spiral or rod shaped. The cooler waters are home to Sulfur-oxidizing filamentous bacteria. These bacterial colonies are responsible for the beautiful colors we see in the pools of water. The colors change based on the water temperatures, flow and the amount of sunlight.
So next time you find yourself peering deep into the colorful pools in Yellowstone, remember, you are looking into the home and lives of millions of bacteria. Don't worry - this type of voyeurism is just fine!