Yellowstone National Park, USA (5)

Emerald Pool is a large steaming pool with thin, solidified sinter around the edge of the pool. Orange thermophiles line the bottom of the pool and throughout the outwash channel. Emerald Pool was once blue with some yellow thermophiles that caused its waters to appear green, hence its name. But like other pools in Black Sand Basin, a drop in water temperatures have resulted in the loss of the bright pools, with them being replaced with orange, murky pools. Emerald Pool has an average temperate of 136.3°F (58°C).

Hot springs are the most common hydrothermal features in Yellowstone. Their plumbing has no constrictions. Superheated water cools as it reaches the surface, sinks, and is replaced by hotter water from below.
This circulation prevents water from reaching the temperature needed to set off an eruption.

Biscuit Basin contains a small collection of thermal features. Many, however, are small, gem-like encrusted pools and geysers, including Silver Globe Spring, Sapphire, and Black Opal pools, Jewel, Cauliflower, and Black Pearl geysers. The Firehole River and a highway divide the basin. A smaller group, located east of the river, contains mainly hot springs. Cauliflower Geyser is the main feature of this group, and it is identified by the cauliflower or biscuit-like sinter masses surrounding the crater.
Sapphire Pool dominates the main group west of the river. The water of this pool, or spring, is crystal clear with a Oriental blue sapphire tint. Other important features include Shell Geyser, which has a golden-lined crater, and Jewel Geyser, known for the shiny, beaded sinter around its vent.