Hugh volcanic eruptions have taken place in the area and much of the park sits in the "Caldera" (crater) of the last one (600,000 years ago).
The area has over 10,000 geothermal features as the earths crust is only 3-6 miles thick compared to a normal area thickness of around 30 miles.
Clear blue, near boiling water in many pools has coloured edges caused by bacterial growths which are adapted to the high temperature and a diet rich in sulphur and iron.
Probably the best known geyser in Yellowstone "Old Faithful", so named because it erupts every 90 minutes or so to a height of 150ft ft or more.
In the north-west is a different type of formation. Mineral laden water is pushed into pools where evaporation has caused multi layered rock (called Travertine) to build up in huge terraces.