There are small vents at the top of the mounds in this photo from which there is a small spring, coming from the depths of the earth where minerals are dissolved in the hot water, and redeposited as it hits the relatively colder surface. Most of what is deposited is travertine, a bland calcareous rock, but the bacteria and algae that live on the mounds add the bright colors you see in the photo. Yellowstone National Park in Wyoming supports an interesting population of such bacteria throughout its various thermal features.
