How do you tell geysers and hot springs apart? Geysers erupt—or shoot their water and steam into the air periodically. But when a fountain geyser which erupt from a pool, isn’t erupting, it can look a lot like a hot spring.
Look for clues around the edges. Hot spring pools usually have a ledge or walls of sinter built around them because as the water level fluctuates, it leaves behind silica deposits. The edges may even be scalloped or lacy.
Beadwork or pebbly-looking sinter indicates a geyser. As the water splashes with each eruption, it deposits silica, creating a bumpy appearance. Cone geysers are often decorated with this beadwork as well. Rounded stones in the water around a thermal feature are good indication that it erupts. Geyser eggs, which look like river stones, are also formed from water movement and silica deposition.
Dark-colored algae and bacterial mats living in a feature tell you that the water isn’t hot enough to be a geyser or that it hasn’t erupted in some time.
So whether or not there is a sign naming the feature as a hot spring or geyser you may be able to figure out for yourself whether or not it erupts by looking at the surrounding sinter and bacteria mats. And even if there is a sign, remember, because features are always changing, signs can be misleading.