Castle Geyser , like Old Faithful, is located within the Upper Geyser Basain. It is one of the largest geysers in the park. The mound of Castle Geyser is composed of geyserite, or siliceous sinter - a form of silica similar to opal. Fumaroles are smaller features that emit steam and other gases, sometimes with a continuous roaring sound. Mud pots are hot springs of boiling mud .
Bubbles rise through the viscous liquid, then pop, leaving rings on the surface.
Algae or particular minerals may tint the mud various colors, as in the Fountain Paint Pots of the Lower Geyser Basin. Springs and geysers in Norris Basin are hotter than any others in the park. This suggests that magma is nearest to the surface in that area. Visitors can follow paths around many of the geysers and hot springs. Here a steaming hot spring lies in front of an erupting geyser.