Between eruptions, Giant Geyser is a steaming hole surrounded by a cone about 12 feet high. The cone consists of sinter (a buildup of siliceous minerals precipitated from hot springs in volcanic areas). Unlike the Mammoth Hot Springs area where carbonate rocks occur in the subsurface, all the rocks within the Yellowstone volcanic caldera region consist of siliceous igneous rocks (basalt and rhyolite). Ongoing hydrothermal activity underground is gradually breaking down the bedrock and supplying dissolved silica to surface springs. When the water reaches the surface, the changing conditions allow silicate minerals to precipitate. Sinter consists primarily of quartz, opal, and clay minerals.