The amount of deposit surrounding a feature does not always determine its age. Drilling and core samples made in the Upper Geyser Basin revealed a sinter layer nearly 20 feet thick. Deposition varies and depends on the amount of silica brought to the surface and this is not always consistent. A large sinter mound does not always mean an old thermal feature. But most sinter accumulation is only a minute fraction of an inch annually.
Waters flowing from hydrothermal springs contain several dissolved minerals and gases.
These minerals in turn determine the water's pH as acidic, neutral or alkaline. Most geyser basins are either acidic or alkaline, and in some, like Norris, acidic and alkaline springs flow side by side. Some of the thermal features emit strong or obnoxious smells and even deadly odoriess gases. These gases, released at the surface after the pressure lowers, include carbon dioxide, hydrogen sulfide, methane, hydrogen, oxygen, nitrogen, ammonia, argon, radon, as well as other noble gases such as helium, neon, krypton, and xenon. Travertine is the mineral formation responsible for the famous Mammoth Terraces.