108 GEYSERS AND HOT SPRINGS announcing the schedule of eruption The Nordic version of Old Faithful is Strokkur, in Iceland (Figure 3) Geysers have individual schedules: a geyser at the south end of Lake Bogoria (formerly Hannington), Kenya, erupts about every 10 minutes, whereas Beehive geyser, in Yellowstone, erupts only once a year, and a geyser at Rotorua, New Zealand, erupts four times a week Changes in rainfall patterns can affect geysers; the Lake Bogoria geyser can temporally cease activity if the lake level gets too high, and the Great Geyser in Iceland, though active for 8000 years, now has to be stimulated by an injection of soap powder Sinter and Travertine Terraces Figure Strokkur geyser, Iceland Reproduced with permis sion from Green J and Short NM (1971) Volcanic Landforms and Surface Features: A Photographic Atlas and Glossary New York, Hei delberg, Berlin: Springer Verlag Hot waters dissolve various chemicals as they traverse underground rock channels When hot waters exit at surface openings, the dissolved chemicals are precipitated and form cowls or cones around the exit fissure or aperture If the waters stream downslope, they may form spectacular terraces The pink terraces at Rotorua, New Zealand, were long famous but were destroyed by earthquake action, and the best example now extant is the magnificent organ-pipe terraces at Mammoth Springs, Yellowstone, Wyoming (Figure 4) Such terraces are commonly composed of calcium carbonate and are referred to as tufa, where they are carious, or as travertine, where they occur in massive layers Where such deposits are siliceous, they are sometimes referred to incorrectly by these terms, and are best referred to as ‘siliceous sinter’ (calcareous deposits are also sometimes referred to as ‘calcareous sinter’) Figure Mammoth Hot Springs terraces, Yellowstone National Park, Wyoming, USA Reproduced from Science Photo Library