BUILDING CASTLE GEYSER – GEOLOGIC AND CHRONOLOGIC INTERPRETATION OF THE DEVELOPMENT OF CASTLE GEYSER, UPPER GEYSER BASIN, YELLOWSTONE NATIONAL PARK, WYOMING, USA

Castle Geyser, due to its large and imposing edifice, has long been interpreted as one of the oldest geysers in Yellowstone. While much attention has been paid to variations in its patterns of eruption, there has been relatively little discussion of its geologic development. The chronology of the development of Castle is also poorly constrained.

Castle Geyser shares, with variations in scale and probably chronology, many geologic features typical of other geysers in the Upper Geyser Basin. The lowest level of the geyser edifice is a gently sloping terrace that is composed of subhorizontally layered silica, in bands that range from about 1-2 mm to 1-2 cm. Some of these layers show relict characteristics of biogenic-rich zones. Similar sub-horizontal terraces are found at Giant Geyser and at Beehive Geyser. The second (and third?) generations of the growth of the geyser are characterized by horizontal terraces. These terraces, which have subsequently been covered in part by the upper cone of Castle, may be from either hot spring or geyser activity (similar to the much larger but morphologically similar terraces at Great Fountain Geyser). The final stage in growth has been the development of the upper cone.

Quantifying the growth of geysers has been difficult. AMS 14C dating of sinter has recently become possible, and allows an approximate chronology to be developed. A date of 928 +/- 40 years BP on sinter from near the top of the geyser is younger than early observers suggested. Further dating (in progress) will help refine the chronology of each of the stages of growth outlined above.

It is interesting to note that, based on morphological development that is similar to several other geysers, Castle Geyser may record a development history that reflects basin-wide changes in geyser activity.