Formation of the Soils
Soils develop over a long period of time. The time needed to fully develop the features that represent a mature soil is measured in tens of thousands of years. Volcanic areas are unique because of the deposition of pumice and ash on the surface. Soils in these areas rarely have sufficient time to develop before being buried during the next eruption.
The last major volcanic eruption in the park was only 7,700 years ago (Bacon and others, 1997). Material from the eruption of Mount Mazama covered about 90 percent of the surface of the park. The oldest soils, those of the Grousehill and Oatman series, are in the remaining 10 percent, mainly along the western and southwestern edges of the park. These soils formed on glacial ground moraines that are about 15,000 to 25,000 years old (Harris, 1988). Soils of the Llaorock series are an example of those that began forming in andesite and dacite bedrock when the last glaciers receded from the park, but thin layers of ash have been added to the profile over time.
The soils in the remainder of the park exhibit only early stages of soil development. The basic nature of these soils is dependent on the parent material in which they are forming. The relative thinness of the solum (A and B horizons) indicates that not much weathering and soil development have taken place below a depth of about 20 inches. Examples of soils in this early stage of development include those of the Maklak, Lapine, Castlecrest, Sunnotch, Timbercrater, and Cleetwood series.