Friday, 14 July 2006

Secondary Mineral Formation in Cool, Dry Andisols of the Eastern Snake River Plain, USA.

Karen Castenson1, Paul McDaniel1, and David Hoover2. (1) University of Idaho, Department of Plant, Soil, and Entomological Sciences, PO Box 442339, Moscow, ID 83844-2339, (2) Natural Resources Conservation Service, 9173 West Barnes Drive, Suite C, Boise, ID 83709

Soils developed from volcanic ejecta have been extensively studied throughout the world in most climates, with little attention given to formation in cool, aridic climates. This study was conducted to determine the secondary mineral synthesis in soils formed from volcanic materials in the cool, dry climate (mean annual temperature and precipitation: 6.1oC and 380 mm) at Craters of the Moon National Monument and Preserve, Idaho, USA. This area is the largest dominantly Holocene basaltic lava field in the contiguous United States. Eruptive periods occurred from 15,000 to 2,100 yrs BP with as many as 3,000 yrs between eruptions. Particle size, mineral identification, and selective dissolution analyses were performed on bulk soil samples. Coarse fragments range from 18-94% cinders, with the remaining fraction consisting of finer particles of volcanic ejecta that have important edaphic implications for plant growth parameters in the high desert. Electron microprobe analyses of unweathered volcanic glass indicate a basaltic andesite (53.5-58% SiO2) to basaltic (45-53.5% SiO2) composition. The local, iron-rich volcanic glass is colored and therefore more susceptible to weathering than noncolored glass derived from rhyolite, dacite, and andesite rock types typical of the Pacific Northwest USA, which range from 58-80% SiO2. The ratio of oxalate-extractable Al:Si ranges from 1.07 to 2.28 with a mean value of 1.35. Average oxalate extractable iron (Feo) (1.07%) is considerably higher than Feo measured on most andic soils in the Pacific Northwest (0.72%). The dominant andic soils in the Pacific Northwest are siliceous and have greater Alo (1.31%) and Sio (0.51%) than soils at Craters of the Moon (0.49% Alo and 0.36% Sio). Due to these differences, Craters of the Moon tephra are expected to have a different suite of secondary mineral products than other Andisols throughout the region.

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