Mass Balance of Soil Evolution along a Chronosequence of Basaltic Cinder Cones.

The investigation of soil evolution across a chronosequence of basaltic tephra at Craters of the Moon National Monument and Preserve allows for the examination of physical, chemical, and mineralogical properties as a function of time. A well-dated series of basaltic cinder cones formed approximately 2.1, 6.9, and 13.9 ka was investigated. Representative parent material, coarse fragments, and soil samples were analyzed using a variety of selective dissolutions and total elemental digests. Coupled with traditional soil laboratory analyses, a mass balance approach provides a means of relating secondary mineral formation, weathering, and elemental dynamics. Additions, losses, transfers, and transformations of elements were examined within each soil profile and across the cinder cones. The highest degree of weathering was identified on the 13.9 ka profile followed by the 6.9 and 2.1 ka soils. Short-range ordered minerals, including allophane and ferrihydrite, dominate the colloidal fraction of all soils, with greatest total amounts in the oldest soil. Vegetation effects on weathering rates were also observed in the youngest soil profiles with greater weathering occurring in well-vegetated landscapes. The dominant weathering and soil forming processes are the accumulation of organic carbon, weathering of rock fragments, desilication and loss of bases, redistribution of Fe, Al, and Ti, and the accumulation of secondary short-range order and crystalline minerals.