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Pedogenesis in soils derived from the tephras of Mount St. Helens
Abstract
Mount St. Helens has been an intermittent, but prolific source of tephra for more than 40,000 years. In between eruptions soils formed and were subsequently buried by ejecta. Soils from the area of Mount St. Helens were sampled and horizons were correlated to known tephra beds ranging in age from the 1980 eruption to 4,000 years B.P. Spodosols and Andisols formed in areas of similar climate, vegetation, landscape position, and time of weathering. Andisols are derived from more mafic tephra deposits where bases help maintain high pH values and large breaks in the pore-size distribution hinder the leaching of bases. Spodosols form in more felsic tephra deposits where bases are fewer and similar textures throughout the profile enhance the leaching of bases. Spodic horizons and andic soil materials have formed in horizons where the parent material is less than 450 years B.P. The scanning electron microscope (SEM) was used to examine weathering trends of minerals utilizing a semiquantitative approach and to examine the morphology and genesis of secondary minerals from natural fabric samples. The weathering of the grains of fine sand is both time and texture dependent. The natural fabric samples revealed tubular and spherical forms of halloysite plus an intermediate of the two, all weathering directly from volcanic glass at some sites. At other locations allophane is present with aluminum to silica ratios of 2:1. Both alterations occur in 2500 years B.P. or less. Halloysite forms in resilication environments, and allophane with Al:Si ratios of 2:1 forms in desilication environments. The coexistence of gibbsite and gypsite nodules is attributed to the dynamic nature of these soils. Gibbsite often forms through a desilication process. The soil environment changes as new tephra is added and the soil is resupplied with bases and silica.
Subject Area
Geology|Mineralogy
Recommended Citation
Ahrens, Robert Joseph, "Pedogenesis in soils derived from the tephras of Mount St. Helens" (1995). ETD collection for University of Nebraska-Lincoln. AAI9528816.
https://digitalcommons.unl.edu/dissertations/AAI9528816