PEDOGENIC RATES WITHIN THE MOUNT SAINT HELENS NATIONAL MONUMENT

On May 18^th, 1980 Mount Saint Helens experienced an eruption that would change the surrounding land irreversibly. Recumbent trees, hummocky topography, and deep sedimentary lahar depositions are visible 20 years later as if the eruption were yesterday. However, on a smaller scale the monument’s sub-alpine soil was affected in a less obvious manner. With vegetation temporarily destroyed by the scorching heat of tephra deposition, soils would no longer develop at rates influenced by species richness due to the rain of the Pacific Northwest. Rather, a layer of tephra ranging in thickness from several centimeters to several meters would serve as the primary influence for new pedogenic rates within the monument.

A paper by Ugolini and LaManna discusses the compositions of soils, namely the presence of vermiculite, as affected by tephra depositions. My study is similar in that my soil samples also contain trioctahedral vermiculite, as proven by XRD analysis. However, because my samples are taken from a variety of microclimates within the monument, I will discuss pedogenesis rates as affected in general by tephra depositons, but more specifically, by current slope, moisture content, sun/shade exposure, and altitude. Further specification of mineral and chemical composition will be enhanced by the ICP and SEM here at Middlebury. Ultimately, chemical and mineral compositions of soils collected in August, 2000 will be compared with pedogenic rates existing prior to May 18^th, 1980.

Thus far, XRD analysis of several locations including both lahar and ash depositions indicate an abundance of fresh plagioclase feldspar. In addition, the ash deposition contains vermiculite and the lahar contains vermiculite and halloysite. The presence of halloysite indicates that the lahar is undergoing more advance weathering. What causes this? SEM analysis will provide the information to determine whether the feldspar is weathering into vermiculite and halloysite, or whether all three are detrital and formed on the volcano prior to the eruption.