MT. RAINIER - THE ODD MAN OUT: ON USING THE GEOCHEMISTRY OF LAHARS TO IDENTIFY SOURCE VOLCANOES IN THE CASCADES

In understanding the history of volcanic eruptions, it is important to study and understand one of the direct results of these eruptions: the lahars. This study addresses whether a lahar can serve as a geochemical fingerprint for a volcano. Multiple methodologies were used in the study to match the character of a lahar to the source volcano from which it came. This process is applicable in areas where volcanoes are closely spaced and the presence of interconnected river valleys can lead to the convergent flow of lahars, which can confuse the identity of the source volcano. This study focuses on three volcanoes in the Cascades: Mount St. Helens, Mount Hood, and Mount Rainier. The order of the deposition of multiple lahars from these different volcanoes was obtained by comparison of chemical and mineralogical data. Samples were obtained from the Hood River, the Toutle River, and from drainage of Mt. Rainier. Mineralogy was identified through X-ray diffraction (XRD) and chemistry was obtained through X-ray fluorescence (XRF). Scanning electron microscopy (ESEM-EDAX) was used to identify crystal habit and as an additional source for chemical identification. Statistical analysis of the data included a Bray-Curtis Matrix and the application of discrimination plots to data from EDAX and XRF analyses. The analyses revealed that iron is the strongest discriminator. The chemical and statistical analyses indicate that three groupings of lahars can be matched to three different source volcanoes. Furthermore, analyses indicate that Mt. St. Helens and Mt. Hood have similar chemistries while Mt. Rainier stands alone.