PETROGRAPHIC ANALYSIS OF THE PAINTED DUNES FLOWS AT CINDER CONE, LASSEN VOLCANIC NATIONAL PARK

Cinder Cone is located in Lassen Volcanic National Park, the southern-most active volcanic center of the Cascade Range. Wood samples and lake diatoms were used to obtain a radiocarbon date for Cinder Cone between A.D. 1630 and 1670 (1). Cinder Cone is a 215 m high volcanic cone that represents one continuous event. After the scoria was deposited around Cinder Cone, blocky lava flows and ash were deposited. The ash deposits are exposed as the Painted Dunes and are different colors because they were variably oxidized as they were deposited. The cinders contain small crystals of olivine, plagioclase and quartz. A recent Natural Resources Conservation Service (NRCS) soil survey of Lassen Volcanic National Park included work at Cinder Cone where several 1.5 meter deep soil pits were dug to identify the near surface cinder layers. In one pit (site # 190), a more resistant layer is observed between 30 and 38 centimeters depth. This project investigates the timing and cause of the cementation of this tephra layer. Two hypotheses have been investigated here: 1) the scoria was fused together upon emplacement because individual cinders were hot enough to weld together; or 2) this layer is held together by secondary mineralization. Preliminary results indicate that the cinders are welded together, so are more resistant than surrounding tephra layers. In thin section, volcanic glass is observed enclosing crystals of olivine, quartz and plagioclase. If secondary mineralization is present we would see secondary minerals filling in pore spaces. Instead, these pore spaces are filled with volcanic glass. These observations suggest that this layer was partially welded during the emplacement of the ash and cinders. The absence of similarly welded layers in the pits suggests that the 30-38 cm depth layer may have been emplaced at higher temperatures than surrounding layers. Additional work will focus on higher resolution petrography to specify the degree of welding of the tephra particles, which should help constrain the temperature and emplacement conditions during the eruption of this layer. Similar analyses of additional tephra layers from other pits at Cinder Cone will also help determine the extent of this layer and identify any additional, similarly welded tephra layers. (1) Clynne et al., 2001