FACTORS CONTROLLING THE EMPLACEMENT OF THE 80-KM-LONG TIETON ANDESITE LAVA FLOW FIELD, SOUTH-CENTRAL WASHINGTON

The 80 km-long Tieton andesite lava flow erupted 1 Ma from the flanks of Goat Rocks Volcano located in the southern Cascades of Washington. The Tieton andesite provides a valuable opportunity to evaluate the factors that contributed to producing the world’s longest andesitic (59-63% SiO₂) lava flow. To accomplish this goal, we document whole-rock and trace element compositional variations along with the crystal size distributions (CSD), viscosity, and morphological changes across and down flow, and attempt to ascribe them to processes that produced the lava flow field. Initial chemical results suggest two larger and a group of smaller proximal lava flows. The first flow ranged in SiO₂ from 61-63%, is up to 60 m thick and traveled the furthest. The second shorter flow ranged from 59-61% SiO₂. Preliminary CSD results show at least two distinct plagioclase populations. One with higher concentrations of smaller crystal lengths and slightly steeper slopes and the other by lower crystal density with greater lengths and flatter slopes, which can be correlated with the two larger flows. Viscosity calculations based on, chemical composition respectively, range from 10-15 Pa sec at 1000°C to 14-20 Pa sec at 1,200 °C (1-5% H₂O respectively for each). Morphologically the lava flowed down a board ancestral valley of the Tieton River and entered a narrow canyon where it eventually entered the wider paleo-Naches valley subsequently spreading out and stopping. Initial results indicate the topography influenced the length of the flow lobes. Both chemical and CSD data support multiple flows. Flows of long lengths can be related to high effusion rates, however, insulated flows even with lower effusion rates could have long lengths if eruption durations are long and continuous. The high SiO₂ content of the Tieton andesite implies that effusion rates, topography, slow heat loss, heat of crystallization, and tube or channel-fed lava emplacement may explain its 80-km length. To constrain the factors more closely, we will apply a numeric model which links equations that describe the lava flow rheology, cooling rates, and dynamics to simulate the emplacement of the Tieton andesite lava flow(s).