THE PHYSICAL VOLCANOLOGY OF THE TIETON ANDESITE LAVA FLOW: LAVA FLOW LENGTH AND COMPOSITIONAL VARIATIONS

Over 1 million years ago, Goat Rocks Volcano in the Cascade Range of Washington State produced one of the longest andesitic lava flows, nearly 80 km in length, with a minimum volume of at least 2 km³ and a thickness of up to 60 m. The flow was mapped by Warren (1941) as the Tieton andesite lava and extends from Goat Rocks Volcano down the broad ancestral Tieton River valley through a narrow canyon until it pooled in the Naches valley at the outskirts of Yakima, WA.

The goal of this study is to document any compositional variations within the Tieton andesite lava flow and attempt to ascribe them to processes that produced this lava flow field. An additional related question is why did a highly evolved (andesitic) lava flow of presumably high viscosity flow ~80 km from its source vent while all other reported cases of andesitic lavas flowed <10 km?

Stratigraphic sections were sampled through the lava flow, bottom to top, from near the source on Pinegrass Ridge to its terminus near Yakima. The samples were analyzed in hand sample and thin section using a petrographic microscope, and chemically using X-Ray Fluorescence and Inductively Coupled Plasma Mass Spectrometry.

Initial results show the highest silica content (61-63% SiO₂) in the lava flow is down valley near Yakima and the lowest silica content (59-61% SiO₂) is up valley closest to the source area. There are two outliers with silica content of ~55% SiO₂ found on Pinegrass Ridge. Overall crystal content appears to also vary depending on location with higher crystal percentages distally than at proximal localities. Phenocrysts of plagioclase, pyroxene, hypersthene and magnetite comprise on average 25 – 35% and reside in a glassy groundmass. Initial conclusions about the Tieton andesite flow include: (1) the length of the flow appears to have been controlled by heat loss, effusion rates, and topography, (2) the distribution of the rock chemistry implies the eruption as driven by more mafic magma injection into the Tieton andesite magma chamber, and (3) multiple smaller volume lava flows erupted during the waning stage of the eruption.