PRELIMINARY CONSTRAINTS ON THE LENGTHS OF THE TIETON ANDESITE LAVA FLOWS

The Tieton andesite lava flows, in the Cascades of Washington, are from 74 to 52 km long and have been classified into a group of long lava flows found worldwide. The volume of the two flows are estimated at 6.6 and 2.5 km³ with a respective estimated effusion rate reported at 11-18 m³/s. Evidence from emplacement features supports flow inflation that occurred over at least a decade at a sustained effusion rate.

Lengths of lava flows and the controlling properties are not yet fully understood. Initially, George Walker (1973) stated that the mean effusion rate was the key constituent that enabled flow length. Later, Pinkerton & Wilson (1994) discussed the effects of marginal cooling and demonstrated more silicic flows are capable of flowing long distances. Subsequently, Harris and Rowland (2009) argued that it was the role of heat loss that inhibited flow length. The focus of this study is on what are the principal influences controlling flow length for valley confined, siliceous flows of andesitic, trachyandesitic (latites), basaltic andesites, and phonolitic compositions.

Using our field observations of the Tieton andesites, supported by additional literature of 177 lava flows with effusion rates, viscosity, composition. We use FLOWGO (Thermorheological model) to calculate viscosities, yield strength and crystal content related to the Tieton andesite to constrain physical properties for comparison with other flow types. Observations indicate the lava flowed down a broad valley where it entered a larger canyon, spread and stopped. The basal breccia, below the columnar jointing, show clast point welding grading upwards into densely welded breccia and finally into the columnar jointing. Modeling using FLOWGO produces a temperature of 1120°C with a viscosity of 6551 Pa s, crystal content of 35%, yield strength of 2x10⁵ Pa, and a velocity of ~300 ms^-1. Not all parameters are reasonable however, the model does show that longer flow lengths are possible if the basal temperature is higher and the crust remains immobile. In addition, the data supports previous work in that low cooling rates (well-insulated) and higher velocities would produce longer flows similar to the Tieton andesite as supported by field data.