MORPHOLOGICAL, FIELD STRUCTURES AND TEXTURES IN THE 1.64 MA AND 1.39 MA TIETON ANDESITE LAVA FLOWS, SW WASHINGTON CASCADE VOLCANIC ARC

The 1.64 ± 0.07 Ma (Qta₁) and 1.39 ± 0.10 Ma (Qta₂) eruptions of the Tieton andesite lavas (Ta) erupted from vents located on Bear Creek Mtn. of the Goat Rocks volcanic complex in the Cascade arc SW Washington. Sporadic remnants of the lavas are found on canyon walls and floors, and ridge tops. These lavas are trachyandesites that followed the ancestral Tieton River valley as intracanyon flows for 74 km (Qta1) and 52 km (Qta2) respectively and are among the longest arc lava flows in the world. The Ta provide an opportunity to study very long cooling-limited lava flows emplaced on fans, deep valleys, and flat valley terminus. This study documents the flow morphology, structures and textures found along of the flows. The goals of these observations are to provide constraint and help facilitate an understanding of the lava flow dynamics of trachyandesites. The main morphological units under consideration are the lava channels and levees, and the frontal zones. The morphology of the frontal zone (FZ) depends on the amount of lava available, debris, and the substratum. Characteristics of the flows FZ are blocky talus in front, and flow lobes along the margin and terminus. The top and basal breccia is blocky typically dark gray to black with a glassy matrix. The channel zone (CZ) shows variation along the length of the flow. However, overall, the CZ has a non-welded to welded basal and marginal breccias grading up into a dense, gray columnar jointed unit overlain by an entablature with a thick flow top breccia. The core is gray and is more cryptocrystalline. The flow is maintained by continued flow of hot lava over a long period, a thick breccia layer as thermal insulation, lava inflation, and basal and marginal reheating. The study supports previous findings that lava morphology, structure, and textures are controlled by variations of effusion rate, down channel flow rate, channel substratum and drainage system, and thermal evolution regarding viscosity changes.