Wrangell Lava. Rhyodacite flow (Pleistocene). Massive to platy and locally vesicular; possibly single flow about 18 km long, 7 km wide, and as much as 350 m thick. Rock is light gray to medium light gray, and moderately porphyritic containing phenocrysts of sodic plagioclase (4-7%, 1-5 mm), rounded green clinopyroxene (trace to 2%, generally less than 1 mm), and rare potassium feldspar and hornblende in a groundmass of micro- to cryptocrystalline felty aggregates (vapor phase crystallization?) dusted with opaque minerals. A dark-gray to brownish-black massive vitrophyre phase (indicated by stipple), showing spectacular swirly columnar joints, occurs at base and locally at top of flow. Source is probably dome along west side of the north Sanford eruptive center in the adjacent Gulkana B-1 quadrangle (Richter and others, 1989). Whole-rock K-Ar age of sample 74-ALe-29 (map locality 18) is 0.53 +/- 0.06 Ma (table 2). Volume and composition of the lava may be unique for the Wrangell volcanic field. Using an average thickness of 150 m, the dimensions indicate a volume of more than 20 cubic km which is unusually large for a single flow of remarkably uniform composition. However, there is no evidence to suggest that the lava was not emplaced as a normal lava flow. Although the flow is referred to as rhyodacite because of its mineralogy, it does plot in the rhyolite field in the K2O/SiO2 variation diagram of figure 1 and the high K2O content places it well within the high-K field.
Geologic map of the Nabesna B-6 Quadrangle, south-central Alaska