Youngest volcanic rocks

Unit symbol: Qv
Age range Quaternary and latest Tertiary? (1.806 to 0 Ma)
Lithology: Igneous - Volcanic
Group name: Young volcanic and shallow intrusive rocks
Volcanic rocks ranging in composition from rhyolite to basalt. Along the Aleutian magmatic arc and the Wrangell Mountains, the rocks are predominantly andesite and lesser dacite and basalt of calc-alkaline and tholeiitic affinity in lava flows, volcanic breccia, lahar deposits, and debris-flow deposits. Lava flows and clasts in other volcanic deposits of unit are porphyritic, typically glassy, gray to black, and commonly vesicular. Unit also includes basaltic, basaltic andesite, and dacite parasitic cinder and spatter cones. Unit typically forms volcanic edifices; it also forms isolated outcrops that cap ridges, providing a good example of topography reversal, which results from erosion of surrounding country rocks, leaving exposed more erosion-resistant flows that formerly had occupied valleys. Individual flows are locally as thick as 30 m and are laterally continuous over large areas. Includes Edgecumbe Volcanics (basalt, andesite, and dacite) on Kruzof Island (Loney and others, 1975; Riehle and others, 1989) and unnamed basaltic to rhyolitic rocks on islands west of Prince Wales Island (Eberlein and others, 1983), and on Zarembo, Kuiu, and Kupreanof Islands (Brew and others, 1984). Rocks of Holocene age were recognized east of Wrangell Island (Elliott and others, 1981) and on Kruzof Island (Loney and others, 1975), and basaltic rocks of Holocene and (or) Pleistocene age are found on southern Kupreanof Island (Brew and others, 1985). On Revillagigedo Island and mainland to the east in the Ketchikan quadrangle (Berg and others, 1978, 1988) and at many other localities in southeast Alaska (Karl and others, 2012), this extrusive unit consists of alkaline-olivine basalt that forms volcanic cones, columnar jointed lava flows, and rubble flows that contain pumice and scoria; it also includes lenses of ash and lapilli a few centimeters to a few meters thick—too small to show on the map. Includes postglacial flows and pyroclastic deposits that overlie glacial deposits and landforms

Source map information

Source map Richter, D.H., Moll-Stalcup, E.J., Duffield, W.A., and Shew, Nora, 1997, Geologic map of the Nabesna A-6 quadrangle, south-central Alaska: U.S. Geological Survey Open-File Report 97-475, 1 pamphlet, 24 p., 1 sheet, scale 1:63,360.
Symbol Qwa
Unit name Wrangell Volcano -- Two-pyroxene high-silica andesite flows
Description Medium- to dark-gray, thin to thick (5-30 m) lava flows that generally exhibit oxidized and slightly scoriaceous tops and bottoms. Rock is conspicuously porphyritic containing phenocrysts (2-7 mm) of plagioclase (15-20%), orthopyroxene (1-10%), clinopyroxene (trace -5%), opaque minerals (trace %), resorbed olivine (0-trace %), amphibole (0-trace %), and locally abundant glomerocrystic clots of plagioclase + orthopyroxene ± clinopyroxene ± opaque minerals in a pilotaxitic to intersertal ground mass of plagioclase and pyroxene microlites in a glass or cryptocrystalline matrix. Orthopyroxene is invaribly the dominant pyroxene. Some phenocrysts may be largely the result of the disaggregation of glomerocrystic clots. Plagioclase shows evidence of as many as 3 different populations; orthopyroxene is locally rimmed by clinopyroxene and olivine by orthopyroxene, all suggestive of a mixed magma origin. Rock contains 61.8-63.4% Si02 (samples 91ARh-11, 91AMl-5, Table 2); in the contiguous Gulkana. A-1 quadrangle, SiO2 content of correlative lavas ranges from 59-62 % (Richter and others, 1994). Nye (1983) reports a whole~rock K/Ar age of 0.5+/-0.4 Ma for a flow in sec. 18, T. 4 N., R.9 E.; in the contiguous Gulkana A-1 quadrangle, K/Ar ages for correlative lavas range between 0.5 and 0.6 Ma (Richter and others, 1 994)
Lithology Igneous

Correlated geologic units

Label Qw
Description Wrangell Lava, younger part
Geologic age Pleistocene to Holocene
Geologic setting Extrusive
Lithology Form Importance
Dacite < Felsic-volcanic < Volcanic < Igneous Flow Major
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Major
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major