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 Patton, W.W., Jr., Wilson, F.H., and Taylor, T.A., 2011, Geologic map of Saint Lawrence Island, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-3146, pamphlet, 13 p., 1 sheet, scale 1:250,000, and database.
Symbol Qb
Unit name Basalt
Description Flat lying to gently dipping lava flows and interlayered tuffs and breccias composed chiefly of alkali olivine basalt, olivine tholeiite, and minor amounts of basanite, hawaiite, and nephelinite (Moll-Stalcup, 1994). Numerous small cinder cones and flows, composed chiefly of basinite with xenoliths of peridotite and gabbro are aligned in an east trending belt along the crest of the Kookooligit Mountains. Unit assigned a Pleistocene age based on K-Ar ages ranging from 0.238 to 1.46 Ma (Patton and Csejtey, 1980). Location-Unit forms the Kookooligit Mountains, a large shield volcano that covers approximately 850 km in the central part of the island. Two smaller basalt fields cap the uplands near Southwest Cape and tiny patches of the basalt are scattered across the lowlands south and west of Niyrakpak Lagoon and at Mugum Peak southeast of Apatiki Camp
Lithology Igneous

Correlated geologic units

Label Qvm
Description Alkali basalt series including mugearite and benmoreite flows
Geologic age Quaternary
Geologic setting Extrusive
Lithology Form Importance
Mafic-volcanic < Volcanic < Igneous Flow Major