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 Waythomas, C.F., 1995, Surficial geologic map of northern Adak Island, Alaska: U.S. Geological Survey Open-File Report 95-128, Plate 1, scale 1: 50,000.
Symbol Qptb
Unit name Tephra mantling bedrock
Description Deposits of tephra that overlie bedrock, but generally do not obscure bedrock topography and structure. Tephra deposits are usually 1.5 to 3.0 m thick and consist of thin beds of fine-grained (mostly silt and clay size particles) ash and 3 to 5 beds of lapilli-sized (2-64 mm) tephra. Locally interbedded with peat. Many of the ash layers are weathered to clay and have A/Cox soil profiles developed on them. Radiocarbon dates on soil organic matter and peat in the tephra sequence indicate that the tephra deposits are of Holocene age (Waythomas and others, 1994; Miller and Kurianov, 1994). Somewhat porous, but permeability is limited by fine particle size. Lapilli are more porous and permeable than the fine-grained tephra layers. Locally water bearing, especially in low-lying areas.
Lithology Unconsolidated

Correlated geologic units

Label Qafd
Description Ash-flow and ash-fall deposits
Geologic age Pleistocene to Holocene
Geologic setting Extrusive
Lithology Form Importance
Dacite < Felsic-volcanic < Volcanic < Igneous Pyroclastic, ash-flow Major
Dacite < Felsic-volcanic < Volcanic < Igneous Pyroclastic, air fall Indeterminate, major
Andesite < Mafic-volcanic < Volcanic < Igneous Pyroclastic, air fall Indeterminate, major
Basalt < Mafic-volcanic < Volcanic < Igneous Pyroclastic, air fall Indeterminate, major