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Source map Karl, S.M., 2001, Unpublished data.
Symbol Qav
Unit name Volcanic deposits, undivided
Description Gray, brown, and locally red-weathering pyroclastic deposits, welded and nonwelded tuff, tuff breccia, flow breccia, basalt, hawaiite, trachyte, andesite, and minor intercalated dacite, flowbanded rhyolite, and obsidian. Basaltic volcanic rocks include columnar jointed flows, cinder cones and deposits, pahoehoe flows, rubbly aa flows, pumice, scoria, ash, lapilli, and fall deposits up to tens of meters thick. Basalt is black to light gray fresh, green to dark brown weathered, massive, blocky, and locally contains vesicles to 25 cm in diameter. On southern Kupreanof Island, basalt consists of dark gray, rusty-weathering, platy, blocky, or columnar jointed flows 50 cm to several meters thick, commonly vesicular and amygdaloidal, and has inclusions of peridotite that range up to 30 cm across. Basalt is locally olivine porphyritic. Amygdule fillings include calcite, epidote, chalcedony, chlorite, and zeolites. Basalt flows are interlayered with flowbanded rhyolite; rhyolite dikes intrude basalt and basalt dikes intrude rhyolite. Vent breccia in Sumner Strait area consists of angular to subangular blocks of fine-grained light gray silicic volcanic rock ranging from 5 mm to 15 cm with either no matrix or little (< 15%) matrix of very fine-grained dark gray volcanic rock or chalcedony. On Suemez Island and Tlevak Strait, basalt, andesite and rhyolite flows and pyroclastic deposits are interlayered. Obsidian has been recovered from the Totem Bay area on Kupreanof Island, from Suemez Island, and from Princess Bay on Revillagigedo Island. Basalt flows are locally olivine or hornblende porphyritic; andesite and hawaiite are trachytic. Extensive basalt flows on Revillagigedo Island follow linear fault structures that may have served as conduits for fissure eruptions. These rocks have alkaline compositions that are regionally consistent throughout southeast Alaska and distinguish them from tholeiitic volcanic rocks on Mount Edgecumbe, which are unique and mapped separately
Lithology Igneous

Correlated geologic units

Label Qv
Description Volcanic rocks, undivided
Geologic age Pleistocene to Holocene
Geologic setting Extrusive
Lithology Form Importance
Felsic-volcanic < Volcanic < Igneous Flow Major
Mafic-volcanic < Volcanic < Igneous Flow Major
Volcanic < Igneous Pyroclastic Indeterminate, major
Mafic-volcanic < Volcanic < Igneous Pyroclastic Indeterminate, major