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Source map Karl, S.M., 2001, Unpublished data.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 Qab
Unit name Basalt flows of Mount Edgecumbe
Description Massive, dark gray, vesicular, plagioclase, olivine, and clinopyroxene porphyritic basalt flows with intercalated breccia and aquagene tuff. A tuff intercalated with flows on the east coast of Kruzof Island contains marine macrofossils and benthic foraminifers that imply Pleistocene deposition in water 50 to 100 m deep (William Sliter, written commun. in Riehle and others, 1989). Flows appear to cluster in 2 groups: 1) a glomeroporphyritic, holocrystalline to vitrophyric plagioclase basalt that contains up to 35 per cent plagioclase, 3 per cent olivine and 1 per cent clinopyroxene and 2) vesicular, diktytaxitic, diabasic, or equigranular to porphyritic olivine basalt that contains up to 50 to 60 per cent plagioclase, 15 to 25 per cent olivine, and 15 to 25 per cent subophitic clinopyroxene (Riehle and others, 1989; 1994). The plagioclase basalt has trace element ratios similar to mid-ocean ridge basalt, and is located closer to the Fairweather fault. The olivine basalt is mapped farther inboard (east) of the Fairweather fault, has trace element ratios that suggest continental affinity, and has higher 87Sr/86Sr ratios than the plagioclase basalt (Riehle and others, 1994). Hornblende diabase and olivine diabase dikes also intrude the Crawfish Inlet pluton in dikes up to 5 m thick. Loney and others (1975, p. 54) suggest that the olivine diabase dikes may be cogenetic with the Edgecumbe volcanics, but note that the olivine diabase dikes are glaciated. Edgecumbe olivine basalt ranges as old as 611 ka, so predate the LGM
Lithology Igneous

Correlated geologic units

Label Qvb
Description Basalt or basaltic-andesite flows
Geologic age Pleistocene to Holocene
Geologic setting Extrusive
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major
Basaltic-andesite < Mafic-volcanic < Volcanic < Igneous Flow Major
Basalt < Mafic-volcanic < Volcanic < Igneous Pyroclastic Minor