Young shallow intrusive rocks

Unit symbol: QTi
Age range Quaternary or late Tertiary (5.333 to 0 Ma)
Lithology: Igneous - Plutonic
Group name: Young volcanic and shallow intrusive rocks
Hypabyssal plugs, domes, and subordinate flows that range in composition from rhyolite to basalt at Quaternary volcanic centers along the Aleutian magmatic arc and are associated with the volcanoes of the Wrangell Mountains. In the Aleutian Islands, compositions tend to be dacite to basaltic andesite; along the Alaska Peninsula the full range in composition is present; and in the Wrangell Mountains, compositions more typically range from rhyolite to dacite. Included are quartz keratophyre dikes on widely separated parts of Attu Island. Dikes consist of chalky white euhedral feldspar (albite) crystals and glassy quartz in a bluish-gray to light-gray groundmass of fine-grained albite, quartz and minor sericite, calcite, epidote, chlorite, magnetite, and apatite (Gates and others, 1971, p. 748). Unit also includes rhyolite domes at Mount Edgecumbe in the Sitka quadrangle (Riehle and others, 1989). Unit tends to be best exposed at older volcanic centers

Source map information

Source map Riehle, J.R., Detterman, R.L., Yount, M.E., and Miller, J.W., 1993, Geologic map of the Mount Katmai quadrangle and adjacent parts of the Naknek and Afognak quadrangles, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-2204, scale 1:250,000.
Symbol Qad
Unit name Domes
Description Dacitic or rhyolitic vitrophyres having plagioclase, pyroxene, and (or) hornblende phenocrysts in a brown to grayish-brown glassy groundmass.
Lithology Igneous

Correlated geologic units

Label Qvd
Description Dacitic to rhyolitic domes
Geologic age Pleistocene to Holocene
Geologic setting Extrusive
Lithology Form Importance
Hypabyssal-dacite < Felsic-hypabyssal < Hypabyssal < Igneous Dome Major
Hypabyssal-rhyolite < Felsic-hypabyssal < Hypabyssal < Igneous Dome Major