Description

From Miller and others (1998) ^[1] : "Kanaga Volcano occupies the northern corner of Kanaga Island, one of the most southerly members of the Aleutian chain. It is a symmetric composite cone 1307 m high and 4.8 km in diameter at sea level, built of interbedded basaltic and andesitic lava flows, scoria layers, and pyroclastic rocks. Mudflow deposits and other volcaniclastic rocks occur on the volcano's lower slopes. A circular summit crater, approximately 200 m across and 60 m deep, contains recent deposits of vent agglomerate, and several active fumaroles.
"A mantle of volcanic ash and pumice, up to 7 m thick, and containing several soil horizons, blankets the northern half of the island. Most of this deposit was probably erupted from Kanaga Volcano, although some may be derived from explosive eruptions on nearby islands (Coats, 1956 ^[2] , p. 74; Fraser and Barnett, 1959 ^[3] , p. 226). A thin layer of andesitic and basaltic pumice, younger than the ash-and-pumice mantle, coats the volcano's upper slopes, and blocks of dense basalt occur across the island. Fragments of the latter material have produced impact craters up to 4 km from the summit. Four young andesitic flows extend from fissures near the summit of the cone on the south, southwest, and northeast flanks (unit Qkb).
"Kanaga Volcano is flanked on the south and east by an arcuate ridge up to 800 m in elevation; a soma lake, 2 km in diameter, is situated between Kanaga Volcano and the southeast corner of the arcuate ridge. The ridge and associated scarp may represent the eroded remnant of a caldera rim. Two observations support the caldera origin of the ridge. First, although dissected, remnants of the ridge are located along 150 degrees of an approximately circular arc and the radially outward dip of the comprising flows indicates a central source near the present summit of Kanaga Volcano. Secondly, a thin (0.6-9 m)) but widespread blanket of andesitic crystal-lithic tuff (unit Qat) occurs over northern Kanaga Island south and east of Kanaga Volcano ^[2] where relative age and lithologic character suggest that it may be the product of a caldera-forming eruption.
"Coats (1950 ^[4] , 1956 ^[2] ) suggested the caldera formed through collapse of a Tertiary volcano (Mount Kanaton) near the end of Pleistocene time. However, Miller and Kirianov (1994) ^[5] suggested periods of caldera formation on Kanaga occurred ~6000, ~4,500, and ~3,000 yBP based on tephrachronology studies on nearby Adak Island.
"Precaldera rocks include flows and pyroclastic rocks and minor intrusive rocks. Coats (1956) ^[2] , and Fraser and Barnett (1959) ^[3] have assigned a late Tertiary to Pleistocene age to these older rocks. Apparently several episodes of volcanism preceded construction of modern Kanaga Volcano. Low outward dips imply that most of the older rocks were part of a broad, shield-shaped volcano with a vent area near the site of Kanaga Volcano. There is, however, some evidence that at least one composite cone was constructed on the site before formation of the caldera, and of other vent eruptions form the flanks of the ancient volcano ^[2] .
"Evidence of glaciation has not been noted on Kanaga Island, although adjacent islands such as Tanaga display signs of glacial erosion down to sea level."