Nunivak Island description and information

From Wood and Kienle (1990) ^[1]: "The volcanic carapace of Nunivak Island is built on Cretaceous sedimentary rocks in the eastern Bering Sea, within 50 km of western Alaska. The surface of the island is dominated by broad thin pahoehoe lava flows, with subsidiary alkalic basalts forming small lava flows, ~60 cinder cones and 4 maars. The pahoehoe flows are 3-15 m thick and build small shield volcanoes. Exposures are best at sea cliffs and among the young flows of the eastern part of the island, which is otherwise covered with tundra and shallow lakes. Permafrost is ubiquitous.

"Alkali basalt lava flows are <3.5 km long, and most erupted from cinder cones. Some flows contain collapsed lava tubes and rafted segments of cinder cones. Although many true cinder cones occur on Nunivak Island, some alkali basalt cones (e.g. Twin Mountain) have small summit craters and steep flanks, much more like small stratovolcanoes. Alkali basalts also erupted phreatomagmatically, forming 4 maars in an east-west line: Binalik, Ahkiwiksnuk, Nanwaksjiak, and "385," which is the elevation in feet of the crater lake of an otherwise unnamed maar. The latter 2 maars have ~200 m of relief, with floors near sea level. Although various types of nodules are found in many of the alkali basalt cones, ejecta from Nanwaksjiak includes lherzolite xenoliths within tholeiite boulders, an apparently unique occurrence.

"Detailed radiometric dating demonstrated that volcanism occurred in distinct episodes, and systematically migrated eastward (~50 km/5 Ma). Additionally, tholeiites and alkali basalts erupted nearly simultaneously. Five pulses of volcanism (some combined in listing above), each lasting ~0.1-0.2 m.y., and separated by quiescent intervals of 1-2 m.y., occurred between 6 and 1.5 Ma. Since 0.9 Ma volcanism has been frequent, with both tholeiite and alkali basalt common, but since 0.3 Ma only alkali basalt has been erupted. The overlapping occurrence of the two magma types differs from the more familiar patter of tholeiite followed by alkali basalt in Hawaii, and suggests that at Nunivak the latter is not derived from the former."