Holocene Tephra Layers on Adak Island in the West-Central Aleutian Islands, Alaska

Adak Island, Aleutian Islands, Alaska is covered with Holocene sequences of soil-tephra complexes. Tephra layers are useful for establishing a chronographic framework on the island. Black (1976) thought that the three conspicuous tephra deposits (Main, Intermediate and Sandwich) erupted from Kanaga volcano. Waythomas et al. (2001), however, inferred that a possible source volcano of the tephras was Mount Moffett, Adak Island. We conducted a tephrostratigraphic study on the island to re-evaluate the chronological framework and to determine the sources of the tephras. Chemical compositions of volcanic glass were determined by electron probe micro-analysis (EPMA). Radiocarbon dates were determined using accelerator mass spectrometry (AMS). The three tephra layers increase in thickness from south to north. Among them, Intermediate and Sandwich tephras contain fresh lithic fragments. Thickness and maximum diameter of lithic fragments (ML) suggests that their source was Adagdak volcano or a now submerged volcano nearby. The presence of a lava dome and crater rim in the northern part of Adagdak volcano makes it a likely candidate for the source of the tephras. Eruptions of the Intermediate, Sandwich, YBO and Forty Year tephra deposits were dated to approximately 7.2, 4.7, 3.6 and 0.4 cal ka BP, respectively.