Are There Spatial or Temporal Patterns to Holocene Explosive Eruptions in the Aleutian Archipelago? A Work in Progress

By examining the existing geological and archeological record of radiocarbon dated Aleutian tephras of the last 12,000 years, this study sought to determine whether there were spatial or temporal patterns of explosive eruptive activity. The Holocene tephra record has important implications because two episodes of migration and colonization by humans of distinct cultures established the Unangan/Aleut peoples of the Aleutian Islands concurrently with the volcanic activity. From Aniakchak Volcano on the Alaska Peninsula to the Andreanof Islands (158 to 178° W longitude), 55 distinct tephras represent significant explosive eruptions of the last 12,000 years. Initial results suggest that the Andreanof and Fox Island regions of the archipelago have had frequent explosive eruptions whereas the Islands of Four Mountains, Rat, and Near Island regions have apparently had little or no eruptive activity. However, one clear result of the investigation is that sampling bias strongly influences the apparent spatial patterns. For example field reconnaissance in the Islands of Four Mountains documents two Holocene calderas and a minimum of 20 undated tephras in addition to the large ignimbrites. Only the lack of significant explosive activity in the Near Islands seems a valid spatial result as archeological excavations and geologic reports failed to document Holocene tephras there. An intriguing preliminary temporal pattern is the apparent absence of large explosive eruptions across the archipelago from ca. 4,800 to 6,000 yBP. To test the validity of apparent patterns, a statistical treatment of the compiled data grappled with the sampling bias by considering three confounding variables: larger island size allows more opportunity for geologic preservation of tephras; larger magnitude eruption promotes tephra preservation by creating thicker and more widespread deposits; the comprehensiveness of the tephra sampling of each volcano and island varies widely because of logistical and financial limitations. This initial statistical investigation proposes variables to mitigate the effects of sampling bias and makes recommendations for sampling strategies to enable statistically valid examination of research questions. Further, though caldera-forming eruptions occurred throughout the Holocene - and several remain undated - four of six dated eruptions occurred throughout the archipelago between 8,000-9,100 yBP, a period coinciding with some of the earliest human occupation (Early Anangula Phase) of the eastern Aleutians.