Abstract

Tree-ring dating (ring-width pattern matching) was used to determine emplacement dates for eight subfossil-tree–bearing, pre-1980 pyroclastic-flow and lahar deposits in stream valleys draining Mount St. Helens, Washington. Limiting dates were also established for nine other pre-1980 flowage deposits from the ages of trees rooted on or near them, or from exhumed trees. The 17 new dates improve understanding of the chronology of events during and after Mount St. Helens's Kalama eruptive period, which extended from a.d. 1479 to the mid-1700s. We assign bracketing dates to the three phases of the Kalama eruptive period: 1479–1510 to the early Kalama, during which Mount St. Helens erupted explosive dacitic tephras, pyroclastic flows, and one or more domes; 1489–1566 to the middle Kalama, during which Mount St. Helens produced andesitic tephras, pyroclastic flows, and lava flows; and 1489–1750 to the late Kalama, during which Mount St. Helens again erupted dacite. Overlap of the brackets reflects the limits of our dating resolution. The revised chronology confirms that little time (perhaps 50 yr) elapsed between the end of Mount St. Helens's Kalama eruptive period and the start of the succeeding Goat Rocks eruptive period (a.d. 1800–1857). Eruptions apparently occurred sporadically during the entire 1479–1857 interval. The record also indicates that a large pyroclastic flow occurred 7–10 yr after the explosive eruptions that began the Kalama eruptive period. The dates further imply that extrusion of the pre-1980 summit dome continued intermittently for ≈ 100 yr and limit the ages of two tephras. These findings demonstrate the usefulness of relict tree-bearing deposits to volcanic history studies.

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