Eruptive history and geochronology of the Mount Baker volcanic field, Washington
Mount Baker, a steaming, ice-mantled, andesitic stratovolcano, is the most conspicuous component of a multivent Quaternary volcanic field active almost continuously since 1.3 Ma. More than 70 packages of lava flows and 110 dikes have been mapped, 500 samples chemically analyzed, and 80 K-Ar and 40Ar/39Ar ages determined. Principal components are (1) the ignimbrite-filled Kulshan caldera (1.15 Ma) and its precaldera and postcaldera rhyodacite lavas and dikes (1.29–0.99 Ma); (2) 60 intracaldera, hydrothermally altered, andesite-dacite dikes and pods—remnants of a substantial early-postcaldera volcanic center (1.1–0.6 Ma); (3) unaltered intracaldera andesite lavas and dikes, including those capping Ptarmigan and Lasiocarpa Ridges and Table Mountain (0.5–0.2 Ma); (4) the long-lived Chowder Ridge focus (1.29–0.1 Ma)—an andesite to rhyodacite eruptive complex now glacially reduced to 50 dikes and remnants of 10 lava flows; (5) Black Buttes stratocone, basaltic to dacitic, and several contemporaneous peripheral volcanoes (0.5–0.2 Ma); and (6) Mount Baker stratocone and contemporaneous peripheral volcanoes (0.1 Ma to Holocene). Glacial ice has influenced eruptions and amplified erosion throughout the lifetime of the volcanic field. Although more than half the material erupted has been eroded, liberal and conservative volume estimates for 77 increments of known age yield cumulative curves of volume erupted vs. time that indicate eruption rates in the range 0.17–0.43 km3/k.y. for major episodes and long-term background rates of 0.02–0.07 km3/k.y. Andesite and rhyodacite each make up nearly half of the 161 ± 56 km3 of products erupted, whereas basalt and dacite represent only a few cubic kilometers, each representing 1%–3% the total. During the past 4 m.y., the principal magmatic focus has migrated stepwise 25 km southwestward, from the edge of the Chilliwack batholith to present-day Mount Baker.