Hawaiian Glacial Ages

Four glacial drifts that are interstratified with lava flows and tephra layers on the upper slopes of Mauna Kea demonstrate that an ice cap formed repeatedly at the summit of the volcano during the middle and late Pleistocene. The oldest drift (Pohakuloa Formation) probably was deposited shortly after eruption of a lava flow having a K/Ar age of 278,500 ± 68,500 yr. Drift of the Waihu Formation, marked by a belt of subdued end moraines, is correlated with hyaloclastite cones and associated lava flows that were erupted beneath an ice cap about 170,000–175,000 yr ago. One of four younger subglacially erupted lavas at the crest of the volcano has a K/Ar age of 41,300 ± 8300 yr. Tephra layers that antedate the last glaciation are about 29,700 to 37,200 14 C yr old and underlie dune sand that is believed to correlate with drift of the Makanaka Formation deposited during the last ice advance. The late Makanaka ice cap, which covered an area of about 70 km 2 and was as much as 100 m thick, is reconstructed from end moraines and limits of erratic stones that encircle the summit region. The ice cap disappeared from the summit before about 9080 yr ago. Postglacial lavas and tephra overlie the youngest drift on the upper south flank of the mountain and buried a widespread post-Makanaka soil on the lower south rift zone about 4500 14 C yr ago. The island of Hawaii is subsiding isostatically due to crustal loading by Quaternary volcanic rocks, with subsidence near the midpoint of Mauna Kea estimated as about 2.5 ± 0.5 mm/yr. A curve depicting an inferred long-term subsidence rate has been used to adjust equilibrium-line altitudes (ELAs) of former ice caps that are calculated on the basis of reconstructed glacier topography and an assumed accumulation-area ratio of 0.6 ± 0.05. The results indicate that ELA depression was greatest during Waihu glaciation, least during Pohakuloa glaciation, and that the ELA during late Makanaka glaciation was somewhat lower than that of the early Makanaka advance. Available radiometric dates show that late Makanaka glaciation correlates with stage 2 of the marine oxygen-isotope record, and suggest that early Makanaka, Waihu, and Pohakuloa glaciations correlate, respectively, with isotope stages 4, 6, and 8. Because ice caps could have formed on Mauna Kea only after the snowline was lowered many hundreds of meters below its inferred present level, episodes of Hawaiian glaciation probably were restricted to times of maximum ice volume on the continents. The asymmetry of the late Makanaka ice cap and the southeast-descending gradient of its equilibrium line are consistent with a southeast (tradewinds) source of precipitation during the last glaciation. Although departures of glacial-age temperature and precipitation from present values are difficult to assess quantitatively, growth of former ice caps on Mauna Kea most likely was due to enhanced winter snowfall and to reduced ablation rates brought about by lower air temperature and increased cloudiness.