Stratigraphy and Chronology of Late Quaternary Tephra along the South Rift Zone of Mauna Kea Volcano, Hawaii

Abstract

Cinder cones and associated tephra layers along the south rift zone of Mauna Kea volcano accumulated during explosive eruptions of alkalic rocks during the late Quaternary Period. Six widespread tephra units, the youngest consisting of at least five subunits, are recognized on the basis of distributional patterns, variations in particle size and thickness, mineralogical parameters, and stratigraphic relations with respect to two buried soils. Four layers underlying the regionally extensive Humuula soil, the formation of which ended about 4,500 yrs ago based on carbon-14 age determinations, consist of scoriaceous cinders and shards of opaque black glass and crystals of olivine and plagioclase. The two major post-Humuula units lack crystals. Although compositions of crystals are too similar to be useful in differentiating the pre-Humuula layers, plagioclase/olivine ratios are diagnostic. Refractive indices of cinders, based on optical measurements of fused glass beads, range from 1.553 to 1.579, the index falling with decreasing age of units. Individual layers reach an observed thickness of 10 m, with thickness varying exponentially as a function of distance from the source vent. Maximum particle size also varies exponentially with distance and provides a measure of relative magnitude of eruptions. The major tephra-producing eruptions along the south rift zone built cinder cones that are among the largest on the mountain. Pre-Humuula tephra layers and associated lava flows fall broadly within the last episode of ice-cap glaciation (more than 9,100 yrs B.P. by radiocarbon dating). Holocene eruptions that largely buried the Humuula soil on the lower part of the rift zone apparently are represented by thin ash layers in Lake Waiau near the summit of the mountain. The youngest ash layer in the lake, inferred to be about 3,600 yrs old, probably records the latest eruption of the volcano.