The Pahala Ash is up to 27 m thick in the Hilina Fault scarps on the south flank of Kilauea, 15 km from the summit, where it consists of interbedded surge and airfall deposits. Airfall layers contain pumice clasts up to two cm in diameter whereas surge layers contain lithic clasts as large as two cm. The entire 27 m of ash was deposited without any erosional hiatus about 39 ka, based on 14C ages on organic-rich soils from the Hilo Drill Hole. The Pahala Ash is roughly 1.5 m thick in Hilo, at least that thick near South Point (where much of the ash is reworked by wind), and approximately 60 m thick in a drill hole located about 2 km from the northeast rim of the cladera. Much of what was previously designated Pahala Ash, including ashes near the town of Pahala, consists of deposits that are younger than 10-12 ka; these are part of the Puna Basalt.
From youngest to oldest, the ash deposits in the Hilina Basalt consist of the Moo Ash Member, seven separate ashes collectively called the Pohakaa Ash Member, the Kahele Ash Member, and the Halepe Ash Member. Each ash bed consists of coarse surge and fine airfall layers, often repetitively interbedded. The Pohakaa Ash contains lithic clasts as large as 5 cm.
Electron microprobe analysis of fresh glass fragments from each ash bed in the Puna and Hilina Basalts and the Pahala Ash demonstrate that all were erupted from Kilauea Volcano.
The thickness of ash layers and the large lithic fragments in the deposits in the Hilina faults suggest that these are proximal surge facies from large-scale phreatomagmatic eruptions of Kilauea Volcano. The present day caldera is located too far away to be the source of such deposits. When these ashes were erupted, the caldera at Kilauea was much larger and extended further to the south; the southern caldera-bounding fault may well be the Kalanakaoiki Pali, the southernmost fault in the Koae fault zone. The caldera is inferred to have been 12 km across during deposition of the Pohakaa and Pahala Ashes. Subsequent calderas, including present-day Kilauea caldera, are nested within this larger structure.
Following the eruption of the Pahala Ash, this ancient caldera was not only larger, but also much deeper than the present one.
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