Abstract
This work describes the Kiwikiwi Formation – a fresh and locally prominent deposit of reworked volcaniclastic sediment in the upper Whangaehu Valley at Mt. Ruapehu. The grain-size distribution and sedimentary features of the unit suggest deposition from a braided to sheetflow fluvial system, but sediment-transport calculations show that normal fluvial deposition is inconsistent with the unit's grain-size on the steep depositional slope. Eruptive activity following the Mangaio Fm. debris avalanche is inferred to have quickly produced an abundant supply of largely sand-grade tephra resulting in the rapid accumulation of material by hyperconcentrated flows and / or streamflows with highly enhanced bedload transport. Similarly anomalous conditions developed following debris avalanche or/and volcanic eruptions at Mount St. Helens and Mt. Pinatubo when river systems became highly charged with sediment.
Petrological as well as textural investigation shows that the compositional range of the Kiwikiwi Formation is almost identical to that reported for Ruapehu’s last eruption in 1995/1996. Furthermore the range of mineral compositions, zoning patterns and textural features are virtually indistinguishable from those of the modern eruption. This suggests that magmas younger 4600 yrs B.P. have been derived from a relatively stable, "steady-state" system, fed by a mature crustal hot zone. While Ruapehu’s extensive mush column imparts highly complex textures resulting in varied hybrid compositions on short timescales, the system is frequently flushed by small magma batches, inhibiting closure and significant loss to cumulate phases within the upper crust. Thus the overall composition and a stable assemblage of mineral phases can persist over extended periods of time.
This scenario is in sharp contrast to climactic 10 ka eruptions of the Tongariro volcanic centre. These events tapped stagnant crustal magma chambers and erupted highly evolved hydrous magma batches. This contrast among products erupted over the last 10,000 years is the result of a crustal configuration that allows small magma batches to either ascend through a frequently flushed mush column, or to be refined "off site" at mid-crustal levels to more evolved hydrous compositions and then rise along new pathways (i.e. not through a mush column) to the surface.