Sj. Cronin et al., INVESTIGATION OF AN AGGRADING PALEOSOL DEVELOPED INTO ANDESITIC RING-PLAIN DEPOSITS, RUAPEHU VOLCANO, NEW-ZEALAND, Geoderma, 69(1-2), 1996, pp. 119-135
Within a sequence of andesitic volcaniclastic deposits on the northeas
tern ring plain of Ruapehu volcano is a ca. 10 m-thick sequence of wea
thered andesitic tephras. Weathering and paleosol development is most
evident in 3.6 m of fine ash in this sequence. The ages of these tephr
as are constrained between ca. 23-70 ka by dated rhyolite tephras erup
ted from central North Island volcanoes. Mineralogy of the fine ash de
posits reveals their origin, and the processes involved in their soil
development. The fine ash deposits are almost totally locally derived
either as primary volcanic ash or fines reworked from the ring plain i
tself by aeolian processes. Aerosolic quartz input associated with reg
ional loess deposition during the cool climatic episodes of mid-delta(
18)O stage 3 and delta(18)O stage 4 is very low, having been diluted b
y rapid accumulation of andesitic tephras in these episodes. The obser
ved weathering features and secondary minerals within the ash sequence
were derived from a complex combination of factors including climate
change, accretion rate, and post-depositional modification. Relatively
strong weathering development in two parts of the ash sequence is cor
related with two widespread soil development episodes during the Last
Glacial observed throughout the southern North Island. The accretion r
ate of the soil surface at these times also affected the expression of
climate-related weathering. Formation of allophane (with an Al:Si rat
io of 2.1) and ferrihydrite occurred near the soil surface as the ash
was accreting. The amount of allophane and ferrihydrite through the se
quence appears to be inversely related to the accretion rate of the so
il surface. Upon burial of the ash materials by a thick (>20 m) sequen
ce of lahar and tephra deposits, halloysite was later formed in the bu
ried ash. The leaching of silica from the thick overburden of volcanic
lastics into the ash material as well as perched water is thought to h
ave decreased the Al:Si ratio in the soil solution and thus promoted t
he formation of halloysite from weathering andesitic glass.