Investigation of an aggrading paleosol developed into andesitic ring-plain deposits, Ruapehu volcano, New Zealand

Abstract

Within a sequence of andesitic volcaniclastic deposits on the northeastern ring plain of Ruapehu volcano is a ca. 10 m-thick sequence of weathered andesitic tephras. Weathering and paleosol development is most evident in 3.6 m of fine ash in this sequence. The ages of these tephras are constrained between ca. 23–70 ka by dated rhyolite tephras erupted from central North Island volcanoes. Mineralogy of the fine ash deposits 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 itself by aeolian processes. Aerosolic quartz input associated with regional loess deposition during the cool climatic episodes of mid-δ¹⁸O stage 3 and δ¹⁸O stage 4 is very low, having been diluted by rapid accumulation of andesitic tephras in these episodes. The observed 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 correlated with two widespread soil development episodes during the Last Glacial observed throughout the southern North Island. The accretion rate of the soil surface at these times also affected the expression of climate-related weathering. Formation of allophane (with an A1:Si ratio of 2:1) and ferrihydrite occurred near the soil surface as the ash was accreting. The amount of allophane and ferrihydrite through the sequence appears to be inversely related to the accretion rate of the soil surface. Upon burial of the ash materials by a thick ( > 20 m) sequence of lahar and tephra deposits, halloysite was later formed in the buried ash. The leaching of silica from the thick overburden of volcaniclastics into the ash material as well as perched water is thought to have decreased the A1:Si ratio in the soil solution and thus promoted the formation of halloysite from weathering andesitic glass.