Andisols of New Zealand and Australia(International symposium on volcanic ash soils and field workshop in the Mt. Fuji area)

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During the Quaternary, much of central North Island, New Zealand, was repeatedly overwhelmed or modified by emplacement of ignimbrites and especially by multiple rhyolitic and andesitic tephra fallout deposits, and reworked derivatives ; that successively mantled landsurfaces forming buried paleosols in multisequal profiles. Relatively thick proximal deposits buried and isolated antecedent soils, forming compound soil profiles, whereas relatively thin tephra fallout at medial and distal sites resulted in composite or aggrading profiles, their character determined by the interplay of upbuilding and topdown pedogenesis. Scoriaceous basaltic tephras erupted in northern North Island were locally distributed. Andisols, of the 'allophanic' type with andic soil properties dominated by short-range-order (SRO) clays rather than Al-humus complexes, cover 〜32, 100 km^2 and comprise about 12.5% of New Zealand soils. They consist of three groups : (1) 'Entic' Udands or Cryands occur on mainly andesitic eruptives <1000 years old near recently active volcanoes (Tephric Recent Soils in New Zealand Soil Classification ; 〜1200 km^2, 〜0.5%) ; (2) Vitrands occur in central-eastern North Island on glassy, rhyolitic pumiceous deposits mainly from the Taupo eruption c. 232 AD and the Kaharoa eruption c. 1314 AD (Pumice Soils ; 17, 200 km^2, -6.5%); and (3) Udands occur typically on composite, multiple tephra deposits of varying ages in Taranaki (mainly andesitic tephras), King Country-western Waikato (mixed andesitic-rhyolitic), eastern Waikato-western Bay of Plenty-Coromandel (mainly rhyolitic), and Auckland-Northland (basaltic), the age span of sola increasing towards distal sites as constituent tephra layers become thinner and shallower (Allophanic Soils ; -13, 700 km2, -5.5%). Moisture?and temperature regimes are mainly udic and mesic, thermic, or cryic. Udands are pre-eminent among New Zealand's most versatile, high-quality soils because they provide valuable soil ecosystem services including sorption, water storage and supply, natural fertility, and foundation support. Andisols are currently known only in the Mt Gambier area of southeast South Australia, and comprise 〜85 km^2 (〜0.001 %) of Australian soils. Intraplate basaltic volcanoes at Mts Gambier and Schank erupted c. 5000 years ago and the resultant localised tephras contain both exotic crystalline material, derived from underlying limestone and calcareous dunes, and juvenile basaltic material. Sand fractions at Mt Gambier are dominated by exotic, silicic crystalline material (*80%) and relatively little glass (<20%) but at Mt Schank unaltered Al-rich glass predominates (*50%). Vitrixerands and Haploxerands with pHs 6.4-8.0 have been formed under xeric moisture and mesic temperature regimes (Andic, Chernic Tenosols in Australian Soil Classification). The parent mineralogies have influenced clay mineral formation : SRO clays at Mt Schank (up to 20% allophane, 7% ferrihydrite, fine-earth basis) are more abundant than at Mt Gambier (*12% allophane, 4% ferrihydrite) because the glass at Mt Schank weathers more rapidly than the Al-poor crystalline material at Mt Gambier, releasing Al. Seasonal Si-leaching has also been influential : where leaching is sufficient to remove silica, and CaCO_3 content is low (in upper soil horizons at Mt Gambier ; in most horizons at Mt Schank), Al-rich allophane has formed ; where leaching is weak (intermediate horizons), Si-rich allophane and layer silicate minerals (but not halloysite) have formed ; and where leaching barely occurs (in lowest horizons), smectite has formed. The Xerands are versatile soils.
2007-02-28

Andisols of New Zealand and Australia(International symposium on volcanic ash soils and field workshop in the Mt. Fuji area)

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