Volcanic ash soils in New Zealand

Ash and lapilli derived from Holocene and Late Pleistocene volcanic cruptions cover approximately half of the North Island of New Zealand. Deposits from 16 separate series of showers are recognised in the surface soils and areas with 3 inches and more of each ash bed have been mapped out during soil surveys. These and other shower deposits are also recognised and described in fossil soils. The effects of the volcanic ashes on soil properties are related to the thickness and frequency of deposition, to mineral composition, and to the period of exposure to soil formation. Soils formed from shower deposits of Holocene age are extensive and have characteristic properties of friability, fine granular structure, deep humic topsoils and slippery non-sticky clays. The clays are chiefly amorphous and responsible for the notable practical properties of free drainage, high moisture retention, low bulk density, and critical limits of stability under pressure. In New Zealand the development of these characteristies increases with time up to about 15,000 years and then decreases with the crystallisation of the clays to halloysite. Soils formed from Late Pleistocene volcanic ashes are high in halloysite clay and with increasing age resemble soils formed from marine sedimentary rocks. Differing degrees of development of the characteristics are shown in the soil classification. The soils from volcanic ash are widely used for crop, pasture or forestry production. More intensive permanent usage depends on more detailed knowledge of the materials which will be obtained through combined volcanological and pedological research.