Rate of clay formation and mineral alteration in a 4000-year-old volcanic ash soil on Saint Vincent, B.W.I

Abstract

The surface of a large fan of pyroclastic deposits erupted about 4,000 years ago has weathered to a clayey soil generally 6 ft. thick. The weathered deposits were originally andesitic ash consisting largely of glass and the minerals anorthite, labradorite, hypersthene, augite, and olivine. Fine vitric ash has altered to halloysite.4H ~2~ O, allophane, and hydrated ferric oxide. The vitric lapilli have decomposed to halloysite.4H ~2~ O. Anorthite crystals are etched in the soil and frosted in the underlying ash. Many hypersthene and fewer augite and olivine crystals in the soil are etched. Labradorite, hornblende, and magnetite are unaltered. The 4,000-year-old soil represents an early stage in the formation of the Yellow Earth soils of St. Vincent, which may themselves be classified with the yellow-brown volcanic-ash soils of Japan. Halloysite.4H ~2~ O has been redeposited in some late Pleistocene ash beds on St. Vincent to form dark brown, waxlike braunlehm. Whether or not the yellow-brown volcanic ash soils of St. Vincent represent an early state in the formation of a lateritic red soil cannot be demonstrated. Clayey soil formed from ash at an average rate of 1 1/2 - 2 ft./1,000 years on St. Vincent, and glass decomposed at a rate of about 15 gm./cm. ^2^ /1,000 years. The slight degree to which the unstable minerals anorthite, olivine, and the pyroxenes are altered contrasts sharply with the decomposition of the glass. Survey of the literature suggests that mineral grains have altered more rapidly in some podsols of the temperate regions than in the volcanic ash soils of St. Vincent. This contrast in weathering rate may perhaps be explained by the low pH of podsols and chemical effect of the rapidly decomposing glass of the ash soils.