TERTIARY WELDED TUFFS IN EASTERN ICELAND

Abstract

The welded acid tuffs that are a minor constituent of the volcanic pile are hard felsitic rocks, usually highly vesicular, with a basal glassy layer. They usually have the characteristic microtexture of welded tuffs, but field criteria are more reliable in distinguishing them from lavas and non-welded tuffs. An isopach map is given of the Skessa tuff, which is the product of a single subaerial eruption. The original extent of the welded parts of this tuff is estimated to have been 100 square miles, and its average thickness 25 to 30 feet ; peripheral outcrops are not welded, and cover an additional 70 square miles. In the welded parts the tuff particles were deformed, roughly aligned, and welded together under their own weight and momentum and the accumulated tuff remained plastic long enough for vesicles to form. A classification of tuff deposits into five grades is proposed which is based on the plasticity and temperature of the tuff particles. The Skessa tuff is a representative of the rather rare grade in which the particles had the highest plasticity and, presumably, temperature. Bubbles of basaltic glass that occur in the tuff appear to indicate simultaneous eruption of acid and basic magma.