Chapter 6 Compaction of ash-flow tuffs

This chapter discusses the compaction of ash-flow tuffs. The compaction of ash-flow tuffs results from a number of different mechanisms operating at several stages. Recognition of two basic types of units is of critical importance in understanding the compaction process in ash-flow tuffs. The ash-flow is the unit of emplacement and the cooling unit represents one complete welding regime. The chapter deals principally with the compaction that occurs within the first few days after the subaerial eruption and emplacement of hot ash. This compaction is due to a progressive decrease in the pore space of the deposited ash. Compaction is measured by changes in bulk density and by the deformation of objects of known original shape. The first technique provides an accurate measure of the average compactional strain. The second allows the determination of the orientation and shape of the finite strain ellipsoid. Because the compactional strain is uniaxial, the resulting strain ellipsoid is oblate, and therefore the essential features of this strain can be fully depicted in two dimensions. The characteristic foliation of welded tuffs is perpendicular to the direction of maximum shortening.