Welding and crystallisation zonation in Whakamaru Ignimbrite, central north island, New Zealand

Abstract

Whakamaru Ignimbrite (Central North Island, New Zealand) cornprises a 2500-km² cornposite sheet that grades from a single compound cooling unit near source to a series of separate cooling units on the distal edges. Degree of welding and crystallisation decreases, and percent porosity increases, towards the distal edges of the ignimbrite, These changes probably reflect both a thinning of flows and loss of heat with increasing distance from source. Local variations in this pattern result from an irregular pre-eruption topography. Thickness, temperature, and volatile content were irnportant in deterrnining the type of devitr ification products formed in the groundmass and pumice fragments in various parts of the cooling unit.