Ignimbrite

Ignimbrite is a pumice-dominated pyroclastic flow deposit formed from the cooling of pyroclastic material ejected from an explosive volcanic eruption. As the pyroclastic material settles it can build up thick layers, and if the temperature is sufficiently high (> 535°C) it can weld into rock. The degree of welding depends on the weight of overlying material, so at the tops of such deposits the rock is commonly vesicular and of low density, while at depth the overlying weight crushes the pyroclastic material forming a dense, non-vesicular rock. While most volcanic rocks are found close to the eruptive source, ignimbrite of reasonable thickness can often be found tens to hundreds of kilometres from the site of eruption.

ignimbrite (welded) ignimbrite (poorly welded)

Other specimens - Click the thumbnails to enlarge

Group - volcanic.
Colour - variable, typically light-coloured (e.g. pinkish-white, pale grey etc).
Texture - aphanitic if not welded, eutaxitic if welded.
Mineral content - pumice clasts in a fine grained glassy matrix, may contain lithic clasts and / or phenocrysts of varying composition.
Silica (SiO ₂) content - NA.
Uses - building stone for decorative walls, paving etc.
New Zealand occurrences - widespread over the Bay of Plenty-Rotorua-Taupo area.