PROCESSES AFFECTING IGNIMBRITIC ROCK TEXTURES

COMPACTION and WELDING -- Because pumice fragments and glass shards in newly emplaced pyroclastic flows are hot and very malleable, they can readily deform in a plastic fashion due to the aggregate weight of the massive flow. Compaction of these vitric pyroclasts results in fattening and alignment of the glass shards, closing of triple junctions, and compression of the vesicles. Where the fragments have remained plastic enough they can become partly or completely welded, in some cases obliterating the original characteristic ash structures. Extreme compaction, in association with welding, can lead to the complete disappearance of vesicles in pumice fragments, generating dark, glassy fiamme in their place. The most extreme conditions of welding and compaction can produce thick zones of dense, obsidian-like glass.

FLOW DISTORTION -- Emplacement of hot pyroclastic flows may be associated with internal stress, causing the plastic aggregate to flow internally as a viscous fluid. The resulting strain may be reflected in the flow-alignment of lithic and crystal fragments, and the stretching of the hot, plastic vitric pyroclasts into unrecognizable forms.

DEVITRIFICATION -- Devitrification is a post-depositional process resulting in the crystallization of microlites along the boundaries of the glass shards or within glass mass. The mineral compositions produced are mainly cristobalite (a high-temperature form of quartz) and alkali feldspar.

VAPOR-PHASE CRYSTALLIZATION -- This is also a post-depositional process, but here crystallization takes place in open spaces, under the influence of a vapor phase. The hot vapors, derived from magmatic gas-exsolution and from heated groundwater, are generally enriched in H2O, CO2, and SO2. They also have the ability to scavange numerous additional elements from the volcanic debris, such as Si, Al, Na, and K, among others. Cooling of these element-rich phases may result in the crytstallization of a variety of minerals into open cavites as the gases ascend upward through the flow. The main phases of vapor-phase crystallization are tridymite, cristobalite,and alkali feldspar. Lithophysae is a hollow, bubble-like structure composed of concentric shells vapor-phase minerals found within the cavities of pyroclastic flows. The advanced product of vapor-phase crystallization is sillar, a whitish, well-cemented, coherent rock with little pore space. Sillar zones are often found in association with abundant fumarole pipes in degassed ignimbrites.