PYROCLASTICS 393 Figure Compositionally zoned ignimbrite (Mount Mazama ignimbrite, Crater Lake, Oregon) (Photo: M Branney.) Figure The origins of pyroclastic density currents Most large volume ignimbrites are derived from B and C Type D refers to transient pyroclastic density currents generated by lateral blasts or small scale phreatomagmatic activity (Surtseyan or Taalian eruptions) Type E commonly produces block and ash flows (From Branney and Kokelaar (2002).) ribbon-like deposits confined to valleys to extensive sheets distributed radially around the vent or across mountain ranges Plinian eruptions are usually sustained for hours to days, and PDCs may be generated for all or part of that time, or intermittently Ignimbrites exhibit a breath-taking diversity in their physical characteristics (e.g., geometry, bedding, sorting, grainsize, clast grading, geometry) Many are massive (Figure 7) and exhibit internal coarse-tail grading patterns the size grading shown by the coarsest particles in the deposit, but bedded, stratified, and crossstratified lithofacies also occur Detailed lithofacies analyses, similar in approach to those applied to sedimentary deposits, have proved most successful in dealing with the complexity of ignimbrites and in elucidating important information about emplacement mechanisms Moderate volume ignimbrites (1–10’s km3) often have complicated lateral, longitudinal, and vertical lithofacies transitions (ignimbrite architecture) Ignimbrites often show evidence for crystal enrichment relative to the juvenile pumice clasts, indicating significant loss of ash during transport (a process termed elutriation) This elutriated ash forms co-ignimbrite plumes, which rise vertically from moving flows and can later deposit extensive ash deposits Progressive evacuation of a chemically zoned magma chamber results in vertical chemical zoning of juvenile pyroclasts in an ignimbrite (Figure 7) Typically, juvenile pyroclasts become more basic with height (e.g., rhyolite to andesite; the inverse of vertical zoning in the magma chamber) Early phases of the eruption tap acidic magma (e.g., rhyolite) at the top of the chamber, while lower and more basic levels (e.g., andesite) are tapped during later phases High temperature emplacement can lead to the agglutination of hot pyroclasts on deposition (welding) Pumice lapilli can flatten and deform to produce bedding-parallel fiamme, and columnar joints, similar to those developed in lavas, can develop In extreme cases, the aggrading ignimbrite can start to flow as a dense viscous liquid and can end up looking superficially like lava