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An ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, which is a hot suspension of particles and gases flowing rapidly from a volcano driven by having a greater density than the surrounding atmosphere. New Zealand geologist Patrick Marshall derived the term 'ignimbrite' from ‘fiery rock dust cloud’ (from the Latin ''igni-'' (fire) and ''imbri-'' (rain)), formed as the result of immense explosions of pyroclastic ash, lapilli and blocks flowing down the sides of volcanoes. Ignimbrites are made of a very poorly sorted mixture of volcanic ash (or tuff when lithified) and pumice lapilli, commonly with scattered lithic fragments. The ash is composed of glass shards and crystal fragments. Ignimbrites may be loose and unconsolidated or lithified (solidified) rock called lapilli-tuff. Proximal to the volcanic source, ignimbrites commonly contain thick accumulations of lithic blocks, and distally, many show meter thick accumulations of rounded cobbles of pumice. Ignimbrites may be white, grey, pink, beige, brown or black depending on their composition and density. Many pale ignimbrites are dacitic or rhyolitic. Darker coloured ignimbrites may be densely welded volcanic glass or, less commonly, mafic in composition. ==Deposition== There are two main models that have been proposed to explain the deposition of ignimbrites from a pyroclastic density current, the ''en masse'' deposition and the progressive aggradation models. The ''en masse'' model was proposed by volcanologist Robert Stephen John Sparks in 1976. Sparks attributed the poor sorting in ignimbrites to laminar flows of very high particle concentration. Pyroclastic flows were envisioned as being similar to debris flows, with a body undergoing laminar flow and then stopping ''en masse''. The flow would travel as a plug flow, with an essentially non-deforming mass travelling on a thin shear zone and the en masse freezing occurs when the driving stress falls below a certain level. This would produce a massive unit with an inversely graded base. Branney et al. 2002 suggest that as an ignimbrite is a deposit, its characteristics cannot completely represent the flow. They suggest that the deposit only records the depositional process. They highlight a number of problems with ''en masse'' deposition. Vertical chemical zonation in ignimbrites is interpreted as recording incremental changes in the deposition and the zonation rarely correlate with flow unit boundaries and may occur within flow units. Branney et al. suggest that the chemical changes are recording progressive aggradation at the base of the flow from an eruption whose composition changes with time. For this to be the case the base of the flow cannot be turbulent. They also suggest that instantaneous deposition of an entire body of material is not possible because displacement of the fluid is not possible instantaneously. Any displacement of the fluid would mobilize the upper part of the flow and en masse deposition would not occur. For a flow to stop simultaneously across its entire length would cause local compression and extension, there would be evidence of this recorded, in the form of tension cracks and small scale thrusting, and it is not seen in most ignimbrites. In response they suggest the ignimbrite records progressive aggradation from a sustained current and that the differences observed between ignimbrites and within an ignimbrite are the result of temporal changes to the nature of the flow that deposited it. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ignimbrite」の詳細全文を読む スポンサード リンク
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