Friday, May 7, 2010

Ignimbrite is a deposit of pyroclastic rock which is formed by ground-hugging flows of hot volcanic fragments and particles, essentially synonymous with pyroclastic-flow deposit, ash-flow tuff, flood tuff, or welded tuff. Ignimbrites are often of dacitic or rhyolitic composition.

Ignimbrites may be loose and unconsolidated, or lithified (solidified) rock called lapilli-tuff. Near source, ignimbrites commonly contain thick accumulations of lithic blocks, and distally, many show m-thick accumulations of rounded blocks (or cobbles) of pumice. The 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 commonly produced during explosive eruptions and are associated with most of the world's volcanic systems. They vary in size by orders of magnitude (10-3 to 103 km3 of erupted material) and have chemical compositions that span the entire range commonly exhibited by igneous rocks (basaltic to rhyolitic). An ignimbrite can be of any form and size, but most deposits have sheetlike shapes and cover many thousands of square kilometers.

Ignimbrite deposits are characterized by a poorly sorted aggregate of ash (crystals and glass shards) and pumice. In the larger deposits, the pumice fragments may be flattened and stretched to yield ovoid-to-lenticular shapes, reflecting the compaction and welding of the deposit after or during emplacement. See also Igneous rocks; Pumice; Pyroclastic rocks; Tuff; Volcanic glass; Volcano.

If sufficiently hot when deposited, the particles in an ignimbrite may weld together, and the deposit is transformed into a 'welded ignimbrite', made of eutaxitic lapilli-tuff. When this happens, the pumice lapilli commonly flatten, and these appear on rock surfaces as dark lens-shapes, known as fiamme. Intensely welded ignimbrite may have glassy zones near the base and top, called lower and upper 'vitrophyres', but central parts are microcrystalline ('lithoidal').

Ignimbrite deposite on Canary Island