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Igneous activity and landforms
Published in Richard J. Chorley, Stanley A. Schumm, David E. Sugden, Geomorphology, 2019
Richard J. Chorley, Stanley A. Schumm, David E. Sugden
The last major constructional feature of acid extrusive activity is that produced by the ashflow. These consist of solid particles of volcanic glass, crystals and pumice suspended in gas which flows like a fluid of low viscosity due to excessive gas expansion. The most striking example of this is the nuee ardente, a glowing avalanche, one of which flowed for some 5.6 km at a speed of up to 160 km per hour following the Mt Pelee eruption, on Martinique, on 8 May 1902. The resulting rock of rhyolite, dacite, andesite, or trachyte composition is termed ignimbrite. Ignimbrites can be relatively soft, occurring as bedded or chaotic tuffs, but are often found in the form of welded tuffs. Welding occurs when the glassy particles are so hot when they come to rest that they become welded together, expelling the gas, and compact to produce a very dense, homogeneous rock of low porosity, susceptible to marked columnar jointing, especially in the lower layers. Ashflows are erupted in great quantity from vents or systems of fissures – one emission of Katmai, Alaska, produced 28 km3 of rock in sixty hours (Figure 6.22). Individual flows may be many tens or even hundreds of metres thick. Some individual Tertiary ignimbrite flows in Nevada covered up to 16,000 km2 and were as much as 60 m thick. Repeated ashflows characteristically produce flat-topped plateaus with abrupt eroded edges. Two of the largest existing ignimbrite plateaus are in the North Island of New Zealand (20,000 km2; 8000 km3) and in Sumatra (20,000 km2; 2000 km3) (see Figure 6.5) (Macdonald, 1972).
Igneous rocks
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
An ignimbrite is a rock formed by the solidification of hot fragments explosively erupted from a volcano and forming a pyroclastic flow. It is composed of volcanic ash, pumice and lithic fragments in a matrix of vitric, crystal and lithic ash. In thin section the textures can be variable due to differences in compaction and welding. If sufficiently hot, the grains may weld together producing a welded ignimbrite containing flattened pumice fragments called fiamme (Italian: flame).
Minerals, rocks, discontinuities and rock mass
Published in Ömer Aydan, Rock Mechanics and Rock Engineering, 2019
Welded tuff or ignimbrite is a product of pyroclastic flows hot enough to fuse, or “weld,” still hot ash into a single uniform layer called a cooling unit. Ignimbrite is primarily composed of a matrix of volcanic ash, pumice fragments and crystals.
Taupō: an overview of New Zealand's youngest supervolcano
Published in New Zealand Journal of Geology and Geophysics, 2021
Simon J. Barker, Colin J.N. Wilson, Finnigan Illsley-Kemp, Graham S. Leonard, Eleanor R.H. Mestel, Kate Mauriohooho, Bruce L.A. Charlier
Pyroclastic deposits of the Taupō eruption (Unit Y of Wilson 1993) are global type examples because of their variable eruptive styles (e.g. Walker 1980, 1981b, 1981c; Froggatt 1981; Wilson 1985; Wilson and Walker 1985; Smith and Houghton 1995; Smith 1998; Houghton et al. 2010, 2014; Mitchell et al. 2018). The deposits are split into seven subunits based on inferred changes in eruptive intensity, degrees of magma-water interaction and shifting vent positions parallel to the Taupō rift on the eastern side of Lake Taupō (Table 1; Figures 2B and 3H; Healy 1964; Froggatt 1981; Wilson and Walker 1985; Smith and Houghton 1995; Smith 1998; Houghton et al. 2010, 2014). The eruption climaxed with the generation of a widespread non-welded ignimbrite that was emplaced by an extremely energetic pyroclastic density current travelling at more than 200–300 m/s and devastating ∼20 000 km2 (Wilson 1985). The Taupō eruption is unique in the post-Oruanui sequence in that it had the largest eruptive volume by ∼5-fold and resulted in further caldera collapse in the northeastern part of Lake Taupō (Table 1; Figure 2A; Davy and Caldwell 1998).
Paleomagnetism of the Carboniferous Gresford Block, Tamworth Belt, southern New England Orogen: minor counter-clockwise rotation of a primary arc segment
Published in Australian Journal of Earth Sciences, 2020
Low-temperature components are identified as L (omnipresent mid-Cenozoic overprint) and high-temperature components as P (primary, Carboniferous) or O (overprint, Permian or Permo-Triassic) rather than with general temperature LT, HT (low-temperature, high-temperature) descriptors. Early, tentative, committal to component origin at the component determination stage pre-empts the interpretation stage but simplifies component description. Early labelling has been retained for the few sites where a component’s origin has been revised during more substantive interpretation, or where directionally consistent high-temperature components of unclear origin have been labelled Q, or W as per their dominant direction. Thermal, and also AF, demagnetisation of ignimbritic samples has produced generally high-quality Zijderveld plots throughout the stratigraphy, facilitating separation and individual determination of subcomponents from subtle, systematic, directional shifts, across low-temperature unblocking/low coercivity ranges (L) and across the high-temperature unblocking/higher to high coercivity ranges (P) for magnetite and hematite. Subcomponents are annotated as L1, L2, or P1, P2, etc. with higher numbers reflecting further progressed demagnetisation.