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Volcanoes and Their Products
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
The principal types of pyroclasts (see Table 7.3) are bombs, blocks, lapilli, and ash, distinguished by their sizes. For the largest size, the term bomb is reserved for clasts formed from material ejected from a volcanic vent while molten or semi-molten. Bombs develop “bread-crust” or other surface textures, suggesting that they were cooled quickly, shaped, and solidified as they traveled through air. In contrast, pyroclastic blocks have an angular or blocky texture, suggesting that they were already solid at the time of eruption (Fig. 7.52b). The term lapilli (lapillus, singular) refers to any pyroclast with a mean diameter of 2 to 64 millimeters, and ash (sometimes divided into fine ash and coarse ash) refers to finer material.
Igneous Rocks
Published in F.G.H. Blyth, M. H. de Freitas, A Geology for Engineers, 2017
F.G.H. Blyth, M. H. de Freitas
In the upper part of the magma chamber beneath the volcano (Fig. 5.3) gases accumulate and build up pressure in the pipe of the volcano. When eruption occurs the expanding gases burst the lava into countless small fragments of dust, ash, or pumice, which ultimately fall around the vent or are blown to a distance by wind. Larger fragments (lapilli) and still larger lumps of magma (bombs) may also be ejected, together with fragments and blocks of rock torn by the force of the eruption from the walls of the volcanic vent.
Volcanic activity
Published in F.G. Bell, Geological Hazards, 1999
The term ‘lapilli’ is applied to pyroclastic material that has a diameter ranging from approximately 10 to 50 mm (Figure 2.6). Cinder or scoria is irregularly shaped material of lapilli size. It is usually glassy and fairly to highly vesicular and represents the ejected froth of a magma. Lapilli can be ejected during an explosive eruption over a radius of several kilometres from a volcano.
Whakaari/White Island: a review of New Zealand’s most active volcano
Published in New Zealand Journal of Geology and Geophysics, 2021
Geoff Kilgour, Ben Kennedy, Bradley Scott, Bruce Christenson, Arthur Jolly, Cameron Asher, Michael Rosenberg, Kate Saunders
Phreatomagmatic and magmatic ash and lapilli tuff units erupted during the 1970s are poorly preserved on the volcano flanks (Figures 4 and 5). Pyroclastic and volcaniclastic facies within the central sub-crater are heterolithic and heterogeneously distributed. Deposits younger than 1976–77 are periodically exposed where dissected by drainage channels and in walls of collapse pits and craters within the central crater (Figure 4). Nearly all of the thinly bedded ash (1970s and 1980s) and lapilli tephra (1977; 2000 AD scoria) units are interpreted as primary phreatomagmatic and phreatic falls and surge deposits (Houghton and Nairn 1991; Kilgour et al. 2019). These beds drape volcaniclastic mass flow deposits and are themselves peppered by ballistic blocks and bombs (Kilgour et al. 2019; Kennedy et al. 2020).
Contemporaneously emplaced submarine volcaniclastic deposits and pillow lavas from multiple sources in the island arc Brook Street Terrane, Southland, New Zealand
Published in New Zealand Journal of Geology and Geophysics, 2020
Jasmine F. Mawson, James White, James Michael Palin
The rocks at Riverton comprise five lithofacies (nomenclature from White and Houghton 2006). Major characteristics of the lithofacies are summarised in Table 1. A geological map (Figure 2), a cross-section along the coast (Figure 3A), and a logged stratigraphic section (Figure 3B) display the relations among lithofacies. A large pillow lava pile (about 1.7 km across, at least 120 m high) dominates the southeastern part of the Riverton Peninsula. This and smaller pillow lava piles intrude into lapilli tuff and tuff. Lapilli tuff underlies and fills channels cut in tuff (Table 1, Figure 4B). Tuff underlies and is interbedded with argillite. The whole section is cut by dikes. Westwards from the large pillow lava pile is a shallowly dipping sequence of lapilli tuff, tuff and argillite, while to the east lapilli tuff dominates and the sequence is less clear.
Intraplate volcanism on the Zealandia Eocene-Early Oligocene continental shelf: the Waiareka-Deborah Volcanic Field, North Otago
Published in New Zealand Journal of Geology and Geophysics, 2020
James M. Scott, James D. L. White, Petrus J. le Roux
All of the studied sections comprise sequences of tuff breccia, lapilli tuff and tuff (Figure 2A) that are in places interbedded with pillow lava and limestone, mudstone or marl (Figure 2B). The contact relationships among different volcaniclastic deposits, often featuring sharp but local erosion surfaces (Figure 2C) and/or cross-bedding, indicate that they formed primarily as deposits of submarine density currents from proximal volcanic edifices, emplaced both as primary deposits during eruptions, and through subsequent post-eruptive redistribution (Cas et al. 1989; Andrews 2003; Maicher 2003; Corcoran and Moore 2008; Moorhouse et al. 2015). Volcaniclasts are mostly now-altered vesicular basaltic glass, or blocks fragmented from dikes associated with the eruptions (Figure 2D). Anomalously, Kakanui deposits also contain a variety of crust and mantle fragments (described below). Palaeontology indicates that water depths were generally shallow (∼ <80 m) during the volcanic field's duration (Lee et al. 1997; Hicks 2014) and it is probable that volcanic edifices breached the sea surface, with those of entirely pyroclastic construction being rapidly planed off to normal wave base (e.g. Thorarinsson 1967). The volcanoes eroded rapidly, with evidence for calcareous algae, bryozoans and serpulids encrusting basalt boulders and cobbles (Lee et al. 1997) indicating the volcanoes formed sites favourable for rhodolith, bryozoan, brachiopod, echinoderm, bivalve and gastropod colonies (Utlley 1918; Cas et al. 1989; Lee et al. 1997; Corcoran and Moore 2008; Hicks 2014).