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Sedimentary Petrology
Published in Supriya Sengupta, Introduction to Sedimentology, 2017
Volcaniclastic sediments are broadly divided into two groups according to grain-size—the coarser volcanic breccia (angular fragments) and agglomerates (rounded fragments), and the finer tuffs. Breccia and agglomerates consist of volcanic bombs and rock fragments. Tuffs are composed of fine crystals, rock fragments and volcanic glass. These are named according to their principal constituent—crystal tuffs, lithic tuffs and vitric tuffs (Fig. 4.19).
Landslide triggers and types
Published in Jan Rybář, Josef Stemberk, Peter Wagner, Landslides, 2018
Robert L. Schuster, Gerald F. Wieczorek
Volcaniclastic deposits, particularly volcanic ash, often exist as loose, easily liquefiable materials, which when on steep slopes, easily can be triggered into debris flows with the addition of intense rain. Such volcaniclastic debris flows, which are referred to by volcanologists as lahars, have occurred on many volcanoes, with tragic consequences; two examples follow:
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
Another group of rocks which can be regarded as igneous are pyroclastic rocks which are formed of clasts of volcanic material (ash, volcanic bombs and lapilli). These may be directly expelled from volcanoes as clasts or transported and re-deposited via sedimentary processes when they are termed volcaniclastic rocks (see Sedimentary rocks).
Zircon U–Pb age, whole-rock geochemistry and Nd–Sr–Pb isotope constraints on petrogenesis of the Eocene Zajkan gabbro–monzogranite intrusion, Tarom-Hashtjin magmatic belt, NW Iran
Published in Australian Journal of Earth Sciences, 2022
M. A. A. Mokhtari, H. Kouhestani, K. Z. Qin
In the Zajkan area, the major geological units are Eocene volcanic–volcaniclastic rocks (>2000 m in thickness; Hosseiny et al., 2016) that were intruded by the Zajkan intrusion (Figure 2). Volcanic rocks are andesite, trachyandesite and porphyritic andesite with local olivine basalt, andesitic basalt and rhyodacite lava flows (Hosseiny et al., 2016; Kouhestani et al., 2019a, 2019b). Andesites and trachyandesites comprise plagioclase, hornblende and biotite phenocrysts within fine-grained groundmass. Porphyritic andesite is composed of centimetre-sized plagioclase phenocrysts along with hornblende and minor small clinopyroxene phenocrysts set in a fine-grained to glassy matrix (Hosseiny et al., 2016). The olivine basalts consist of olivine, plagioclase, and clinopyroxene phenocrysts set in an intersertal and fine-grained groundmass (Hosseiny et al.,2016). Andesitic basalts show a porphyritic texture and consist principally of distinctive zoned phenocrysts of plagioclase and pyroxene (Hosseiny et al., 2016). The rhyodacite lavas are composed of largely plagioclase, sanidine, and quartz phenocrysts and rare biotite with a porphyritic texture (Hosseiny et al., 2016). The volcaniclastic strata is mainly tuff (i.e. crystal tuff, crystal lithic tuff, lapilli tuff, and tuff breccia), ignimbrite, agglomerate, sandy tuff and tuffaceous sandstone, with minor conglomerate intercalations (Hosseiny et al., 2016).