Volcanic eruptions produce two main classes of material: lavas and pyroclastic rocks. Lavas are formed by the eruption of molten material (magma) that flows out of a vent or fissure over the surface and solidifies to form compact crystalline, partly glassy, or glassy rock. Pyroclastic rocks are composed of fragmental materials produced during more explosive types of eruption: the finer-grained material makes up volcanic ash and tuff and the coarser material volcanic agglomerate. Lava flows may have a relatively uniform composition, but pyroclastic rocks are more heterogeneous and may contain large amounts of comminuted wall-rocks from the vent as well as fragments of solidified lava.
Lying somewhat between lavas and pyroclastic rocks in their mode of origin are various eruptive products termed pyroclastic flows, including welded tuffs and ignimbrites. They are erupted as fluidized systems in which the liquid phase is dispersed in a gas medium. Such systems flow as discrete bodies...
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Cross-references
Acid(ic) Igneous rocks; Basic Igneous rocks; Explosive volcanic eruptions— classification; Flood basalt; Igneous rocks— classification and nomenclature; Igneous rocks— tectonic setting; Intermediate igneous rocks; Kimberlite; Lava and lava eruptions ; Mafic and ultramafic inclusions in igneous rocks; Magma; Magmatic differentiation; Mid-ocean ridge and ocean-floor petrology; Ocean-island igneous rocks; Petrochemical calculations; Plutonic rocks— classification and nomenclature; Pyrolite; Volcanoes and volcanism.
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Cox, K.G. (1989). Volcanic rocks . In: Petrology. Encyclopedia of Earth Science. Springer, Boston, MA. https://doi.org/10.1007/0-387-30845-8_257
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