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Department of Mineralogy and Petrology
Faculty of Natural Sciences
Comenius University in Bratislava

Pyroclastic rocks

Pyroclastic rocks are formed by accumulation of material ejected during explosive fragmentation of magma and/or by pre-existing solid rocks destructed during the volcanic explosion. Their name comes from Greek pyr – fire and klastós – broken-down. These rocks exhibit fragmental texture. Individual fragments of disintegrated material are called pyroclasts. Pyroclasts can be composed of fragments of older rocks (prevailingly volcanic, plutonic or metamorphic) pulled out of the basement, or they are solidified fragments of lava, volcanic glass and/or minerals. If 75 % of pyroclastic rock is composed of non-altered pyroclasts and the rock is consolidated, it is called pyroclastic. If the same rock is non-consolidated, it is called tephra. If the pyroclasts are altered, or their pyroclastic origin is ambiguous, the rocks are epiclastics.

Several types of pyroclastic rocks are distinguished according to size of pyroclasts. Bombs are pyroclasts with diameters larger than 64 mm, having an oval shape that indicates their plastic behaviour during their origin and transport. Blocks are pyroclasts larger than 64 mm with angular shape, which indicates solid state during their origin and accumulation. Lapilli are pyroclasts of various shapes, 2-64 mm in size. Volcanic ash is composed of pyroclasts less than 2 mm in diameter. Despite polymodal character of most pyroclastic rocks, their classification is descriptive, and is based on proportional abundance of individual types of pyroclasts.

Pyroclastic rocks exhibit typical pyroclastic (fragmental) structure. If containing glass porphyroclasts, they have hyaloclastic structure. If the glass porphyroclasts are cemented by glass, we speak about specific vitroclastic structure. The most important pyroclastic rocks are: agglomerate, pyroclastic breccia, tuff breccia, lapilli tuff, lapillistone and tuff. They are classified according to abundance and size of individual types of pyroclasts.