Pyroclast
Pyroclasts , singular pyroclasts, (from ancient Greek πῦρ pyr "fire" and κλαστός klastós "broken") are (rock) fragments that were created by tearing or breaking (fragmentation) or by direct crystallization as a result of volcanic activity from a solid or liquid volcanic source material are. The grain size, the internal structure and the chemical-mineralogical composition are variable. The individual shape only plays a role in the designation of some types of pyroclasts. Pyroclasts are transported by ballistic ejection (with possible subsequent drifting by wind and air currents) or by flowing on the ground in density currents ( gas-particle mixtures ), in contrast to the formation of volcanic rocks from lava flows. By definition, pyroclasts form the main part (> 75%) of pyroclastic deposits .
definition
Pyroclasts are single crystals, crystal fragments, glass or rock fragments that were created during an explosive eruption of a volcano. The pyroclasts may already have been solid or still liquid when they erupted. The internal structure (but see below) and the mineralogical-chemical composition play no role in the definition. The individual shape, on the other hand, is important for the classification of individual pyroclast types.
Pyroclasts in the narrower sense must not show any signs that they have been reprocessed, i.e. H. the external shape that these fragments assumed during their formation or transport (ballistic transport or transport in pyroclastic density currents) must not show any signs of reprocessing. However, their shape can be changed during or shortly after the deposition, for example by partial melting and deformation by the load of superimposed pyroclasts or by internal grain / grain movements in the deposits.
Relocated pyroclasts, ie pyroclasts that have already been deposited and transported again, are also called “reprocessed pyroclasts”, clasts of uncertain origin are called “epiclasts”.
classification
Pyroclastic be based on the grain size classified . The IUGS ( International Union of Geological Sciences ) recommends the following classification:
- Bombs (> 6.4 cm). The external shape and surface show signs that the pyroclast melted during formation and transportation
- Blocks (> 6.4 cm). The angular or rounded-angular shape indicates that the pyroclast was solid at the time it was formed
- Lapilli (0.2 to 6.4 cm), small pyroclasts of any shape
- Ash (<0.2 cm), very small pyroclasts of any shape. They can be further divided into coarse ash grains (0.2 mm to 1/16 mm) and fine ash grains or ash dust (<1/16 mm).
There is also an older classification by Sohn & Cough (1989), which is still used today:
- fine ash (finer than 1/16 mm)
- medium ash (1/16–1 / 2 mm)
- coarse ash (1 / 2–2 mm)
- fine lapilli (2–4 mm)
- medium lapilli (4–16 mm)
- coarse lapilli (16–64 mm)
- fine blocks (64–256 mm)
- coarse blocks (larger than 256 mm)
Blistering and fragmentation
Most pyroclasts are more or less vesicular (vesicular). Bubble germs form when a gas-rich magma rises and the pressure is released. The magma can become oversaturated for one or more gas phases and the bubble nuclei grow rapidly through diffusion and decompression. This leads to the formation of a magma foam near the surface of the earth which, when released, can fragment explosively and form pyroclasts.
Phreatomagmatic processes also fragment the invading magma very strongly. In contrast to the explosive degassing of magma, these particles are significantly less vesicular and therefore an important indication of this formation process.
Pyroclastic deposits
Pyroclastic deposits or pyroclastics are so named if they consist of more than 75% pyroclasts. The rest can consist of epiclasts, chemical, biogenic or other sedimentary rocks . A type of volcano in which predominantly or almost exclusively pyroclastic loose products are extracted is the tuff volcano . In contrast, shield volcanoes produce predominantly lava and only a few pyroclasts.
Pyroclastic deposits are distinguished on the one hand according to the grain size of the pyroclasts and on the other hand according to the type of transport into pyroclastic flowing and falling deposits .
Loose, as yet unsolidified pyroclastic deposits (sediments) are also known as tephra . If pyroclastic loose sediments solidify, one speaks of pyroclastic rock.
swell
Individual evidence
- ↑ YK Son and SK Chough: depositional processes of the Suwolbong tuff ring, Cheju Iceland (Korea). Sedimentology 36, 837-855, 1989
literature
- Roger Walter Le Maitre: Igneous rocks: IUGS classification and glossary; recommendations of the International Union of Geological Sciences, Subcommission on the Systematics of Igneous Rocks. 2nd ed., 236 pp., New York, Cambridge University Press 2002, ISBN 0-521-66215-X
- Hans-Ulrich Schmincke: Pyroclastic rocks: In: In: Hans Füchtbauer (Ed.), Sediment-Petrology, Part 2, Sediments and Sedimentary Rocks . 4th edition, pp. 731-778, E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart 1988 ISBN 3-510-65138-3 .