Pyroclastic sediment
As pyroclastic sediments , also pyroclastic deposits or pyroclastics , (from the Greek πῦρ, fire; and κλαστός, broken) are deposits in volcanology that consist of more than 75% pyroclasts . The remaining 25% can consist of other rocks, for example chemical , biogenic or clastic sedimentary rocks .
description
Pyroclasts are (rock) fragments that are formed by tearing or breaking (fragmentation) of rock or magma during volcanic eruptions or other volcanic processes. Unsolidified pyroclastic deposits are called tephra (Greek for ash ); predominantly solidified pyroclastic deposits are called pyroclastic rocks. Different transport and deposition processes of the pyroclasts can be used to further subdivide the pyroclastic sediments. Besides the pyroclasts and their deposits, lavas are the most important volcanic extraction products. In contrast to the pyroclasts and their deposits, they are the product of direct volcanic activity. A possible (self-) brecciation of lava in lava flows, for example, is not counted as pyroclastic deposits. The investigation of the degree of fragmentation of pyroclasts and the transport and deposition process of pyroclastic deposits or rocks are therefore the most important tools for reconstructing the events of volcanic eruptions in the history of the earth .
Differentiation according to components and grain sizes
Most pyroclastic deposits are polymodal , i.e. they consist of material of different grain sizes . They are classified according to the predominant proportion of their pyroclast types. This classification is independent of the type of transport and can be applied to all types of pyroclastic deposits.
- Agglomerate , a pyroclastic rock made up of more than 75% volcanic bombs ; Welding slag is a special form of agglomerate .
- Pyroclastic breccia , a pyroclastic rock made up of more than 75% volcanic blocks ; A special form of the pyroclastic breccia is the litter slag .
- Tuff breccia, a pyroclastic rock composed of between 25% and 75% volcanic bombs and blocks.
- Lapilli tuff, a pyroclastic rock that contains less than 25% bombs and blocks and more than 75% lapilli and ash.
- Lapilli stone, a pyroclastic rock that contains more than 75% lapilli.
- Tuff, or ash tuff, a pyroclastic rock that contains more than 75% volcanic ash . A further distinction is made between coarse ash tuff and fine ash tuff. The fine ash tuff can also be called dust tuff.
Tuffs and ashes can be further distinguished according to their composition. Lithic tuff consists mainly of rock fragments ( Greek líthos 'stone'), vitric tuff mainly of pumice and glass fragments ( Latin: vitrum 'glass'), crystal tuff mainly of single crystals or crystal fragments .
Each of these pyroclastic rock types can be further subdivided according to the origin (genesis) or the petrographic composition, for example chimney agglomerate, rhyolitic tuff, basaltic lapilli tuff, etc. These terms can also be replaced by exclusively genetic terms if the origin of the pyroclastic Deposition is known and the genesis is in the foreground. Mixed pyroclastic-epiclastic deposits (proportion of pyroclasts 25% to 75%) are called tuffites . The names for clastic rocks are given the addition Tuffitic, for example Tuffitic breccia, Tuffitic conglomerate, Tuffitic sandstone, Tuffitic siltstone and Tuffitic claystone.
Differentiation according to the type of transport
In the case of explosive volcanic eruptions, two types of transport systems can be distinguished. These are mainly controlled by the density, direction and speed of the eruption jet over the vent; d. H. whether the systems are buoyant or not. If the systems are buoyant and / or if the main trajectory is directed upwards, then large, vertical, wind-influenced eruption clouds with internal turbulence are created that produce pyroclastic fall deposits. Systems moving sideways, whose main trajectory was originally directed sideways and / or which have no buoyancy, generate turbulent pyroclastic density currents on the ground that are controlled by gravity and the local relief . However, changes in lift and turbulence can result from initially vertical transport systems later laterally directed transport systems and vice versa. A special case of predominantly vertical transport is the ballistic ejection of larger pyroclasts, the deposition of which is hardly influenced by the atmosphere. However, they only represent a small fraction of the pyroclasts ejected.
According to the two transport systems, two groups of pyroclastic deposits can be distinguished:
- Pyroclastic deposits case ( Engl. Pyroclastic deposits falling ), this includes all deposits that were transported by ballistic transport from Auswurfsort the deposition site and deposits by rainout and by atmospheric leaching of a plume. Pyroclastic fall deposits cover the relief (mountains and valleys) more or less evenly.
- Pyroclastic flow deposits (English pyroclastic flow deposits iw S.). Deposits from pyroclastic density flows, on the other hand, are mostly restricted to valleys. They can usually not overcome higher barriers (depending on the density) and are limited to morphologically lower lying areas.
The following are not pyroclastic deposits in the actual sense, as they are not directly related to a volcanic eruption, but can also be formed independently of it:
- Lahars ; Deposits from volcanic mud flows . They are also restricted to valleys and can only overcome minor morphological barriers.
- Debris avalanche (English debris avalanche ); are basically landslides or debris flows that can arise from a partial collapse of a volcanic building.
Tephrochronology
Pyroclastic deposit layers can often be clearly assigned to individual volcanic eruptions. If a temporal classification is possible, pyroclastic sediments in rock layers serve as calibration horizons in chronostratigraphy . This tephrochronology is limited to recent volcanic activity within the Quaternary . A relatively well-known calibration horizon is the tephra of the Laacher See volcano, which dates back to 10,982 BC. Erupted and covered large parts of Central Europe with a layer of ash.
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 Pichler and Thomas Pichler: volcanic areas of the earth. 261 p., Spektrum Akademischer Verlag, Heidelberg 2007 13: 978-3-8274-1475-5
- Haraldur Sigurdsson (Ed.): Encyclopedia of Volcanoes. 1417 pp., Academic Press, San Diego et al., 2000 ISBN 0-12-643140-X
Web links
Individual evidence
- ↑ B. Weninger, O. Jöris: Use of Multi-Proxy Climate Date at the Middle-Upper Palaeolithic Boundary. Powerpoint presentation of a lecture at the UISPP conference in Lisbon, 2006; 2.26 MB.