Pyroclastic flow deposition
In volcanology, pyroclastic flow deposits are those volcanic deposits that were created during a volcanic eruption from the components (pyroclasts) of a pyroclastic density current flowing on the subsurface . A pyroclastic density flow is a turbulent gas (liquid) particle mixture ( dispersion ) that can arise in an explosive volcanic eruption. Due to its high density, it does not rise, but rather flows along the ground following the topography . Solidified pyroclastic flow deposits are known as ignimbrites . In contrast to pyroclastic flow deposits, pyroclastic fall deposits arise from those pyroclasts that fall from an eruption column or have been ballistically ejected from the volcanic crater .
Emergence
A pyroclastic density flow is a laterally moving transport system whose main trajectory was originally directed laterally and / or which could not produce any lift. Due to changes in lift and turbulence, however, primarily vertical transport systems can also develop into secondary lateral transport systems (e.g. when an eruption cloud collapses). These are turbulent gas (liquid) particle mixtures that flow on the ground and are controlled by density, gravity and relief. Pyroclastic flux deposits arise during the movement of pyroclastic density flows and when they come to a standstill.
Classification according to grain size
Pyroclastic flow deposits, like pyroclastic fall deposits, are further subdivided according to grain size and genesis. A distinction is made between agglomerates , pyroclastic breccias , tuff breccia , lapilli tuff , lapilli stones , and ash tuff .
Due to the high temperatures found in many pyroclastic density flows, the components can melt during transport or shortly after deposition. "Melts" or ignimbrites are formed . This process takes place at temperatures of over 550 °. Cooler streams form stratified, unconsolidated deposits with a characteristic inventory of sedimentary structures. This distinguishes them from unconsolidated pyroclastic falling deposits of the same grain size. But only the deposits from pyroclastic surges are called ignimbrites; the deposits from pyroclastic currents i. e. S. as block and ash stream deposits. However, this definition of the term ignimbrite is not widely accepted.
Pyroclastic case deposits, on the other hand, form loose sediments (tephra) in the great majority of cases, as the components are usually cooled down by ballistic, turbulent or aeolian transport to such an extent that they are already solid when they are deposited.
Classification according to the density of the depositing stream
The deposits from the pyroclastic density flows are divided in the more recent literature according to the density of the gas-particle mixture of the flow into:
- Pyroclastic flow deposit; massive, poorly sorted, ash-rich deposit of a density flow with high particle concentrations (> 10%)
- Pyroclastic surge deposit; Stratified deposition (lamination, cross stratification) of a density flow with low particle concentration (<1%).
Related deposits
Related deposits, but no pyroclastic deposits in the strict sense are the debris avalanches (debris avalanches) and lahars . Although they mainly consist of pyroclasts, they are not directly linked to a volcanic eruption; they can also arise completely independently much later. However, by definition they are tied to a volcano or its immediate vicinity.
literature
- Haraldur Sigurdsson (Ed.): Encyclopedia of Volcanoes. 1417 pp., Academic Press, San Diego et al., 2000 ISBN 0-12-643140-X
- Elisabeth A. Parfitt and Lionel Wilson: Fundamentals of Physical Volcanology. 230 pp. Malden, MA, Oxford & Carlton, Victoria, Australia, Blackwell Publishing, 2008. ISBN 978-0-63205443-5
- Hans Pichler and Thomas Pichler: volcanic areas of the earth. 261 p., Spektrum Akademischer Verlag, Heidelberg 2007 13: 978-3-8274-1475-5