Pyroclastic drop deposition
As pyroclastic case deposits in the are Vulkanologie both those pyroclastic deposits referred to by the rain-out and leaching of pyroclastic from a volcanic eruption cloud by atmospheric processes or the gravity caused, and that in a such explosive volcanic eruption of ballistic be formed ejected pyroclastic. In contrast to pyroclastic fall deposits, pyroclastic flow deposits arise from a pyroclastic density flow .
Emergence
Pyroclastic fall deposits arise when the pyroclasts ejected by a volcanic eruption fall to the earth after a ballistic transport, or when ejected pyroclasts are initially held in suspension by the turbulence in the eruption cloud and, after more or less further drifting by the wind, sink to the ground by gravity or washed out by atmospheric processes (rain).
They arise mainly from primary, relatively stable Plinian eruption clouds . But pyroclasts can also rain out from convection clouds (“phoenix clouds”), which are secondary to pyroclastic density currents . Since these secondary convection clouds usually only contain finer material, these pyroclastic case deposits are always relatively fine-grained. A typical characteristic of these falling deposits formed from secondary convection clouds is the alternating position and interlocking with pyroclastic flow deposits .
Pyroclastic drop deposits are sorted according to the size of the particles over the entire area of the deposits of an outbreak. In general, the largest components fail near the site of the eruption; coarse ash particles can be transported many kilometers. Fine ash particles can be thrown into higher areas of the atmosphere and orbit the earth for months and years. The finest ash particles often form condensation nuclei for the aerosols , small water droplets in which volcanic gas has dissolved.
In the immediate vicinity of volcanoes, however, pyroclastic fall deposits are almost always poorly (er) sorted in profile and in relation to other clastic deposits, as the high turbulence within the eruption cloud allows pyroclasts of different sizes to rain out. This means that larger components can rain out much sooner than would be expected from the ejection speed, the height of the ejection and the size of the component. The reverse is also true; the turbulence can prevent them from raining “normally” and they are therefore transported further away from the eruption site than would only be possible by gravity, depending on the ejection speed, the height of the ejection and the size of the component. In addition, there is the drift due to the wind, which usually increases in speed as the height of the eruption cloud increases. Wind-drifted pyroclastic fall deposits are therefore usually relatively well sorted, since the greater the distance from the eruption the turbulence decreases and this no longer plays a major role in the deposit. The sorting by the wind or the wind speed predominates. However, only finer particles can be displaced by the wind and only ash tuff is deposited . Wind-drifted pyroclastic fall deposits can have hundreds, u. It can even be deposited thousands of kilometers from the site of the eruption. The pyroclasts are often mixed with other sediment particles and tuffites are deposited .
Pyroclastic fall deposits are almost always loose sediments, also known as tephra . The ballistic, turbulent or aeolian transport usually cools the pyroclasts to such an extent that they are already solid when they are deposited; Melting (or even melting) of components or deformation of components that are still plastic is therefore very minor. Only in the transition area to the effusive activity of a volcano, for example during lava-throwing activity, can basic melts e.g. B. Flat lava be thrown out, which is deformed when it hits the ground or can bake together ( welding slag ).
Classification
Pyroclastic case deposits are further subdivided according to grain size and genesis. Be distinguished pyroclastic agglomerates , pyroclastic breccia , tuff breccia , lapilli tuffs , Lapillisteine and ash tuffs .
The terms according to grain size and according to the components are not only applied to the pyroclastic fall deposits, but also to the description of the pyroclastic flow deposits that arise from pyroclastic density flows . Although these deposits can also form loose sediments, they are usually so hot that the transported and deposited pyroclasts are melted again and baked together (melting tuffs or ignimbrites ).
Special forms of pyroclastic fall deposits
Large volcanic eruptions can trigger violent thunderstorms and precipitation due to the rapid rise of moist air to great heights and the further "sucking in" of moist air masses. The fine ash particles act as condensation nuclei when drops form. A coincidental meeting of a tropical (eddy) storm or a low pressure area with an eruption cloud can also result in heavy precipitation in the vicinity of the eruption cloud. This precipitation can wash away large amounts of fine pyroclastic material and cause mud rain .
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