Pyroclastic rock

As pyroclastic sediments, also pyroclastic deposits or pyroclastic deposits, deposits are referred to in volcanology, consisting of more than 75 % of pyroclastics. The remaining 25 % can be made of other rocks, such as chemical, biogenic and clastic sedimentary rocks.

Description

Pyroclastics are ( rock ) fragments produced by tearing or breakage ( fragmentation) of rock or magma during volcanic eruptions or other volcanic processes occur. Unconsolidated pyroclastic deposits are called tephra (Greek ash); mainly solidified pyroclastic deposits are called pyroclastic rocks. Different transport and deposition processes of pyroclastic can be used for further subdivision of pyroclastic sediments. In addition to the pyroclastic deposits and their lavas are the most important volcanic conveyor products. In contrast to the pyroclastic deposits and their they are the product of direct volcanic activity. A possible (self) brecciation of lava in lava flows, for example, but is not taken into pyroclastic deposits. The investigation of the degree of fragmentation of the pyroclastic and the transport and deposition process of pyroclastic deposits or rocks are thus the most important tools for reconstructing the events during volcanic eruptions in Earth's history.

Distinction by components and grain sizes

Most pyroclastic deposits are polymodal, ie consist of material of different grain size. They are classified according to the predominant part of their Pyroklastentypen. This classification is independent of the type of transport and can be applied to all types pyroclastic deposits.

• Agglomerate, a pyroclastic rock consisting of more than 75 % of volcanic bombs; a special form of the agglomerate is the welding slag.
• Pyroclastic breccia, a pyroclastic rock consisting of more than 75 % of volcanic blocks; a special form of pyroclastic breccia is the throw slag.
• Tuff breccia, a pyroclastic rock composed of between 25 % and 75 % of volcanic bombs and blocks.
• Lapilli tuff, a pyroclastic rock that contains less than 25% of bombs and blocks and more than 75 % lapilli and ash.
• Lapillistein a pyroclastic rock, which contains more than 75% lapilli.
• Tuff or ash tuff, a pyroclastic rock that contains more than 75 % volcanic ash. It is further divided into coarse ash tuff and fine ash tuff. The fine ash tuff can be referred to as Staubtuff.

Tuffs and ash can be further distinguished according to their composition. Lithic tuff consists predominantly of rock fragments (Greek líthos ' stone '), predominantly of pumice tuff vitrischer and glass fragments (Latin vitrum ' glass '), Kristalltuff predominantly of crystals or crystal fragments.

Each of these pyroclastic rock types can after the creation (Genesis ) or the petrographic composition can be further divided, eg vent agglomerate, rhyolitic tuff, basaltic lapilli tuff, etc. These terms can also be replaced by solely genetic terms, if the origin of pyroclastic deposition is known and the genesis is in the foreground. Mixed - epiklastische pyroclastic deposits ( share of pyroclastics 25% to 75 %) are referred to as tuffites. The terms for clastic rocks are provided tuffitisch with the addition of, for example tuffitische breccia, conglomerate tuffitisches, tuffitischer sandstone, siltstone and tuffitischer tuffitischer mudstone.

Distinction according to the type of transport

With explosive volcanic eruptions are two types of transport systems can be distinguished. These are mainly controlled by the density, direction and speed of the eruption beam above the vent; that is, whether the systems have lift or not. Do the systems lift and / or is the main path of movement upwards, so results in large, vertical, wind influenced eruption clouds with internal turbulence that produce pyroclastic fall deposits. Sideways moving systems whose main path of movement was originally directed laterally and / or do not have a lift, produce on the ground flowing, controlled by gravity and the local relief, turbulent pyroclastic density currents. However, may emerge later laterally directed by changes in buoyancy and turbulence from first vertical transport systems transport systems and vice versa. A special case is predominantly vertical transport of ballistic ejection of larger pyroclastic whose deposition is hardly affected by the atmosphere. However, they represent only a small fraction of the ejected pyroclastics.

Corresponding to the two transport systems, two groups of pyroclastic deposits can be distinguished:

• Pyroclastic deposits case (English pyroclastic deposits falling ), this includes all deposits that were transported to the site of deposition by ballistic transport from Auswurfsort and deposits by rainout and by atmospheric leaching from an eruption cloud. Pyroclastic deposits cover case the relief ( hills and valleys ) more or less evenly.
• Pyroclastic flow deposits (English pyroclastic flow deposits in the broad sense ). Deposits of pyroclastic density currents, however, are mostly confined to valleys. You can usually higher barriers not overcome ( depending on the density) and are limited to morphologically low -lying areas.

No pyroclastic deposits in the strict sense are the following mentioned because they are not directly related to a volcanic eruption, but can also be formed regardless of:

• Lahars; Deposits of volcanic mud flows. They are also restricted to valleys and overcome only minor morphological barriers.
• Debris avalanches (English debris avalanche ); are landslides or debris flows, which can occur when a part breakdown of a volcano building in the background.

Tephrochronologie

Can often be pyroclastic deposit layers clearly individual volcanic eruptions assign. If a temporal classification possible, serve pyroclastic sediments in rock layers as calibration horizons in chronostratigraphy. This Tephrochronologie limited to younger volcanic activity within the Quaternary. A relatively well-known calibration horizon is the tephra of the Laacher See volcano, which erupted 10,982 BC, and large parts of Central Europe covered with a layer of ash.