Tephra
Pyroclastics (singular Pyroklast ) (Greek πῦρ " fire" and κλαστός " broken " ) are ( rock ) fragments produced by tearing or breakage ( fragmentation) or by direct crystallization as a result of volcanic activity are the result of a solid or liquid volcanic starting material. The grain size, the internal structure and the chemical- mineralogical composition are variable. The individual shape plays a role only in the name of some Pyroklastentypen. Pyroclastics are ballistic ejection transported ( with possible following drifting by wind and air currents ) or by flowing on the ground in density flows ( gas-particle mixtures), in contrast to the formation of volcanic rocks from lava flows. Pyroclastics form the major portion (> 75%) of pyroclastic deposits by definition.
Definition
Pyroclastics are single crystals, crystal fragments, glass or rock fragments that have emerged in an explosive eruption of a volcano. The pyroclastics at the outbreak may have been already fixed or still liquid. The internal structure (but see below) and the mineralogical and chemical composition play no role in the definition. The individual form is, however, important for the classification of individual Pyroklastentypen.
Pyroclastics in the strict sense may not show any signs that they have been re- worked, that is, the external shape, have accepted these fragments during the formation or transport ( ballistic transport or transport in pyroclastic density currents ) must not show any signs of reprocessing. However, they can be changed at or shortly after deposition in shape, such as partial melting and deformation by the applied load of overlapping pyroclastic or by internal grain / grain movements in the deposits.
Migrated pyroclastics, so already deposited and again transported pyroclastics are also called " remanufactured pyroclastics " clasts of uncertain origin are called " Epiklasten " means.
Classification
Pyroclastics are classified according to the grain size. The IUGS (International Union of Geological Sciences) recommends the following classification:
- Bombs ( > 6.4 cm). The outer shape and surface exhibit signs that the Pyroklast was melted during formation and transport
- Blocks ( > 6.4 cm). The angular or rounded - square shape indicates that the Pyroklast was fixed at the time of emergence
- Lapilli (0.2 to 6.4 cm), small pyroclastic any form
- Ash (< 0.2 cm ), very small pyroclastic any form. They can be further divided into coarse ash grains ( 0.2 mm to 1/16 mm), and fine ash particles or ash dust (< 1/16 mm).
In addition, there is an older classification of Son & Cough (1989 ), which is sometimes 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)
- Rough blocks (greater than 256 mm )
Blistering and fragmentation
Most pyroclastics are more or less bubbly ( vesicular ). Bubble nuclei are formed when a gas-rich magma and it comes to pressure relief. The magma can become supersaturated for one or more gas phase and the bubble nuclei are by diffusion and decompression rapidly growing. This leads near the surface to form a magma foam which can fragment explosively on exit and form pyroclastics.
Phreatomagmatic processes also fragment the magma aufdringende very strong. In contrast to the explosive degassing of magma, these particles are much less bubbly and thus an important indication of this development process.
Pyroclastic deposits
Pyroclastic deposits or pyroclastics are so named if they consist of more than 75 % of pyroclastics. The remainder may consist of Epiklasten, chemical, biogenic and other sedimentary rocks. A volcano type, should be encouraged in the predominantly or almost exclusively pyroclastic Locker products, is the Tuffvulkan. In contrast, shield volcanoes promote predominantly lava and pyroclastic few.
Pyroclastic deposits are distinguished on the one hand by the grain size of pyroclasts, on the other hand, according to the mode of transportation.
Loose, still unconsolidated pyroclastic deposits ( sediments ) are also called tephra. Become pyroclastic loose sediments solidified, it is called pyroclastic rocks.