Ignimbrite

Ignimbrite (Latin ignis - fire, imber - Rain ) is a relatively fuzzy term from the sedimentology, volcanology and petrology. He called pumice - or ash-rich deposits of pyroclastic density currents that initially deposited loose, or baked at high temperatures later ( " melted" ) have been. Alternative z.T. even more precise terms Bimsstrom, or if the ash fraction is predominant and ash stream, or if the deposits ignimbrite fused or welded.

History of the term

The name was introduced in 1932 by Patrick Marshall for the first time in the literature. He interpreted the ceiling shaped overlapping, rhyolitic rocks in New Zealand not as far as lava flows, but as deposits of "Fire rain" consisting of hot pyroclasts. His definition of the term was quite narrow, compared to the current view:

• Igneous rocks of acid or intermediate composition Perhaps Which havebeen Formed from material ejected from orifices did HAS BEEN in the form of a multitude of highly incandescent particles Which were Mainly of a minute size ..

Today, a different formation mechanism for the ignimbrites is assumed.

Use of the term

In the older literature, the term generally to the welded by high temperatures after the deposition of sediments of a pyroclastic density current ( " Schmelztuffe " ) is limited. But not all pyroclastic density currents are hot enough so that it comes after the deposition of the merger of the components. Therefore, the term in today's literature fused and unfused, pumice and ash- rich, unconsolidated and consolidated deposits of pyroclastic density currents; that is, after this extended definition are ignimbrites ash and pyroclastic flow deposits bimsreiche. However, this rather broad and vague definition is controversial and not uniform in volcanology. In the petrography is under a consolidated ignimbrite deposit or a rock understood that arose from a pyroclastic density current.

Formation

Pyroclastic density currents are preferentially formed in volcanoes, promote the gas-rich and acidic, so very siliceous lava mainly rhyolitic composition. Acid lava is very viscous and can therefore clog the volcanic vent and eg form a lava dome. If the gas pressure of the upcoming magma too large, it comes to an explosive eruption. The lava dome breaks off and can be almost completely destroyed by the explosion, which partly already semi-solidified and consolidated rocks are highly fragmented and mixed with glazed lava and crystals. This particle -air mixture flows down the flanks of the volcano; can be up to 150 km move far from the center of explosion. Pyroclastic density currents can be very hot; will be referred to more than 800 ° C temperatures.

Structures

The deposits of pyroclastic density currents or ignimbrites are mostly massive layers of glass fragments Bimslapilli, crystals and rock fragments. They are poorly sorted, large and small fragments are present side by side. The ash fraction predominates. At low temperatures below 500 to 600 ° deposited ignimbrites are not welded and form massive unconsolidated rock, or in the case of subsequent cementation hard rocks. Was the flow of hot as 500 to 600 ° C, bake in the great heat, the pyroclastic components into a solid mass with a high glassy component. You are welded or sinter. In a most fine-grained groundmass (matrix) hardly assorted fragments of rocks, crystals, and flattened, short strips or patties from pumice ( fiamme structures), a frothy glass rocks are embedded. In petrography one also speaks of a ignimbritischen or eutaxitischen structure. The resulting from the sintering or welding rock reminds by its density more like lava.

Flow units

Ignimbritserien almost always consist of several flow units, ie Individual streams, followed in rapid succession (usually in minutes or hours apart) and which together form a cooling unit. The individual flow units or individual streams have a characteristic structure:

• A lower bottom layer consisting of coarse particles. It occurs when at the end of the pyroclastic flow, air is sucked in and diluted the current. Coarse particles can no longer be transported.
• The main part having a gradation, specifically heavy particles at the base and at the top of light pumices
• A fine-grained ash layer on top. It arises from the deposition of ash from the ash clouds that are released during the transport of the currents of the actual current. The ash particles are entrained by the escaping hot gases from the stream.

Large-scale deposition forms

According to their large-scale deposit forms two types of ignimbrites or Ignimbritserien be distinguished:

• Small volume, limited to valleys ignimbrites caused by ash and block flows.
• Large-volume, plateau -forming ignimbrites, pyroclastic ash flows arising from

In particular, the large-volume ignimbrites can trigger eruptions secondary so-called "co- ignimbrite ". The eruption clouds can thereby occupy a much larger area than a fixed to a small area Plinian eruption cloud.

Examples

In addition to the flood basalts of the plateau forming large-volume ignimbrites are the most common volcanic rocks. Examples of these large-volume, plateau -forming ignimbrites are:

• Yellowstone National Park, several outbreaks: 600,000 years ago was the Lava Creek Tuff, deposited with a volume of 1000 km ³; 1.2 million years before depositing the Mesa Falls Tuff with a volume of 280 cubic kilometers, 2 million years ago, depositing the Huckleberry Ridge Tuff with a volume of 2500 km ³.
• Campanian ignimbrite ( Campanian ignimbrites ), > 200 km ³, trachytisch - phonolithischer pyroclastic flow, 39,300 ± 0100 years old, within the Campi Flegrei west of Naples, Italy.
• Taupo ignimbrite, 30 km ³, 186 AD, central part of the North Island of New Zealand, expansion in the radius of 80 ± 10 km from the center of the outbreak

Economic Importance

Ignimbrites are important natural stones and natural stones, which were used for construction of buildings and still are. The Brohltal Trass was an important additive to cement. Porphyry from Rochlitz and Bozen quartz porphyry have been or are used as building stone in the construction of buildings and the erection of monuments.

In the U.S., the Yucca Mountain ignimbrite is intended as a repository for radioactive waste.