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Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. [1] [2] Rock that contains greater than 75% ash is considered tuff, while rock containing 25% to 75% ash is described as tuffaceous (for example, tuffaceous sandstone). [3]
Ignimbrites are made of a very poorly sorted mixture of volcanic ash (or tuff when lithified) and pumice lapilli, commonly with scattered lithic fragments. The ash is composed of glass shards and crystal fragments. Ignimbrites may be loose and unconsolidated, or lithified (solidified) rock called lapilli tuff. Near the volcanic source ...
Pyroclasts include juvenile pyroclasts derived from chilled magma, mixed with accidental pyroclasts, which are fragments of country rock.Pyroclasts of different sizes are classified (from smallest to largest) as volcanic ash, lapilli, or volcanic blocks (or, if they exhibit evidence of having been hot and molten during emplacement, volcanic bombs).
Once clasts have fallen to the ground, they remain as tephra unless hot enough to fuse into pyroclastic rock or tuff. When a volcano explodes, it releases a variety of tephra including ash, cinders, and blocks. These layers settle on the land and, over time, sedimentation occurs incorporating these tephra layers into the geologic record.
Pyroclastic rocks are often the result of volcanic debris, such as ash, bombs and tephra, and other volcanic ejecta. Examples of pyroclastic rocks are tuff and ignimbrite. [citation needed] Shallow intrusions, which possess structure similar to volcanic rather than plutonic rocks, are also considered to be volcanic, shading into subvolcanic.
Lapilli tuffs are a very common form of volcanic rock typical of rhyolite, andesite and dacite pyroclastic eruptions, where thick layers of lapilli can be deposited during a basal surge eruption. Most lapilli tuffs which remain in ancient terrains are formed by the accumulation and welding of semi-molten lapilli into what is known as a welded tuff.
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An ash flow from this explosion was recognised 2,500 km west of the volcano. The total area of recognisable pyroclastic fall was greater than 800,000 km 2. The pyroclastic ash encircled the globe in 13.5 days and at altitudes of between 30 and 50 km the average velocity was 12 km/h. The ash remained in the upper atmosphere and produced ...