Nuees ardentes were first described by Lacroix in connection with the 1902 Mt Pelee eruption, these comprise a mixture of everything from fine ash to massive blocks. They are associated with Dome collapse – the instantaneous collapse of a developing lava dome in a vent, dome development being usually associated with the more acidic lavas, also these have a high volatile content and are thus have the potential for highly explosive eruptions. The avalanche comprises two parts, the lower incandescent rock fall and the dust cloud rising above; in general the density of the fall tends to confine the avalanche to valleys, but can extend for many miles from the source.
Eruption column collapse
Eruption column collapse is usually the result of the gravity collapse of a column where the density of the column becomes greater than the explosive strength of the eruption and the thermal convection lift effect. It was a combination of collapse of the Plinian eruption column (named in honour of Pliny the Younger who first described the appearance of the column in the AD79 Vesuvius eruption – as looking like a Stone Pine) and heavy ash fall that buried Pompeii and Herculaneum, many of the remains showing evidence of high-heating.
Lateral base surge or pyroclastic surges are formed when an eruption releases large volumes of pressurised high gas content material. these have a high gas:solids ratio and are highly turbulent enabling them to cross all terrains, (unlike nuees confined to valleys). Temperatures range up to 1000°C (hot surge); interaction of magma with water may form cold surges with temperatures around 100°C…
Unwelded ignimbrite, Argentina — This photo is a typical example of an unwelded ignimbrite. It consists of fist-sized dacitic pumice fragments and small, sparse lithic fragments in a fine-grained matrix of dacitic lapillia and ash. This example is from the 4.6 million year old Real Grande ignimbrite which has a volume of over 55 km3.
The picture below is from Santorini – see Carl’s post ‘Santorini – the end of civilisation’ and is taken from http://volcano.si.edu/world/volcano.cfm?vnum=0102-04=&volpage=photos&photo=001008
This spectacular outcrop shows light-colored deposits from the 3500-year-old Minoan eruption of Santorini volcano filling a valley cut in darker, bedded ashfall layers of Pleistocene age. The lower, beige-colored unit filling the ancient valley is a pumice-fall deposit from vertical explosions early in the eruption. It is overlain by laminated pyroclastic-surge deposits produced when water gained access to the magma reservoir as the volcano collapsed into the sea. The upper whitish layer truncating both these deposits is a pyroclastic-flow deposit.
The pictures of ignimbrites above show unwelded tuffs where the rock fragments are relatively poorly cemented (in recent deposits, older deposits that have undergone diagenesis will obviously be compacted and ‘solid’ but still retain each fragment’s form) Welded tuffs are those formed where the pyroclastic flow retained sufficient heat for the fragments, particularly those of vessicular pumice, to partially melt and collapse; experimentally, temperatures of c600°C are sufficient to produce collapse. These collapsed and elongaed pumice fragments are Fiamme (welded tuffs give a Eutaxitic texture to the rock) and are characteristic of welded tuffs of all geological ages.
Welded tuff with Fiamme – in centre – in thin section. From http://geology.isu.edu/geostac/Field_Exercise/Cassia_mtns/thinsect.html
Whilst most ignimbrites are confined to the area around the source, occasional very large eruptions yield enormous amounts of ignimbrite. The June 1912 eruption of Katmai and Novarupta in Alaska, the most powerful eruption of the 20thC produced an ignimbrite ‘sheet’ up to 200m thick near the source and 120sq km in extent. Miocene eruptions in central Turkey produced similar large deposits, some may be associated here with caldera collapse.
The yellow line delineates the extent of the ignimbrite deposits from http://geology.com/novarupta/
You never know what’s behind you!!