Category: Pyroclastic

Type Ignimbrite

Commons Lapilli tuff is a consolidated pyroclastic rock containing more than 75% lapilli. Lapilli are pyroclasts ranging from 2 to 64 mm in size, formed by the fragmentation of surrounding material during a volcanic eruption. They may consist of individual crystals, crystal aggregates, volcanic glass, or rock fragments. The finest-grained pyroclasts are ash grains, with particle sizes smaller than 2 mm. In addition to ash, tuff may contain coarser pyroclastic material, such as volcanic bombs—rounded pyroclasts larger than 64 mm with a "bread-crust" surface, which were partially or completely molten during formation and transport—and blocks—angular pyroclasts larger than 64 mm that were solid at the time of formation. Lapilli tuff can form through the fallout of pyroclasts during an eruption or through deposition from pyroclastic flows. Ignimbrite is the genetic term for deposits resulting from ash-pumice pyroclastic flows. Depending on the temperature at the time of deposition, ignimbrites can be classified as welded or non-welded. Welded ignimbrites are characterized by the presence of flattened “fiamme”- typically deformed pumice fragments that underwent plastic deformation due to heat and the weight of overlying pyroclastic material. The lapilli tuff from Ipolytarnóc is part of the massive tuff complexes found in the Pannonian Basin region. It originates from a 10–40 m thick stratified sequence of ignimbrites, referred to in older literature as the Gyulakeszi Rhyolite Tuff Formation or the "Lower Rhyolite Tuff" (Lukács et al., 2018; Pálfy et al., 2007). According to the latest lithostratigraphic classification, the Gyulakeszi Rhyolite Formation was renamed to Tihamér Rhyolite Lapilli Tuff Formation (Lukács et al., 2022). Its spatial extent reaches into Slovakia, where lapilli tuffs occur within the Bukovina Formation (Vass et al., 2002). The best-known localities are Mučín and Lipovany, which were dated by Šarinová et al. (2021). The ignimbrites at the Ipolytarnóc site are products of explosive silicic volcanism dating to the Miocene (17.4–17.2 Ma) (Karátson et al., 2022; Pálfy et al., 2007; Lukács et al., 2018). They overlie sandstones in which fossilized tracks of vertebrates and birds were discovered in 1900. The site is renowned for these paleontological finds, which were preserved on an ancient riverbank thanks to a volcanic eruption that instantly blanketed the sand with a thick layer of ignimbrite. A total of 3,000 fossil tracks representing 11 animal species have been documented there (Kordos, 1985). The lapilli tuffs originated from the Eger volcanic center in present-day Hungary; this eruption generated multiple pyroclastic flows, at least three of which reached the Ipolytarnóc area. Classified as a Plinian eruption with a Volcanic Explosivity Index (VEI) greater than 7 (on a scale where the maximum is 8), the event at the Eger volcanic center resembled the eruption of Vesuvius in 79 AD (Karátson et al., 2022). However, the Vesuvius eruption had a slightly lower VEI, in the range of 5–6. The temperature of a pyroclastic flow decreases during transport; in Pompeii and Herculaneum, located near Vesuvius, many people perished beneath hot pyroclastic flows. The Eger volcanic center, considered the source of the material found at Ipolytarnóc, is currently located 75 km away from the site. By the time the pyroclastic flows reached present-day Ipolytarnóc, they had already cooled. Consequently, intact, uncharred tree trunks and branches, leaves (Hably, 1985), and animal tracks (Kordos, 1985) have been preserved at the site. It is estimated that the eruption covered an area of 1,650 km² and produced a total ignimbrite volume of 99 km³, corresponding to 58 km³ of magma (Karátson et al., 2022). Recent historical eruptions with a VEI of 7 include the Taupo eruption in New Zealand in 232 AD (30 km³ of magma) and the Late Bronze Age Minoan eruption of Santorini (82 km³ of magma). Today, Ipolytarnóc is rightly regarded as an "ancient Pompeii"; it is a protected natural area and part of the Novohrad-Gemer Geopark.

Name origin The name "tuff" originates from the Italian word tufo and the French word tuf. The term tuff was first defined by Phillips in 1818 (Phillips, 1815). The word "lapilli" derives from the Latin “lapillus”, meaning "small stone" (Lyell, 1835). The term "ignimbrite" was coined by Marshall (1932) and is a compound of two Latin words: “ignis” (fire) and” imber (shower).

Locality Ipeľský Trnovec (Hungarian Ipolytarnóc), Novohrad County, Hungary (sample from the V. Konečný archive)

GPS: 48° 14' 12" N, 19° 39' 25" E

Major minerals The pyroclasts consist of glass fragments (pumice and glass shards), minerals -crystaloclasts (quartz, plagioclase - An_30–50, biotite) and rock fragments - lithoclasts of both volcanic origin (vitrophyric lithoclasts and recrystallized glass) and non-volcanic origin (siltstones, mudstones, and claystones). The pyroclasts are cemented by fine volcanic ash composed of glass fragments, which are often altered to clay minerals.

Accessory minerals Apatite, zircon, allanite, ilmenite.

Classification Classification of lapilli tuff is based on modal composition within the polymodal classification scheme for pyroclastic rocks defined by the content of blocks/bombs, lapilli, and ash (Fisher, 1966). Lapilli tuff from Ipolytarnóc is plotting into the field characterized by an ash and lapilli content less than 75 vol. % and a block/bomb content less than 25 vol. %. Giordano & Cas (2021) published a new classification of ignimbrites based on the volume of ash-pumice flow material and their areal extent in the field.