The ground layer of the Campanian Ignimbrite: an example of deposition from a dilute pyroclastic density current

Abstract

A thin, fines-poor, and lithic- and crystal-rich layer\ud locally present, from proximal to distal areas, at the base of the\ud pyroclastic density current deposits of the Campanian\ud Ignimbrite eruption (39 ka), sourced from Campi Flegrei\ud (Italy), is interpreted as a ground layer (GL). It has an average\ud lithic and crystal content of 57 and 25 wt%, respectively. The\ud GL rests on a paleosol or an early fall deposit and is capped by\ud a stratified ash deposit. It is bounded by erosion surfaces and\ud its thickness does not change systematically from the source.\ud The occurrences of GL in places separated from the source by\ud a stretch of sea and the angular to sub-angular shape of the\ud lithic clasts are consistent with deposition from a dilute current.\ud The presence of the GL up to 971 m above sea level and\ud beyond 1400–1500-m-high mountain ridges, allows estimation\ud of the thickness of the PDC as having been at least\ud 1.5 km. To investigate the parameters influencing the transport\ud and emplacement mechanisms that produced the GL, we use\ud field and sedimentological data (grain size and componentry),\ud from samples collected between 30 and 70 km from the\ud source. The progressive decrease in maximum lithic clast\ud and median grain-size with distance from the vent and increasing\ud altitude corresponds to lateral and vertical grading of lithic\ud clasts within the current. The coarser clasts moved toward the\ud basal part of the current, settling in more proximal areas and at\ud low altitude. Farther from the vent (45–50 km), the lower\ud part of the pyroclastic current impacted against the reliefs\ud surrounding the Campanian Plain, causing the emplacement\ud of coarser and more poorly sorted deposits on slopes oriented\ud toward the source

A chemostratigraphic study of the Campanian Ignimbrite eruption (Campi Flegrei, Italy): Insights on magma chamber withdrawal and deposit accumulation as revealed by compositionally zoned stratigraphic and facies framework

“…It was accompanied by a huge volcano-tectonic collapse, giving birth to the Campi Flegrei caldera (CFc; Rosi and Sbrana, 1987;Perrotta et al, 2006;Scarpati et al, 2013). The CI plinian eruption emplaced a large volume of pyroclastic fall and pyroclastic density current products, which resulted in a very complex sequence in proximal, medial and distal outcrops [e.g., Barberi et al (1978);Di Girolamo et al (1984); Rosi and Sbrana (1987);Fisher et al (1993); Scarpati (1994, 2003); Orsi et al (1996); Rosi et al (1996Rosi et al ( , 1999; Cappelletti et al (2003); Perrotta et al (2006); Fedele et al (2008); Perrotta (2012, 2016); Engwell et al (2014); Scarpati et al (2015aScarpati et al ( , 2015b; Sparice (2015)]. …”

“…The CI proximal deposits can be observed in limited exposures along the CFc rim, directly overlying the PPF. The entire sequence, up to 70 m thick, makes up the Breccia Museo formation (BM), consisting of 6 well-defined units [following Fedele et al (2008), from base to top]: 1) the Unconsolidated Stratified Ash Flow (USAF), a stratified and unconsolidated ash to sandy deposit (recognizable up to medial exposures) containing fine pumice lapilli, with a laterally discontinuous lithic-and crystal-rich basal layer (ground layer; Scarpati et al, 2015b); 2) the Piperno, a high-grade tuff consisting of alternating beds of welded ash with flattened scoria clasts (fiamme) and a monolithologic coarse breccia made of lava fragments; 3) the Lower Pumice Flow Unit, a poorly sorted pumice-rich deposit; 4) the Breccia Unit, a crudely stratified coarse lithic-rich breccia with sporadic pumice, spatter and obsidian clasts; 5) the Spatter Unit, a welded spatter deposit interbedded in the basal and middle parts of the Breccia Unit; 6) the Upper Pumice Flow Unit, an unlithified poorly sorted deposit showing a faint normal grading of pumice and lithic clasts and abundant degassing structures.…”

The Magnitude of the 39.8 ka Campanian Ignimbrite Eruption, Italy: Method, Uncertainties and Errors

“…The CI eruption emplaced both pyroclastic fall and PDC deposits in a complex sequence currently exposed in proximal, sporadic medial, distal and ultra-distal outcrops ( Figure 1) (Barberi et al, 1978;Rosi et al, 1988;Fisher et al, 1993;Perrotta and Scarpati, 1994;Orsi et al, 1996;Rosi et al, 1996;Rosi et al, 1999;De Vivo et al, 2001;Cappelletti et al, 2003;Perrotta and Scarpati 2003;Perrotta et al, 2006;Fedele et al, 2008;Engwell et al, 2014;Scarpati et al, 2015a, Scarpati et al, 2015bSparice, 2015;Scarpati and Perrotta, 2016;Smith et al, 2016). The first phase of the eruption generated Plinian columns up to 44 km high (Rosi et al, 1999;Marti et al, 2016), producing a widespread fall deposit dispersed by winds to the east (Rosi et al, 1999;Perrotta and Scarpati, 2003;Marti et al, 2016;Scarpati and Perrotta, 2016).…”

Anticipating future Volcanic Explosivity Index (VEI) 7 eruptions and their chilling impacts

“…Pyroclastic flows from VEI 7 eruptions can travel 20, 30, or even 100 km in any or all directions from the volcano. Pyroclastic flows from Kikai traveled as far as 100 km, across water and land, those from the Taupo 1.8 ka eruption traveled at least 80 km, and those from the 39 ka Campanian eruption of Campi Flegrei reached distances of at least 70 km through what is now a densely populated urban area (Scarpati et al, 2015). Any town or city in the path would be destroyed, and death tolls could reach millions unless mass evacuations had been made.…”