Origin of accretionary lapilli within ground-hugging density currents: Evidence from pyroclastic couplets on Tenerife

Abstract

Aggregation of airborne particles is an important way in which the atmosphere is cleansed of fi ne dust particles, such as following explosive eruptions and meteorite impacts. We identify successive stages in the growth history of particle aggregates based upon well-preserved ash aggregate-bearing pyroclastic layers on Tenerife. The layers are persistently organized into couplets made up of a lower ignimbrite layer and an upper , widespread coignimbrite ash-fall layer. The upper part of each ignimbrite contains whole and fragmented concentric-laminated accretionary lapilli, whereas the overlying coignimbrite ash-fall layer lacks accretionary lapilli and is composed of frameworksupported smaller and nonlaminated ash pellets , sometimes slightly deformed or partly disaggregated. The pellets resemble the cores of the larger accretionary lapilli in the underlying ignimbrite layer. These fi eld relations are repeated numerous times in several different successions, and they indicate that ash pellets, not accretionary lapilli, form within the coignimbrite ash plumes. Some pellets fell directly to the ground, producing coignimbrite ash-fall layers, but others settled into pyroclastic density currents, where they accreted successive concentric laminations of fi ne ash as they circulated through the variously turbulent levels of the stratifi ed current, and heat of the lower part of the current dried and partly lithifi ed them into brittle accretionary lapilli. The fully formed whole and broken accretionary lapilli were then deposited from the current along with ash and pumice lapilli. Numerous ignimbrite veneers on Tenerife have the form of ash layers, a few centimeters thick, that drape topography and locally contain matrix-supported accretionary lapilli. Most volcanoes lack laterally continuous fi eld exposure, and such accretionary lapilli-bearing layers might be mistaken for ash-fall deposits. We highlight the value of careful distinction between different types of ash aggregate facies when interpreting the origin of pyroclastic deposits, for example , during hazard assessments.

A review of volcanic ash aggregation

“…3A). Broken fragments of AP2 aggregates are common alongside whole ones (Brown et al, 2010). Other types of ash aggregates in PDC deposits are less commonly reported (e.g., ash pellets and clusters).…”

“…Most occurrences are of AP2 aggregates (e.g., McPhie, 1986;Schumacher and Schmincke, 1991;Ui et al, 1992;Sohn, 1996;Baer et al, 1997;Colella and Hiscott, 1997;De Rita et al, 2002;Palladino et al, 2001;Scolamacchia et al, 2005;Branney et al, 2007;Brown et al, 2007;Andrews et al, 2008;Brown et al, 2010;Ellis and Branney, 2010). AP2 aggregates are commonly found within the matrices of PDC deposits or clastsupported in discontinuous layers or lenses within bedded PDC deposit (Fig.…”

“…The unstructured cores of the AP2 aggregates in the PDC deposits are closely similar to the AP1 aggregates in the overlying fall layer suggesting a genetic link. Brown et al (2010) concluded that the AP1 aggregates grew in buoyant co-PDC plumes during the passage of the PDCs and that these subsequently accreted layers-evolving into AP2 aggregates-as they passed through lower parts of the buoyant plume above a PDC.…”

“…Stratigraphic context of accretionary pellets (AP) in tephra fallout layers AP-bearing fall layers derived from eruption clouds or co-PDC clouds ('co-PDC deposit clouds') are common in the geological record and recent past (e.g., Self, 1983;Hayakawa, 1990;Schumacher and Schmincke, 1991;De Rita et al, 2002;Branney et al, 2007;Brown et al, 2010;Ritchie et al, 2002). Typically these deposits contain AP1 aggregates or, less commonly, AP2 aggregates (Table 1).…”

“…Later studies of aggregation processes brought to light the complexities of particle aggregation and the greater variety of particle aggregates from other types of eruptions, and also included more focus on laboratory study (e.g., Sorem 1982;James et al 2002;James et al 2003;). As a consequence, aggregation terminology has evolved and a number of different classification schemes have been proposed (Reimer, 1983;Schumacher and Schmincke, 1991;Thordarson, 2004;Brown et al, 2010). No classification scheme has yet been widely adopted and, over the past twenty years, the term 'accretionary lapilli' has been used to describe unstructured aggregates (e.g., Hayakawa, 1990;Rosi, 1992;Sisson, 1995;Watanabe, 1999;Trusdell et al, 2005), multiple concentric-laminated aggregates (e.g., Cole and Scarpati, 1993;JunqueiraBrod et al, 2005;Edgar et al, 2007), aggregates with a single fine-grained coating around a massive ash core (Branney, 1991) and ash-coated lithic clasts (Bednarz and Schmincke, 1990;Palladino et al, 2001).…”