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 fine 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 framework-supported 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 field 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 fine ash as they circulated through the variously turbulent levels of the stratified current, and heat of the lower part of the current dried and partly lithified 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 field 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.