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Origin of accretionary lapilli within ground-hugging density currents; evidence from pyroclastic couplets on Tenerife

R. J. Brown, M. J. Branney, C. Maher and P. Davila-Harris
Origin of accretionary lapilli within ground-hugging density currents; evidence from pyroclastic couplets on Tenerife
Geological Society of America Bulletin (January 2010) 122 (1-2): 305-320

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.


ISSN: 0016-7606
EISSN: 1943-2674
Coden: BUGMAF
Serial Title: Geological Society of America Bulletin
Serial Volume: 122
Serial Issue: 1-2
Title: Origin of accretionary lapilli within ground-hugging density currents; evidence from pyroclastic couplets on Tenerife
Affiliation: Open University, Volcano Dynamics Group, Milton Keynes, United Kingdom
Pages: 305-320
Published: 201001
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 88
Accession Number: 2010-015589
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2009050
Illustration Description: illus. incl. 2 tables, sketch map
N28°15'00" - N28°15'00", W16°34'60" - W16°34'60"
Secondary Affiliation: University of Leicester, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201010
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