Transport and deposition of pyroclastic material from the ∼1000 A.D. caldera-forming eruption of Volcán Ceboruco, Nayarit, Mexico

• Published in: Bulletin of volcanology 2005-06, Vol.67 (5), p.469-489
• Main Authors: Browne, B. L., Gardner, J. E.
• Format: Article
• Language: English
• Subjects: Calderas; Density currents; Escarpments; Flow; Geology; Lithology; Topography; Volcanoes; Volcanology
• ISSN: 0258-8900; 1432-0819
• DOI: 10.1007/s00445-004-0390-6

The complex eruption sequence from the 1000 A.D. caldera-forming eruption of Volcán Ceboruco, known as the Jala Pumice, offers an exceptional opportunity to examine how pyroclastic material is transported and deposited from pyroclastic density currents over variable topography. Three main pyroclastic surge deposits (S1, S2, and S3) and two pyroclastic flow deposits (Marquesado and North-Flank PFDs) were emplaced during this eruption. Pyroclastic surge deposits are massive, planar, or cross-bedded, poor-to-well sorted, and display fluctuations in thickness, median diameter, sorting, and lithology as a function of distance, topography, and flow dynamics. Marquesado pyroclastic flow deposits reveal lateral variations from massive, poorly sorted deposits located within 5 km of Ceboruco to planar bedded, moderately well sorted deposits located >15 km away over the nearly horizontal topography to the south of Ceboruco. North-Flank pyroclastic flow deposits also reveal lateral variations from massive, poorly sorted deposits located within 4 km of Ceboruco to planar bedded, moderately well sorted deposits located 8 km away atop an escarpment that steeply rises 230 m from the northern valley floor. Field observations, granulometric analyses, component analyses, and crystal sedimentation calculations along flow-parallel sampling transects all suggest that both surges and flows were density stratified currents, where deposition occurred from a basal region of higher particle concentration that was supplied from an overlying dilute layer that transports particles in suspension. This supports the idea of a transition between "flow" and "surge" end members with variations in particle concentration. Topography greatly affects the transport and depositional capacity of the pyroclastic density currents as a result of "blocking", either by topographic obstacles or by abrupt breaks at the base of volcano slopes, whereas the origin of Jala Pumice surge deposits (phreatomagmatic versus magmatic) appears to have little impact on their flow dynamics.[PUBLICATION ABSTRACT]