The 0.57 Ma plinian eruption of the Granadilla Member, Tenerife (Canary Islands): an example of complexity in eruption dynamics and evolution

The Granadilla Member is one of the most widely dispersed and largest volum e pyroclastic units at Tenerife (Canary Islands) and represents the culmina ting eruption to a second cycle of explosive volcanism of the Las Canadas e difice. The member, dared at 0.57 Ma, comprises a plinian fall deposit. the Granadilla pumice, which is overlain by ignimbrite up to 30 m thick. The G ranadilla pumice is up to 9 m thick approximately 10 km from sourer (Pyle b (t) value is 5.35 km), and is subdivided into four Fail units. Unit 1 is up to 1.2 m thick and is further divisible into another four pumice fall subu nits, based on bedding and grainsize differences. Unit 2 is a thin but dist inctive ash layer (similar to2 cm thick), and its wide dispersal (>550 km(2 )), constant thickness. planar laminations and ash aggregate textures colle ctively indicate a phreatoplinian fall origin. The lithic-rich nature and a bundance of unaltered lithic fragments reflect magma interaction with aquif er-derived water at depth. Unit 3 (less than or equal to1.8 m thick), recor ds a reversal to dry plinian eruptive activity. Unit 4, the thickest of the fall units (up to 6.3 m thick), records the maximum dispersal and intensit y of the eruption (Pyle b(t) and b(c) values are 5.7 and 6.3 km, respective ly), best illustrated by the presence of large pumice bombs up to 30 cm dia meter (at distances up to 20 km from vent). and reverse grading of lithic a nd pumice clasts. The widespread (>500 km(2)), nonwelded and pumice-rich Gr anadilla ignimbrite (unit 5) records the collapse of the plinian eruption c olumn. The ignimbrite has a simple sheet-like geometry. but exhibits a comp lex internal stratigraphy. The base of the ignimbrite locally cuts down thr ough the underlying Granadilla pumice removing it entirely, indicating up t o 9 m of erosion by the pyroclastic flows. A coarse, vent-derived lithic br eccia horizon towards the top of the ignimbrite is interpreted to record th e onset of caldera collapse late in the eruption. Minimum volume estimates for the Granadilla pumice and ignimbrite are 5.2 a nd 5 km(3), respectively. The dispersal area, deposit characteristics. and exponential thickness and clast size decay relationships with (isopach area )(1/2) are consistent with dispersal and fallout from the umbrella region o f a moderately high (similar to 17 to greater than or equal to 25 km) plini an column. We propose that the eruption involved two vents, probably aligne d along a NE-SW fissure within the Las Canadas caldera. (C) 2000 Elsevier S cience B.V. All rights reserved.