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

Abstract

The Granadilla Member is one of the most widely dispersed and largest volume pyroclastic units at Tenerife (Canary Islands) and represents the culminating eruption to a second cycle of explosive volcanism of the Las Cañadas edifice. The member, dated at 0.57Ma, comprises a plinian fall deposit, the Granadilla pumice, which is overlain by ignimbrite up to 30m thick. The Granadilla pumice is up to 9m thick approximately 10km from source (Pyle bt value is 5.35km), and is subdivided into four fall units. Unit 1 is up to 1.2m thick and is further divisible into another four pumice fall subunits, based on bedding and grainsize differences. Unit 2 is a thin but distinctive ash layer (∼2cm thick), and its wide dispersal (>550km2), constant thickness, planar laminations and ash aggregate textures collectively indicate a phreatoplinian fall origin. The lithic-rich nature and abundance of unaltered lithic fragments reflect magma interaction with aquifer-derived water at depth. Unit 3 (≤1.8m thick), records a reversal to dry plinian eruptive activity. Unit 4, the thickest of the fall units (up to 6.3m thick), records the maximum dispersal and intensity of the eruption (Pyle bt and bc values are 5.7 and 6.3km, respectively), best illustrated by the presence of large pumice bombs up to 30cm diameter (at distances up to 20km from vent), and reverse grading of lithic and pumice clasts. The widespread (>500km2), nonwelded and pumice-rich Granadilla ignimbrite (unit 5) records the collapse of the plinian eruption column. The ignimbrite has a simple sheet-like geometry, but exhibits a complex internal stratigraphy. The base of the ignimbrite locally cuts down through the underlying Granadilla pumice removing it entirely, indicating up to 9m of erosion by the pyroclastic flows. A coarse, vent-derived lithic breccia horizon towards the top of the ignimbrite is interpreted to record the onset of caldera collapse late in the eruption.Minimum volume estimates for the Granadilla pumice and ignimbrite are 5.2 and 5km3, 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 of a moderately high (∼17 to ≥25km) plinian column. We propose that the eruption involved two vents, probably aligned along a NE–SW fissure within the Las Cañadas caldera.