SPECIAL Tectonic influences in piecemeal caldera collapse at Glencoe Volcano, Scotland

Glencoe Volcano is renowned as the archetype for caldera collapse that results from piston-like subsidence of a coherent crustal block on a ring fault. However, the caldera-floor rocks, and the geometry of seven intracaldera units that record large volume explosive eruptions, show that the collapse involved incremental and haphazard subsidence of numerous crustal blocks before development of the ring fault. Some subsidence involved flexure (downsag) with associated development of extensional crevasses hundreds of metres deep. A system of orthogonal faults and related grabens records the influence of two intersecting basement discontinuities, one of which is inferred to have linked with the active Great Glen fault. Substantial extensional and/or transtensional faulting, with distinct related sedimentary responses, occurred without eruptions. The tectonic framework substantially controlled the magmatic plumbing and the locations of vents, caldera depocentres, and through-going rivers. The related sediments influenced contrasting eruption styles (e.g. transitions between phreatomagmatic and magmatic) and the shallow-level emplacement of sills