A proximal record of caldera-forming eruptions: the stratigraphy, eruptive history and collapse of the Palaeogene Arran caldera, western Scotland

Abstract

Caldera-forming volcanic eruptions are among the most dangerous, and can generate extensive pyroclastic deposits and deliver ash into global atmospheric circulation systems. As calderas collapse, the eruptions can deposit thick proximal ignimbrite sequences and thinner ignimbrites more distally. However, the proximal record of caldera collapse is often obscured by later intrusions, volcanism, faults, alteration, water and sediments, which significantly limits our understanding of these eruptions. A Palaeogene caldera system in central Arran, western Scotland, preserves a rare proximal caldera-fill succession, the Arran Volcanic Formation. This caldera largely comprises highly heterogeneous ignimbrites and minor intra-caldera sedimentary rocks. The current level of erosion, and the general absence of faults, intrusions and sediments, allows a complex stratigraphy and collapse history to be determined, which can be linked to changing eruptive styles at a constantly evolving volcano. The first recorded phase was eruption of a homogeneous rhyolitic lava-like tuff, deposited from high temperature, high mass-flux pyroclastic density currents generated from low fountaining columns that retained heat. A succeeding phase of highly explosive Plinian eruptions, marked by a thick blanket of massive lapilli tuffs, was then followed by piston-like caldera collapse and erosion of steep caldera walls. Volcanism then became generally less explosive, with predominantly lava-like and eutaxitic tuffs and cognate spatter-rich agglomerates interbedded with non-homogenous lapilli tuffs. High topographic relief between distinct units indicate long periods of volcanic quiescence, during which erosive processes dominated. These periods are, in several places, marked by sedimentary rocks and evidence for surface water, which includes a localised basaltic-andesitic phreatomagmatic tuff. The caldera-forming eruptions recorded by the Arran Volcanic Formation provide an important insight into caldera collapse processes and proximal ignimbrite successions. The lack of thick autobreccias and lithic-rich lapilli-and block-layers indicates that subsidence was relatively gradual and incremental in this caldera, and not accompanied by catastrophic wall collapse during eruption. The relatively horizontal nature of the caldera-fill units and paucity of intra-caldera faulting indicate that piston subsidence was the dominant method of collapse, with a relatively coherent caldera floor bounded by a steeply dipping ring fault. Possible resurgence may have caused later doming of the floor and radial distribution of subsequent ignimbrites and sedimentary rocks. Our work emphasises the continued need for field studies of caldera volcanoes.

Catastrophic caldera-forming eruptions and climate perturbations: the result of tectonic and magmatic controls on the Paleocene-Eocene Kilchrist Caldera, Isle of Skye, NW Scotland

“…Whilst much BPIP research has concentrated on the gabbroic and granitic central complexes (Figure 1B-D) together with the basaltic lava fields [Bell 1966;1976;Bell and Williamson 1994;Williamson and Bell 1994;Goulty et al 1996], less attention has been paid to silicic volcanic rocks. However, detailed mapping, logging, and re-interpretation of silicic volcanic sequences has recently been conducted on Eigg [Brown and Bell 2013], Skye [Drake and Beard 2012], Arran [Gooday et al 2018],…”

Stratigraphy and eruptive history of Gedemsa caldera volcano, Central Main Ethiopian Rift

“…The evolution of silicic calderas is generally characterized by three major steps: (1) regional uplift and formation of ring fractures, (2) large-volume volcanic eruption and subsequent collapse, and (3) post-caldera eruption or intra-caldera products and/or resurgent doming (Newhall and Dzurisin, 1988;Acocella et al, 2000;. The typical silicic caldera-forming eruptions often result in the deposition of a combination of extensive ignimbrite sheets and widespread pumice and ash fall deposits (Self and Rampino 1981;Self et al 1984;Newhall and Dzurisin 1988;Hildreth and Fierstein 2012;Gooday et al, 2018). Different processes of formation, morphology and structure and caldera types (piston, piecemeal, and funnel types) have been proposed at different calderas (e.g.…”

Geological, geophysical, and geochemical constraints on the time-space evolution of Akan composite caldera, Hokkaido, Japan

“…Processes of caldera formation from explosive silicic volcanism have been well studied because of their association with large-scale and widely distributed ignimbrites, vigorous geothermal activity, mineralization, and potential to cause catastrophic damage to society on regional to global scales (Maeno and Taniguchi, 2007;Geyer and Marti, 2014;Gooday et al, 2018;Takarada and Hoshizumi, 2020). Since the documentation of classification schemes for caldera formation under piston, piecemeal, and funnel types by Williams (1941), Williams and McBirney (1979), and Lipman (1997), many calderas have been investigated and assigned according to these categories.…”