Jack Whattam

• A reappraisal of explosive–effusive silicic eruption dynamics: syn-eruptive assembly of lava from the products of cryptic fragmentation

• Silicic conduits as supersized tuffisites: Clastogenic influences on shifting eruption styles at Cordón Caulle volcano (Chile)

• Degassing-induced chemical heterogeneity at the 2011-2012 Cordón Caulle eruption

• Whattam, Jack William; Midtkandal, Ivar; Jerram, Dougal Alexander; Callegaro, Sara & Svensen, Henrik Hovland (2024). The episodic onset of explosive and silicic-dominated volcanism in a continental rift; insights from the Permian Oslo Rift, Norway. Volcanica. 7(2), p. 925–951. doi: 10.30909/vol.07.02.925951. Full text in Research Archive Show summary

  The nature of rifting episodes and their associated volcanism mark key stratigraphic events in the evolution of volcano-sedimentary basins. Following the final assemblage of Pangea, the European region was subject to rifting and magmatism during crustal re-equilibration throughout the Permian. The aborted, partially eroded Oslo Rift is an excellent archive of this Permian magmatism, but the late-stage volcanological evolution is poorly understood. We present the first detailed documentation of a succession covering this period, with mafic lavas and volcaniclastic (tuff breccia) deposits, trachy-andesitic tuffs, and rheomorphic to moderately welded ignimbrites. Contrary to previous ideas, we show evidence suggesting the development of Strombolian-type scoria-cones, silicic fissure-fed eruptions, and major periods of rift-wide volcanic quiescence. Our observations highlight episodic silicic volcanism characterised by rapid evolution from high-grade ignimbrites to moderately and poorly welded ignimbrites. We infer rapid emptying of large, shallow silicic reservoirs and frequent source switching is responsible for the observed characteristics.

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• Andersen, Gard Lilledal; Svensen, Henrik Hovland; Deegan, Frances M.; Callegaro, Sara; Whattam, Jack William & Jeon, Heejin [Show all 8 contributors for this article] (2026). Small crystals, big eruptions: Quartz phenocrysts as archives of caldera-forming processes in the Permian Drammen Caldera, Oslo Rift, Norway. Full text in Research Archive Show summary

  Caldera-forming eruptions are among the most dangerous and catastrophic volcanic phenomena, yet the related magma reservoir processes are poorly understood. In the Permo-Carboniferous Oslo Rift in Southeastern Norway, there are at least 15 calderas, several of which have preserved pyroclastic deposits. Here we focus on a >450 m thick rhyolitic ignimbrite succession in the 7 x 7 km Drammen Caldera, located 40 km southwest of Oslo. We present field observations, petrography, and geochemical analyses, with an emphasis on primary quartz phenocrysts. Ignimbrite stratigraphy in the Drammen Caldera comprises the high-grade, phenocryst-rich Lian Formation (25–30% crystals) and the moderate- to low-grade, phenocryst-poor Sørumlia Formation (1–2% crystals). The Lian Formation records a large, sustained pyroclastic eruption that tapped a crystal-rich part of the reservoir, whereas the Sørumlia Formation was deposited over several smaller, highly explosive eruptions sourced from crystal-poor magma batches. In both formations, primary quartz phenocrysts occur as euhedral, rounded, and embayed crystals, and range in size from 50 to 2000 μm. Cathodoluminescence (CL) imaging of primary quartz phenocrysts reveals widespread oscillatory zoning, and several crystals show a distinct core with resorption margin, indicating changing magma reservoir conditions during crystal growth. To investigate the origin of the zoning, we conducted in situ oxygen and silicon isotope analyses. The primary quartz has an average δ18O value of 8.03 ± 0.23‰, suggesting assimilation of high-δ18O Lower Palaeozoic sedimentary rocks surrounding the caldera. As most intracrystal values are indistinguishable within the analytical uncertainty, we interpret assimilation to have occurred prior to quartz growth. Quartz phenocrysts yield an average δ30Si value of -0.08 ± 0.43‰. Like oxygen, silicon isotopes show no significant inter- or intracrystal variation, indicating isotopic homogeneity of the melt prior to the ignimbrite-forming eruptions. The combination of textural complexity and isotopic homogeneity in primary quartz phenocrysts provides new insights into felsic magma reservoir processes in the Oslo Rift.

• Andersen, Gard Lilledal; Svensen, Henrik Hovland; Augland, Lars Eivind; Callegaro, Sara; Whattam, Jack William & Deegan, Frances M. [Show all 8 contributors for this article] (2025). Unravelling the eruption history of the Drammen Caldera: A multidisciplinary study of caldera-forming processes in the Permian Oslo Rift, Norway. Full text in Research Archive Show summary

  During the mature stages of the Permo-Carboniferous Oslo Rift, at least 15 calderas formed. Thick ignimbrite deposits are preserved in several of these calderas but remain poorly studied with modern methods. This study focuses on a rhyolitic ignimbrite succession in the 7x7 km Drammen Caldera, located 40 km southwest of Oslo. Here, we present field observations, petrographic results, and geochemical analyses of the deposits. Despite significant glacial erosion and partial Quaternary coverage, we show that over 450 m of ignimbrite stratigraphy is preserved in the Drammen Caldera. In addition, we have identified ashfall and lahar deposits interbedded between ignimbrite units. The ignimbrites range from high- to low-welding grade, and we interpret them to record at least five explosive eruptions resulting in two caldera collapse events. The ignimbrites are altered but retain primary volcanic textures. Cathodoluminescence (CL) reveals complex zoning patterns in quartz phenocrysts, indicating multiple growth and dissolution stages. Phenocrysts are heavily fragmented, with fractures healed by quartz cementation. Oxygen isotopes measured in-situ in zoned quartz phenocrysts suggest assimilation of high-δ18O Lower Palaeozoic sedimentary rocks surrounding the caldera. Ongoing work on silicon isotopes and U-Pb geochronology aims to further refine the timing and petrogenetic processes. We conclude that the ignimbrites preserved in the Drammen Caldera hold information about complex magma reservoir dynamics, leading to several explosive eruptions. We present a six-stage evolution model for the caldera. This study highlights the potential for detailed multidisciplinary ignimbrite studies in other Oslo Rift calderas, as well as other eroded calderas globally.

• Svensen, Henrik Hovland; Callegaro, Sara; Kjøll, Hans Jørgen; Midtkandal, Ivar; Whattam, Jack William & Dalslåen, Bjørgunn Heggem [Show all 10 contributors for this article] (2024). Rhomb porphyry lavas from the Oslo Rift revisited: New insights from construction-related boreholes and cores. Full text in Research Archive

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