Large Silicic Eruptions in the BPIP
The origin of the Palaeocene Sgurr of Eigg pitchstone has long been a matter of debate. The pitchstone dominates the south of the island, and was initially interpreted as a valley-constrained lava flow by Geikie back in 1871. Later, in 1906, Harker considered the pitchstone to be intrusive, but the lava flow theory gained traction again in 1914, when Bailey supported Geikie’s original interpretation. Allwright subsequently reinterpreted the basal portion of the pitchstone as being a welded tuff and the upper, columnar jointed portion as being intrusive. However, a more recent investigation by Brown and Bell in 2013, suggests that the entire structure is of pyroclastic origin, and these authors tentatively suggested Skye as the source of the activity.
This year a new study, published in Nature Scientific Reports by Troll and co-workers, pulls together new petrographic, mineralogical and geochemical data along with all the previously published data, and finally pulls together a coherent story. Their work on the Sgurr of Eigg pitchstone and pitchstone found on the Òigh-sgeir skerries some 30km away, show that the two outcrops display identical mineral textures and compositions, major- and trace elements, and Sr-Nd-Pb-O isotope ratios, implying that they represent the remnants of a single, formally extensive, pyroclastic deposit. Isotopic data and the available radiometric ages, also suggest a source for the eruption some 40km further north on the Isle of Skye. The estimated eruption volume for the deposit is some 5km3 DRE, suggesting an event with a volcanic explosivity index (VEI) in excess of 5. The implications of these estimates are that large volume silicic explosive activity in the BPIP, long considered to be of little importance, were in fact an integral part of the activity during the opening of the North Atlantic.
This study is a fascinating read, and is available on open-access on the Nature website:
A large explosive silicic eruption in the British Palaeogene Igneous Province
Allwright, E. A.1980. The stucture and Petrology of the volcanic rocks of Eigg, Muck and Canna, N. W. Scotland (Doctoral dissertation, Durham University).
Bailey, E. B. 1914. II.—The Sgùrr of Eigg. Geol. Mag. 1, 296–305.
Brown, D. J. & Bell, B. R. 2013. The emplacement of a large, chemically zoned, rheomorphic, lava-like ignimbrite: the Sgurr of Eigg Pitchstone, NW Scotland. J. Geol. Soc. London. 170, 753–767
Geikie, A. 1871. On the Tertiary volcanic rocks of the British Islands. Q. J. Geol. Soc. London 27, 279–310.
Harker, A. 1906. The geological structure of the Sgùrr of Eigg. Q. J. Geol. Soc. London 62, 40–67.
Troll, V. R., Emeleus, C. H., Nicoll, G. R., Mattsson, T., Ellam, R. M., Donaldson, C. H. & Harris, C. 2019. A large explosive silicic eruption in the British Palaeogene Igneous Province. Nature Scientific Reports 9, 494