The Lake District’s tuff hailstones – accretionary lapilli AKA birds eye tuff, also known as pheasant‘s eye tuff, by Clive Boulter
Clive Boulter explained using animations and video clips that the bird’s eye tuff of the BVG may not necessarily have been formed in thunderstorms created by volcanic explosions. A classification proposes three types; pellets, coated pellets and accretionary lapilli. An elongated distortion of the pellets in the well-known ‘bird’s eye tuff’ in Kentmere has enabled a calculation of 50-60% shortening of the Earth’s crust in the ‘slate belt, where cleaved’, as plates converged in the period of Ordovician volcanic activity in the Lake District.
A 1820s painting based on Pliny’s account of the Vesuvius eruption of AD 79 was used in an animation to show how lapilli tuff forms in explosive volcanoes with lots of water; a Phreatoplinian type of eruption. Magma is shredded into ash in a strong thrust of gas from the volcanic vent. This rises in a column, eventually spreading and the particles drop, some as ash pellets. Accretionary lapilli may also form when part of the pyroclastic flow moving down the side of an explosive volcano lifts up into a ‘Phoenix cloud’. Ash pellets become coated as they sink into the flow, stick together and are lithified.
BVG vulcanicity began with the eruption of andesite lavas from fissures, which opened due to crustal extension, despite overall plate convergence, to build up as a plateau. Tens of cubic kilometres of water entered the magma chamber to create a Phreatoplinian eruption, with repeated pyroclastic eruptions and caldera collapse. The Scafell caldera was 20 x 20 km in size. Later a lava dome, caldera lake and tuff ring finished the vulcanicity. The volcanoes of BVG of the Lake District are interpreted now as low plateaux, rather than the steep-sided composite volcanoes which featured in earlier reconstructions.
Thanks to Sylvia Woodhead for this write-up of the 20th January event.
