Quantitative characterisation of a blocky peperite : 2D image analysis of clast shape and distribution in an andesite sill at Honister, Cumbria

Robinson, Aidan and Tuffen, Hugh and Jones, Thomas (2026) Quantitative characterisation of a blocky peperite : 2D image analysis of clast shape and distribution in an andesite sill at Honister, Cumbria. Masters thesis, Lancaster University.

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Abstract

Peperites record complex interactions between magma and wet sediments that are fundamental to hydromagmatic volcanism. However, interpretations of these processes have historically been limited by a reliance on qualitative field descriptions. We present the first quantification of clast shape and distribution in a peperite exposure, applying a novel morphometric approach to a well-exposed upper margin of a peperitic andesite sill at Honister, Cumbria, England. Here, blocky peperite formed where magma intruded and mingled with a wet and unconsolidated volcaniclastic sediment during the Ordovician volcanism that formed the Borrowdale Volcanic Group. We applied Structure-from-Motion photogrammetry to create a high-resolution 2D image of the exposure, enabling outcrop-scale quantification of clast characteristics through image analysis. Based on our field observations and a complementary dataset of clast morphological and textural characteristics, we identify four syn-magmatic processes governing peperite formation at Hopper Quarry: (1) shallow emplacement of poorly vesicular magma into wet, unconsolidated sediment, with magma propagation accommodated by non-brittle processes and the formation of a vapour film at the interface, as evidenced by fluidisation cross-cutting undisturbed sediment; (2) localised vesiculation of the sill margin driven by a volatile saturation gradient at the magma-wet sediment interface, recorded by heterogenous clast vesicularity unrelated to clast dimensions; (3) brittle cooling-contraction granulation of the sill margin, forming an initial population of closelypacked blocky, in situ clasts characterised by angular margins and jigsaw-fit arrangements, with fine fragmentation driven by localised instabilities of the vapour film; (4) clast dispersion driven by fluidisation of the host sediment and bulk magma-sediment density contrasts, producing a transition from framework-supported to matrix-supported clast domains with distance from the sill. A quantitative baseline for blocky peperite clast shape and distribution has been established here, providing abundant scope for comparison with a broader suite of peperites and blocky lavas.