Detailed seismic reflection mapping 100 km N of Shetland has revealed a large subcrop of Palaeocene basalts proved by exploration drilling. The geometry of internal reflectors within the buried basalt suggests the existence of two partially eroded volcanoes. A vent of about 2 km in diameter, surrounded by radially outward-dipping basalts, is centred over the Erlend igneous complex, whose existence had been postulated previously on gravity and magnetic evidence alone. A new volcano, the more deeply eroded West Erlend centre, is postulated by interpretation of seismic data and by gravity modelling. An approximately cylindrical mafic or ultramafic pluton is inferred from the gravity modelling to underlie each of the two centres; the diameter of the Erlend pluton is about 14 km and that of West Erlend about 5–7 km. The seismic stratigraphy of volcanic rocks can now be recognized as a valid new field of study with particular relevance to hydrocarbon exploration.

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