Groups and Networks

NW Britain was the site of intense igneous activity during the Palaeocene and early Eocene (c. 64–52 Ma) which accompanied continental separation and lithospheric attenuation during the early stages of North Atlantic opening. Volcanism was most vigorous in the Inner Hebrides, NW Scotland mainland and Northern Ireland, but also extended to southern Scotland, NE England, Lundy in the Bristol Channell and the Outer Hebrides. Dykes of similar age in North Wales and the English Midlands are outlying representatives of the contemporaneous activity in tertiary igneous map. Scientific investigation over the last 200 years has made the British Tertiary Volcanic Province one of the most historically important and intensely studied igneous provinces in the world.

Large accumulations of basaltic lava flows cover most of Skye, Mull and NE Northern Ireland, with extensive swarms of basaltic dykes, most intense near Skye, Rhum, Mull, and Arran but extending to the Outer Hebrides, southern Scotland, north Yorkshire and parts of North Wales. Intrusive complexes may consist of granite, gabbro, peridotite and other rock types forming a linear chain from Skye to the Bristol Channel and also occur at several places in the NE Atlantic; these are the deeply-dissected roots of major volcanoes. Gabbro and peridotite have given rise to the rugged mountain scenery of St Kilda, Skye (Cuillins) and Rum. The less rugged, mountains of northern Arran and the Skye Red Hills are composed of granite, while the piles of essentially flat-lying lavas form tabular, ‘trap featured’ hills in northern Skye and much of Mull.

Volcanism extended over ~11 million years, largely within the Palaeocene Epoch. The life span of individual central intrusive complexes was short, of the order of 2 or 3 million years (or less) and the thick lava accumulations built up over shorter periods. Volcanic activity in each area often started with the formation of small amounts of volcaniclastic accumulations, that was rapidly followed by voluminous subaerial eruptions of basaltic lavas which covered the surfaces of older rocks. Occasionally, the lavas covered landscapes of considerable relief, filling valleys, burying hills and flowing into lakes, where pillow lavas and hyaloclastites formed. Rare sediments within the lava successions, may be fluviatile conglomerates, sandstones and fine-grained plant-bearing horizons which provide valuable stratigraphic and palaeogeographical information. The lava flow tops often display intense Palaeocene age weathering, with the formation of bright red lateritic deposits.

Lavas were principally fed from fissure eruptions, the feeders are among the multitude of dykes forming the swarms extending across the Province. The thick sequences of lavas preserved in Mull, Skye, and the Small Isles built up between about 63 Ma and 60 Ma, early in the life of the Province. Occasionally, lavas must have been erupted as the central complexes developed; there is good evidence from several centres that silicic and intermediate lavas were closely associated with central complex formation but there are few substantiated examples of basaltic lavas, with the exception of pillow lavas within the Mull centre. The central complexes generally post-date the adjoining lavas, and were intruded by later members of the dyke swarms. The central complexes record varied intrusive sequences in which basaltic and granitic magmas have been intimately associated (extracts from Emeleus & Gyopari, 1992).

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