The Outer Hebrides: a geological perspective

Abstract

The Outer Hebrides form part of a horst block between the Atlantic margin of Europe and the marine troughs which occupy the Minch and Sea of the Hebrides. The geological history of the islands from late Palaeozoic times until the present day has been dominated by events connected with the break-up of the Laurasian supercontinent and the opening of the Atlantic Ocean—events which involved rifting and volcanic activity to the east and sea-floor spreading to the west. During an earlier period, from late Precambrian to mid-Palaeozoic times, the Outer Hebrides occupied a marginal position with respect to troughs flanking an older ocean which opened and subsequently closed on the site of the Caledonian orogenic belt in mainland Scotland. For much of this period also they functioned as a horst block. The behaviour of the Outer Hebrides as a geological unit has been controlled largely by the history and structure of its Lewisian basement which developed as a deep-seated metamorphic gneiss complex during the period 3000–1800 million years in early Precambrian times. The formation of a large region enriched in granite and the development of a grid of deep dislocations are among the events whose influence on the subsequent evolution of the Hebridean area are discussed.