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Landscape of St Kilda

Explosive Beginnings

The dramatic coastal scenery of the St Kildan islands is a result of their being formed of resistant crystalline rock. Some 60 million years ago volcanic activity was a feature of what is now the western coast of Scotland. A chain of volcanoes stretched from south of Arran northwards across the continental margin into the Atlantic. Molten rock, generated by the stretching of the Earth's upper mantle and overlying crust, ascended through the crust to erupt from large central volcanoes or elongated fissures.

St Kilda marks the site of one of the central volcanoes and the rocks which now form the spectacular group of islands resulted from the slow cooling of molten magma deep beneath the erupting volcano. Magmas of different composition formed the dark grey gabbros which run from Soay, Hirta and Dun to Boreray, and the light pink granite of Conachair and Oiseval.


Photograph: Graham Durant
The two magma types had different compositions, the gabbros being relatively richer in magnesium and iron and the granites being relatively richer in silicon. From time to time the two magmas, existing at significantly different temperatures and densities, mixed together, triggering volcanic eruptions at the surface. Evidence for this process is preserved in the rocks and boulders at the western end of Village Bay.

Photograph: Graham Durant
Dark pillow-like blobs of hot gabbroic magma mixed with cooler lighter-coloured granite to form this striking rock exposed adjacent to Caolas an Duin. Elsewhere, more thorough mixing of the two magma types formed a range of hybrid diorite crystalline rocks.

Photograph: Graham Durant
Thin inclined sheets and dykes of fine-grained basalt cut through the various gabbros and granophyres. These are conspicuous features in the granite cliffs of Oiseval.

Photograph: Graham Durant
Sixty million years of erosion following the end of volcanic activity has left the resistant heart of a volcano standing proud close to the edge of the north-west European continental margin.

Photograph: Graham Durant

© The National Trust for Scotland