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 elongate 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
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.
pillow-like blobs of hot gabbroic magma mixed with cooler lighter
coloured granite to form this striking rock exposed adjacent to Dun
Passage. Elsewhere, more thorough mixing of the two magma types formed
a range of hybrid diorite crystalline rocks.
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.
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 northwest European continental margin.