by Mike McNamee Published 01/08/2016
Mull is the third largest of the Hebridean islands, behind Skye and Lewis. It shares a small fragment of pre-Cambrian Lewisian gneiss with Lewis, but only on Iona, the island just off the Ross of Mull. Most of the geological complexity comes from the two volcanoes centred in the middle of the island. These erupted some 65 million years ago when Greenland moved away from Scotland and are part of a line of volcanic activity which stretches from Ailsa Craig in the south up through Arran, Ardnamurchan, Rum, Skye, then bearing westwards to St Kilda.
The glaciation of ice ages has planed down the landscape around the original, conical, volcanoes to form classic ring dykes.
The 20 or so outpourings of molten lava have created a stepped landscape (called 'trap' after the Swedish for step) of sills of hard rock topped by layers of softer larva to form a layer cake that runs right to the sea in places. This layering is further complicated by raised beaches, formed when cooling climates recaptured the water and locked it in to the polar ice caps leaving the coastline high, dry and rebounding upwards. The abbey of Iona is built on one such raised beach.
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