A fascinating combination of ancient continental and ocean-floor crustal rocks on Britain’s most northerly inhabited island
As continents drift across the globe, the actions of plate tectonics normally mean that ocean-floor crust forms and then disappears again, recycled back into the mantle. Sometimes however, a sliver of ocean-floor rocks escapes the process of subduction, and ends up being added to continental crust. That’s what happened here. On the beautiful and remote island of Unst, you can walk over layered ocean-floor rocks that started life deep beneath the Iapetus Ocean. You can also visit the junction where ocean floor meets continental crust.
Much of the western side of Unst is made up of continental rocks formed in the Caledonian Orogeny, with links to the Scottish Highlands. These rocks started out as sediment deposited under the sea, and were later transformed into tough metamorphic rocks – gneiss, quartzite and psammite – which you can see at Hermaness and the most northerly islands of Out Stack and Muckle Flugga.
The rocks of eastern Unst had a very different origin; these are ocean-floor igneous rocks formed at an ocean ridge. The rock layers visible here represent different types of rock formed at different depths, including rocks from both the crust and upper mantle, making this one of the most accessible and complete examples of an ‘ophiolite’ in the world. Take a walk across the ophiolite from west to east, and you are travelling down through the ocean crust (made of rocks such as gabbro and dunite) until you reach the Mohorovičić discontinuity, or ‘Moho’, named after the Croatian seismologist who discovered it. To the north-west there are two craggy hills, the Heogs, made of tougher harzburgite: the rock left behind in the mantle as the magma melted out. Harzburgite contains the tough mineral orthopyroxene, which is why the Heogs are higher than the dunite to the east.
These rocks are more than just a curiosity to geologists, for there is chromite within the dunite layer – the mineral ore that is the source of chromium metal. Unst was an important source of chromium during the 19th Century, and today you can still see remains of the quarries and the horse mill where the rock was crushed.
At Norwick beach you can see the junction between the two main rock units of Unst, with the continental rocks in contact with the ophiolite. Around the corner is the Skaw granite, formed as a magma intrusion into the continental crust about 430 million years ago. The granite was later intensely deformed, probably during the same activity that pushed the ophiolite rocks into their present position adjacent to continental rocks.
Text contributed by Jennifer Roberts and Angus Miller
Find out more
Geopark Shetland, a UNESCO Global Geopark: https://www.shetlandamenity.org/geopark-shetland
The Shetland ophiolite trail: https://www.shetlandamenity.org/trails-and-exhibits
100 Great Geosites: https://www.geolsoc.org.uk/GeositesHeogsMoho
This web page is published by the Scottish Geodiversity Forum under a Creative Commons ‘Attribution Non-commercial’ (CC BY-NC) licence, which permits non-commercial reuse provided the original work is properly cited.