The Shetland Ophiolite

Muckle SHETLAND HERITAGE TRAILS Flugga The Shetland ophiolite Loch Haroldswick Explore the floor of an ancient ocean... of Cliff Unst Baltasound Intrusions Skaw Granite Balta Ophiolite (upper nappe) Dunite Loch of Huney Watlee Harzburgite Serpentinite / Steatite A968 Mid imbricate zone Phyllite Funzie Conglomerate Uyeasound Ophiolite (lower nappe) Belmont Gabbro Uyea Wehrlite / Pyroxenite O Dunite t Harzburgite s c u e Serpentinite / Steatite r a Continental rocks C n l i Hamars Greenschist a c Ness t Quartzite n C e r Schist in u Fetlar Calcareous rocks t s Gneiss on Point of contact, t C Norwick beach, Unst Fault Fault © Robina Barton Robina © Houbie Thrust Loch of Funzie Funzie Projected line Major road 1km Minor road In Shetland, though, this did not happen. Layered ocean crust forms within a magma Beneath an ancient ocean Instead a thick slab of ocean crust and chamber at a mid-ocean ridge and spreads mantle was thrust up over the continent Mid-ocean ridge and ‘emplaced’ upon it. The crust bent 420 million years ago, an ancient ocean called ‘Iapetus’ closed Pillow lavas upwards so the horizontal layers of rock and the continents on either side (early versions of Europe and Sheeted dykes in the ophiolite ended up almost vertical. Oceanic North America) collided. As the landmasses came together, a Gabbro Later erosion exposed a section through crust section of oceanic crust (the rock beneath the ocean floor) was the ophiolite at the surface, making this Pyroxenite Magma chamber one of the best places in the world to see caught between them and thrust on top of the North American Wehrlite continent. This stranded section of oceanic crust, called an a compact vertical section through ancient oceanic crust. Dunite ophiolite, now forms the east side of Unst and much of Fetlar. ‘Moho’ As you travel along the trails you will Harzburgite Magma melting up discover both kinds of crust. The ancient (mantle rock) from mantle Keen of Hamar Central Unst Heogs North American continent is found in the Fetlar Dunite Wehrlite / Pyroxenite / Gabbro Harzburgite west of the islands, while you can walk You may wish to refer to BGS Geological Balta Upper Nappe Gabbro across the vertically tilted layers of rock map of Scotland Sheet 131 (Unst & that formed the ocean crust in the east. Fetlar) to help you. Geology of Unst in relation to the layers within the magma chamber shown above Unst cross section: ocean crust is bent Granite CONTINENTAL ROCKS Continental intrusion upwards as it is thrust over the continent rock E © Hazel Prichard Hazel © W Panorama looking south from Saxa Vord showing geology of Unst and Fetlar Thrust WehrlitePyroxeniteGabbro Phyllite HarzburgiteDuniteDunite Oceanic crust is one of two types of crust apart. Magma rises from the mantle to W E Continental that make up the Earth’s surface. The fill the gap, then cools and solidifies, Moho Thrust rock land we live on is continental crust. forming new ocean crust. Different Serpentinite‘Moho’ Ophiolite Oceanic crust is much denser and forms minerals crystallise at different UNST rocks dips in the surface of the Earth that fill temperatures and pressures within the The continental rocks are older than the with water and form oceans. The Earth’s cooling magma, resulting in layers of ophiolite and have been metamorphosed crust is divided into several plates that different rock within the crust. (changed) by high temperatures and float on top of a thick semi-molten layer When spreading ocean crust cools it pressures. This caused their minerals to called the mantle. becomes denser and normally it sinks recrystallise, forming quartz and garnet- Oceanic crust is continually being back into the mantle and is recycled. We bearing gneisses. The closing of Iapetus recycled. New crust forms under the call this subduction. Phyllite and emplacement of the ophiolite caused oceans at mid-ocean ridges. Currents in alteration of the oceanic rocks too, the mantle pull the plates of the crust changing some of them to serpentinite. Billion Years Ago Million Years Ago 4.6 2.8 600 420 400 350 300 60 0 Age of the Oldest rocks Iapetus Iapetus Ocean closes to form Devonian period. Further continental Pangaea splits, separating Earth in Shetland are Ocean opens Euramerica. Deformation of Funzie Conglomerate. collisions result in North America from formed. Emplacement of ophiolite begins. Emplacement of ophiolite completed. supercontinent Pangaea. Europe, and forming the Formation of Funzie Conglomerate. Atlantic Ocean..

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