With rocks of every era from the Precambrian to the Carboniferous, Shetland’s geology spans almost 3 billion years and is more diverse than any similar sized area in Europe. Where else can you walk on ancient oceanic crust, explore an extinct volcano and stroll across shifting sands in the space of a day?
Over the past 700 million years Shetland has travelled from a location close to the South Pole across the Equator to its current latitude of 60° north. On the journey it experienced oceans opening and closing, mountains forming and eroding, tropical seas, volcanoes, deserts, ice ages and ancient rivers. Evidence of these earth-shaking events can be found throughout the isles, including a number of unique geological features.
Where it all began
600 million years ago, Shetland and Scotland were part of the continent we now call North America. Northern and Western Europe (including England) formed another two continents, and all three were separated by an ocean – the Iapetus Ocean. The ancient North American continent was made partly of gneiss. This rock, seen in northern parts of Shetland today is about 2.8 billion years old! Slowly, the ancient North American continent eroded and thick layers of sand and mud built up on the seabed around its coast.
When continents collide
Tectonic plates carrying the Earth's continents are constantly shifting, (which explains why Shetland was once to be found near the South Pole!) Between 500 and 420 million years ago the North American continent collided with those carrying Northern and Western Europe to form a huge new landmass called Pangaea. The layers of sediment on the seabed, which had now turned into rock, were crumpled during this massive collision. They were forced upwards to form an enormous mountain chain. This Caledonian mountain chain would have been similar in size to the Himalayas we know today.
Death of an ocean
When Pangaea formed, part of the crust (sea-bed) of the Iapetus Ocean and the mantle beneath it were forced up over the top of the North American continental crust. In Unst and Fetlar, to the far north, serpentine rocks that originally formed on this ocean floor were driven up over continental rocks along two major thrusts. An exposed ocean crust is called an ophiolite and they occur very rarely. The Shetland ophiolite has been described as "the most compact, best exposed, complete and accessible in the world" by the late Professor Derek Flinn of Liverpool University.
Pangaea was a vast desert. By 400 million years ago Shetland was lying close to the equator and it experienced a tropical climate that varied from wet and humid to dry and arid – hard to believe today! Streams and rivers ran down from the Caledonian mountains and fed temporary lakes on the desert plain. During wetter periods the lakes could be long-lived and aquatic life evolved and thrived. Sandstones and mudstones built up from layers of sediments in these watery environments. When the creatures of the lake died, they sank to the bottom to be buried among the sediments. The remains of these strange primitive fish would become fossilized, and these fossils can be found today in areas around the south and west Mainland.