About 580 million years ago the supercontinent of Vendia began to break up. Magma welled up from the Earth’s mantle into the widening rift and solidified to form oceanic crust – the floor of a new ocean called Iapetus.
About 480 million years ago the Iapetus Ocean began to close and by 420 million years ago the continents on either side – early versions of North America, Europe and Scandinavia – had collided to form the continent of Euramerica (uniting the rocks of Scotland and England for the first time).
Oceanic crust is denser than continental crust. Normally when oceans close, the oceanic crust beneath them sinks back into the mantle and melts.
When Euramerica formed, part of the oceanic crust beneath the Iapetus Ocean was thrust up over the top of the Laurentian (or North American) continent. A section of this ocean floor can be seen in the eastern parts of Unst and Fetlar. An exposed ocean crust such as this is called an ophiolite, and Shetland is one of the best places in the world to see one.