About 580Mya 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.
Oceanic crust forms under the oceans at mid-ocean ridges. As currents in the mantle pull the plates of the crust apart, magma rises from the mantle to fill the gap, then cools and solidifies, forming new ocean crust. Different minerals crystallise at different temperatures and pressures within the cooling magma, resulting in layers of different rock within the crust. After the ophiolite was emplaced on the Laurentian continent the crust was tipped up vertically so the once horizontal layers are now exposed on edge at the surface.