Basement rocks
Map to show locations of Shetland's Lewisian, Moine and Dalradian rocksZoomUntil about 420 million years ago, Shetland and Scotland were part of the continent we now call North America. Many of Shetland’s rocks share the history of this ancient continent which is known as Laurentia.
Shetland’s oldest rocks formed between 2900 and 2500 million years ago – these Archaean gneisses are over half the age of the Earth! They probably began as igneous rocks within an early continent which collided with others to form Laurentia. During this collision and those that followed the rocks were metamorphosed to their present condition due to heat and pressure. They spent much of their history buried deep within the crust of Laurentia and can be seen in northern parts of Shetland today. They are mainly coarse grained crystalline rocks like gneiss and are known as Lewisian gneiss after the Isle of Lewis where rocks from this period are commonly found. They are the same age as the Lewisian gneisses of Scotland and similar rocks are also found in Northern Canada and Greenland.
Laurentia eroded and thick layers of sand and mud built up on the seabed around its coast. Eventually it collided with Baltica (Scandinavia), Siberia and Gondwana (Africa), to form a supercontinent called Rodinia. During the collision (between 1000 and 870 million years ago), massive temperatures and pressures were generated, which altered the sand and mud that had built up around Laurentia into a range of metamorphic rocks. We call these the rocks of the Moine Supergroup. They can be seen in parts of the Shetland Mainland and throughout the island of Yell. Moine rocks include coarsely layered gneisses and schists, sometimes containing minerals such as garnets which form when sandstones are metamorphosed at high temperatures. They make up much of the Scottish Highlands to the west of the Great Glen Fault.
Rodinia began to split apart about 800 million years ago leaving a smaller continent called Vendia. By 730 million years ago Vendia was beginning to stretch and thin. In the same way that the Moine rocks formed, layer upon layer of sands, silts and iron and aluminium rich muds were deposited in marine basins within the continent. 580 million years ago Vendia split and Laurentia drifted south as a new ocean called Iapetus was born. Magma welled up from the Earth’s mantle into the rift and solidified to form the floor of the new ocean.
About 480 million years ago the Iapetus Ocean began to close and Laurentia collided again, this time with Baltica (Scandinavia) and Avalonia (Western Europe) to form the continent of Euramerica. It was at this time that the rocks of England and Scotland were first united. The sands, silts and muds were metamorphosed to form the rocks of the Dalradian Supergroup which make up much of Shetland’s Central Mainland and the western part of Unst. The Dalradian rocks include psammites (metamorphosed impure sandstones), schists, quartzites and limestones. Dalradian rocks make up most of the Central Highlands of Scotland.
The rocks of the Moine Supergroup were further metamorphosed and thrust over Lewisian rocks when the Iapetus Ocean closed.
