Geological Overview
Gneiss in Parts
Lewisian Gneiss, Uyea - half the age of the Earth! Copyright Allen Fraser.Zoom600 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. We call it Lewisian Gneiss, after the Isle of Lewis, where it is commonly found. This rock can be seen in northern parts of Shetland today. It is about 2.8 billion years old!
The ancient North American continent eroded and thick layers of sand and mud built up on the seabed around its coast.
Terra not so Firma
The Iapetus Ocean closes. Copyright Robina BartonZoomTectonic plates carrying the Earth’s continents are constantly shifting. In fact, 600 million years ago ‘Shetland’ was 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, serpentine rocks that originally formed on this ocean floor have been driven up over continental rocks along two major thrusts.
An exposed ocean crust is called an ophiolite. Ophiolites do not occur in many places around the globe because ocean crust is heavier than continental crust and usually sinks beneath it. The Shetland ophiolite has been described as ‘the most compact, best exposed, complete and accessible in the world’ (Professor Derek Flinn, Liverpool University).
Fossil Fish. Copyright Jonathan SwaleZoomIsland in the Sun
Pangaea was actually 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 fossilised, and these fossils can be found today in areas around the south and west Mainland.
A Blast from the Past!
Continental collisions generate massive amounts of heat. This causes rocks to melt and huge amounts of molten magma to build up deep below the surface of the Earth. This is exactly what happened when Pangaea formed. The magma forced its way to the surface as volcanoes. At Eshaness in Northmavine you can walk through the flank of one such 360 million year old volcano!
Lost Continent
300 million years ago it was time for another major change. A rift valley formed and filled with water, creating what we now know as the North Sea. A second rift 60 million years ago began to the west of the U.K and created today’s Atlantic Ocean. Pangaea was no more. Scotland was left attached to England (and the now single European continental plate) as America drifted away westwards.
But the story continues, as Europe and America continue to move further away from each other at the rate of 3cm per year!
