Dr Ian Millar and Dr Tom Bradwell were in front of the camera in episodes one and three.
The oldest rocks in all of Britain are found in the North West Highlands of Scotland, and on the islands of the Outer Hebrides.
These ancient, twisted stones are called the Lewisian gneisses, named after the Isle of Lewis. They are up to three billion years old, or, in other words, two-thirds of the age of the Earth itself.
The Lewisian gneisses have had a long and complex history. They were formed as igneous rocks, cooling from molten magma deep within the Earth. Then, for hundreds of millions of years, they were buried far beneath the surface, where intense heat and pressure led to the process of metamorphism and the growth of new minerals.
The high pressure meant that these new minerals formed in parallel layers, known as gneissic layering. Subsequently, those layers were compressed still further, so that they were folded and squeezed in some areas, but sheared out in others.
By about 1.6 billion years ago, the Lewisian gneisses lay in the middle of a much larger continental mass known as the North Atlantic Craton, and were less affected by geological events. For the last third of Earth’s history, as life exploded and the surface of the Earth was drastically altered, the Lewisian gneisses have hardly changed at all.
The Lewisian gneisses have been studied by geologists over many years, ever since they were first mapped in the late nineteenth century. But despite all the work that has been done, many questions remain.
Recently, debate has focused on how the continents were built, billions of years ago:
Were the Lewisian gneisses formed as a single mass of continental crust, with new magmas being added periodically?
Or were they formed in several separate smaller continents and island chains, which then collided to build up the larger continental mass?
Research over the last few years has suggested that the latter is true.
Modern laboratories, such as those at the NERC Isotope Geosciences Laboratory (NIGL) in Nottingham, can date tiny fragments of a mineral called zircon to work out not just when a rock was formed, but also when it was affected by metamorphism. BGS staff and collaborators are using these techniques to understand the history of the Lewisian gneisses — and hence to study how the early Earth worked.
For further information contact Dr Ian Millar
Two thirds of the British Isles were covered by an ice sheet during the peak of the last glaciation, around 25 000 years ago.
Until recently the exact shape of this ice sheet, its thickness and geographical extent were largely unknown. Only recently, with the advent of remarkable new sea bed imagery, have we been able to fully discover the former extent of this enormous mass of ice.
During glacial periods, when global temperatures are typically 5 to 10°C colder than today, continental ice sheets grow on the northern portions of North America, Europe and Asia.
The growth of ice locks up huge volumes of liquid water, lowering global sea levels by over 100 m. Collectively, these northern hemisphere ice sheets greatly exceed the volume of ice currently present in Antarctica.
When these ice masses retreat they leave an unmistakable imprint in the landscape record, like a gigantic geological footprint. This ‘ice sheet footprint’ is made up of glacial features such as moraines, drumlins, grooves and meltwater channels. Moraines formed at the ice front are crucial, as they tell us where glaciers paused on their retreat back to higher ground.
New work in collaboration with the Scottish Association for Marine Science (SAMS) is finding previously undiscovered moraines on the sea bed in the waters around western Scotland.
Our new data shows that the fjord of Loch Linnhe hosts a wealth of features relating to the former extent of Scotland’s glaciers at the end of the last Ice Age.
Sea bed soundings and multibeam ‘swath’ bathymetry, have revealed the sea bed in unprecedented detail. This technology allows offshore surveys to be carried out more quickly and more systematically than ever before.
The data shows that large thick glaciers flowed out from the south-west Scottish Highlands along the Loch Linnhe trough as far as the Isle of Lismore. These glaciers existed around 12 000–13 000 years ago, but would have looked and behaved the same as those along the coast of western Greenland at the present time. Sudden warming around 11 500 years ago caused Scotland’s glaciers to retreat rapidly, with those terminating in the sea responding most quickly.
Ongoing work is seeking to explore how quickly these changes occurred and whether glacier behaviour in Scotland was similar to that being experienced in polar regions today.
For further information contact Dr Tom Bradwell