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Geology of Great Britain
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Geology of Great Britain
The geology of Great Britain is renowned for its diversity. As a result of its eventful geological history, Great Britain shows a rich variety of landscapes across the constituent countries of England, Wales and Scotland. Rocks of almost all geological ages are represented at outcrop, from the Archaean onwards.
Seismographic research shows that the crust of the Earth below Great Britain is from 27 to 35 km (17 to 22 mi) thick. The oldest surface rocks are found in northwest Scotland and are more than half as old as the planet. These rocks are thought to underlie much of Great Britain (although boreholes have only penetrated the first few kilometres), but next appear extensively at the surface in Brittany and the Channel Islands. The youngest rocks are found in southeast England.
The bedrock geology consists of a complex mix of generally older metamorphic rocks overlain by varying sequences of sedimentary rocks into both of which igneous rocks have been intruded at different times. The complexity of Great Britain's geology is due ultimately to its being subject to a variety of plate tectonic processes over a very extended period of time. Changing latitude and sea levels have been important factors in the nature of sedimentary sequences, while successive continental collisions have affected its geological structure with major faulting and folding being a legacy of each orogeny (mountain-building period), often associated with volcanic activity and the metamorphism of existing rock sequences.
Great Britain does not have any active volcanoes now, but has had an active volcanic past. The last active volcanoes stopped erupting about 60 million years ago and the islands are no longer located upon any tectonic boundary or active volcanic region, nor are there other geological phenomena in the island group able to give rise to active volcanism. However, on a geological timescale, a considerable degree of active and large-scale volcanism occurred in Great Britain, and many famous landmarks are formed of the eroded igneous rocks formed during these orogenies (mountain-building periods). The British Geological Survey lists Snowdonia and the Lake District as having extremely large volcanic eruptions around 450 Ma (million years ago), Edinburgh Castle lying upon the remains of a volcano dating back 350 Ma, and some islands of western Scotland as being remnants of volcanoes from around 60 Ma.
Overlain on this bedrock geology ("solid geology" in the terminology of maps) is a varied distribution of unconsolidated material of more recent origin. It includes material deposited by glaciers (boulder clay and other forms of glacial drift such as sand and gravel). "Drift" geology is often more important than "solid" geology when considering building works, drainage, siting water boreholes, sand and gravel resources and soil fertility. Although "drift" strictly refers to glacial and fluvio-glacial deposits, the term on geological maps has traditionally included other materials including alluvium, river terraces, etc. Recent maps use the terms "bedrock" and "superficial" in place of "solid" and "drift".
This description of the geological history of Great Britain is based on that of P. Toghill.
The Lewisian gneiss, the oldest rocks in Great Britain, date from at least 2.7 billion years ago in the Archaean eon, the Earth itself being about 4.6 billion years old. They are found in the far north west of Scotland and in the Hebrides, with a few small outcrops elsewhere. Formed from rock originally deposited at the surface of the planet, the rocks were later buried deep in the Earth's crust and metamorphosed into crystalline gneiss.
South of the gneisses are a complex mixture of rocks forming the North West Highlands and Grampian Highlands in Scotland. These are essentially the remains of folded sedimentary rocks that were originally 25 km thick, deposited over the gneiss on what was then the floor of the Iapetus Ocean. The process started about 1,000 Ma, with a notable 7 km thick layer of Torridon Sandstone being deposited about 800 Ma, as well as the debris deposited by an ice sheet 670 Ma.
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Geology of Great Britain
The geology of Great Britain is renowned for its diversity. As a result of its eventful geological history, Great Britain shows a rich variety of landscapes across the constituent countries of England, Wales and Scotland. Rocks of almost all geological ages are represented at outcrop, from the Archaean onwards.
Seismographic research shows that the crust of the Earth below Great Britain is from 27 to 35 km (17 to 22 mi) thick. The oldest surface rocks are found in northwest Scotland and are more than half as old as the planet. These rocks are thought to underlie much of Great Britain (although boreholes have only penetrated the first few kilometres), but next appear extensively at the surface in Brittany and the Channel Islands. The youngest rocks are found in southeast England.
The bedrock geology consists of a complex mix of generally older metamorphic rocks overlain by varying sequences of sedimentary rocks into both of which igneous rocks have been intruded at different times. The complexity of Great Britain's geology is due ultimately to its being subject to a variety of plate tectonic processes over a very extended period of time. Changing latitude and sea levels have been important factors in the nature of sedimentary sequences, while successive continental collisions have affected its geological structure with major faulting and folding being a legacy of each orogeny (mountain-building period), often associated with volcanic activity and the metamorphism of existing rock sequences.
Great Britain does not have any active volcanoes now, but has had an active volcanic past. The last active volcanoes stopped erupting about 60 million years ago and the islands are no longer located upon any tectonic boundary or active volcanic region, nor are there other geological phenomena in the island group able to give rise to active volcanism. However, on a geological timescale, a considerable degree of active and large-scale volcanism occurred in Great Britain, and many famous landmarks are formed of the eroded igneous rocks formed during these orogenies (mountain-building periods). The British Geological Survey lists Snowdonia and the Lake District as having extremely large volcanic eruptions around 450 Ma (million years ago), Edinburgh Castle lying upon the remains of a volcano dating back 350 Ma, and some islands of western Scotland as being remnants of volcanoes from around 60 Ma.
Overlain on this bedrock geology ("solid geology" in the terminology of maps) is a varied distribution of unconsolidated material of more recent origin. It includes material deposited by glaciers (boulder clay and other forms of glacial drift such as sand and gravel). "Drift" geology is often more important than "solid" geology when considering building works, drainage, siting water boreholes, sand and gravel resources and soil fertility. Although "drift" strictly refers to glacial and fluvio-glacial deposits, the term on geological maps has traditionally included other materials including alluvium, river terraces, etc. Recent maps use the terms "bedrock" and "superficial" in place of "solid" and "drift".
This description of the geological history of Great Britain is based on that of P. Toghill.
The Lewisian gneiss, the oldest rocks in Great Britain, date from at least 2.7 billion years ago in the Archaean eon, the Earth itself being about 4.6 billion years old. They are found in the far north west of Scotland and in the Hebrides, with a few small outcrops elsewhere. Formed from rock originally deposited at the surface of the planet, the rocks were later buried deep in the Earth's crust and metamorphosed into crystalline gneiss.
South of the gneisses are a complex mixture of rocks forming the North West Highlands and Grampian Highlands in Scotland. These are essentially the remains of folded sedimentary rocks that were originally 25 km thick, deposited over the gneiss on what was then the floor of the Iapetus Ocean. The process started about 1,000 Ma, with a notable 7 km thick layer of Torridon Sandstone being deposited about 800 Ma, as well as the debris deposited by an ice sheet 670 Ma.
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