Gravity anomalies of Britain and Ireland
Encyclopedia
Variations in the strength of gravity occur from place to place according to the density distribution of the rocks beneath the surface. Such gravity anomalies
have been mapped across the British Isles
and the surrounding seas and they reveal aspects of these islands’ geological structure
.
than the average. Typically these anomalies are associated with granite
pluton
s and with sedimentary basin
s. Gravity anomaly maps of Britain and Ireland are interpreted as indicating the presence of granite plutons in the following areas. The list is not exhaustive:
A batholith
some 60 km by 25 km in extent underlying the Alston Block of the northern Pennines was first detected by gravity survey. Drilling of the Rookhope borehole subsequently confirmed the existence of this substantial body of granite. A combination of gravity anomaly data and boreholes has permitted geologists to work out the shape of this pluton
. A further borehole was drilled at Raydale to investigate the origin of the gravity anomaly underlying the Askrigg Block. This proved the existence of the Wensleydale granite - a body of rock which is nowhere exposed at the surface.
Similarly, negative anomalies coincide with the following sedimentary basins (amongst others):
The negative anomaly associated with the Worcester Basin/Graben has sharp margins and is particularly pronounced. Work by researchers, Cook and Thirlaway and also by Brooks in the 1950s and 60's resulted in estimates of the thickness of the sedimentary rock sequence in the basin. Models for the structure of the basin are in part based on their interpretation of the gravity anomaly together with data from seismic surveys and a borehole at Kempsey.
basement rocks are at or near the surface or where volumes of basalt
or other similarly dense rocks have been put in place. Intense gravity highs coincide with a series of Palaeogene
volcanic centres associated with the opening of the Atlantic Ocean
and including those at:
The discovery of another volcanic centre, the wholly submarine Blackstones Bank Central Complex, was achieved through combining the results of magnetic, seismic and gravity surveys; its large positive gravity anomaly suggesting that it was a candidate site for further research.
In addition, intense gravity highs are associated with the Anton Dohrn
and Hebrides Terrace seamounts. Gravity anomalies also accompany major lineaments, notably those associated with the Scottish Caledonides and including Great Glen Fault
, Walls Boundary Fault, Highland Boundary Fault
and Southern Uplands Fault
.
Gravity anomaly
A gravity anomaly is the difference between the observed acceleration of Earth's gravity and a value predicted from a model.-Geodesy and geophysics:...
have been mapped across the British Isles
British Isles
The British Isles are a group of islands off the northwest coast of continental Europe that include the islands of Great Britain and Ireland and over six thousand smaller isles. There are two sovereign states located on the islands: the United Kingdom of Great Britain and Northern Ireland and...
and the surrounding seas and they reveal aspects of these islands’ geological structure
Geological structure of Great Britain
The geological structure of Great Britain is complex, resulting as it does from a long and varied geological history spanning more than two billion years...
.
Negative Anomalies
Negative gravity anomalies are associated with local deficiencies in mass due to volumes of rock which are less denseDensity
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
than the average. Typically these anomalies are associated with granite
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
pluton
Pluton
A pluton in geology is a body of intrusive igneous rock that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies...
s and with sedimentary basin
Sedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
s. Gravity anomaly maps of Britain and Ireland are interpreted as indicating the presence of granite plutons in the following areas. The list is not exhaustive:
- GallowayGallowayGalloway is an area in southwestern Scotland. It usually refers to the former counties of Wigtownshire and Kirkcudbrightshire...
- Cheviot HillsCheviot HillsThe Cheviot Hills is a range of rolling hills straddling the England–Scotland border between Northumberland and the Scottish Borders.There is a broad split between the northern and the southern Cheviots...
- Mountains of MourneMountains of MourneThe Mourne Mountains are a granite mountain range in County Down in the south-east of Northern Ireland. It includes the highest mountains in Northern Ireland and the province of Ulster. The highest of these is Slieve Donard at...
- ConnemaraConnemaraConnemara is a district in the west of Ireland consisting of a broad peninsula between Killary Harbour and Kilkieran Bay in the west of County Galway.-Overview:...
- DonegalCounty DonegalCounty Donegal is a county in Ireland. It is part of the Border Region and is also located in the province of Ulster. It is named after the town of Donegal. Donegal County Council is the local authority for the county...
- South West EnglandSouth West EnglandSouth West England is one of the regions of England defined by the Government of the United Kingdom for statistical and other purposes. It is the largest such region in area, covering and comprising Bristol, Gloucestershire, Somerset, Dorset, Wiltshire, Devon, Cornwall and the Isles of Scilly. ...
(Cornubian batholithCornubian batholithThe Cornubian batholith refers to the group of associated granite intrusions which underlie the south-western peninsula of Great Britain. The main exposed masses of the batholith are seen at Dartmoor, Bodmin Moor, St Austell, Carnmenellis, Land's End and the Isles of Scilly. It formed during the...
and Haig FrasHaig FrasHaig Fras is a 45 km long submarine granitic rocky outcrop in the southern part of the Celtic Sea, lying about 95 km northwest of the Isles of Scilly. It covers an area of . At one point it reaches within 38 m of the sea surface. It is protected as a Special Area of Conservation because of the...
graniteGraniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
) - Askrigg BlockAskrigg BlockThe Askrigg Block is the name applied by geologists to the crustal block forming a part of The Pennines of northern England and which is essentially coincident with the Yorkshire Dales. It is defined by the Dent Fault to the west and the Craven Fault System to the south whilst to the north it is...
(Yorkshire DalesYorkshire DalesThe Yorkshire Dales is the name given to an upland area in Northern England.The area lies within the historic county boundaries of Yorkshire, though it spans the ceremonial counties of North Yorkshire, West Yorkshire and Cumbria...
) - Alston BlockAlston BlockThe Alston Block is a term used by geologists to describe the geological structure of the North Pennines of northern England and which forms a part of the Pennine Block & Basin Province which originated during the Carboniferous period. It is defined by the Stublick and Ninety Fathom Faults to the...
(North PenninesNorth PenninesThe North Pennines is the northernmost section of the Pennine range of hills which runs north-south through northern England. It lies between Carlisle to the west and Darlington to the east...
) - Grampian Highlands (CairngormsCairngormsThe Cairngorms are a mountain range in the eastern Highlands of Scotland closely associated with the mountain of the same name - Cairn Gorm.-Name:...
etc.)
A batholith
Batholith
A batholith is a large emplacement of igneous intrusive rock that forms from cooled magma deep in the Earth's crust...
some 60 km by 25 km in extent underlying the Alston Block of the northern Pennines was first detected by gravity survey. Drilling of the Rookhope borehole subsequently confirmed the existence of this substantial body of granite. A combination of gravity anomaly data and boreholes has permitted geologists to work out the shape of this pluton
Pluton
A pluton in geology is a body of intrusive igneous rock that crystallized from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes, sills, laccoliths, lopoliths, and other igneous bodies...
. A further borehole was drilled at Raydale to investigate the origin of the gravity anomaly underlying the Askrigg Block. This proved the existence of the Wensleydale granite - a body of rock which is nowhere exposed at the surface.
Similarly, negative anomalies coincide with the following sedimentary basins (amongst others):
- Moray Firth Basin
- North Channel Basin
- Solway Basin
- Kish Bank Basin
- Munster Basin
- Vale of EdenVale of EdenThe Vale of Eden is formed by the course of the River Eden, one of the major rivers of Northwest England. It is however of much greater extent than the actual valley of the river, lying between the Cumbrian Mountains , and the northern part of the Pennine Range. It lies entirely within the county...
- Vale of ClwydVale of ClwydThe Vale of Clwyd is a tract of low-lying ground in the county of Denbighshire in northeast Wales. The Vale extends south-southwestwards from the coast of the Irish Sea for some 20 miles forming a triangle of low ground bounded on its eastern side by the well-defined scarp of the Clwydian Range...
- Cheshire BasinCheshire BasinThe Cheshire Basin is a late Palaeozoic and Mesozoic sedimentary basin which extends under most of the county of Cheshire in northwest England. It extends northwards into the Manchester area and south into Shropshire...
- Worcester Graben
- Hampshire BasinHampshire BasinThe Hampshire Basin is a geological basin of Paleogene age in southern England, underlying parts of Hampshire, the Isle of Wight, Dorset, and Sussex...
- Cardigan Bay Basin
- St George’s Channel Basin
- Wessex Basin
- Rockall BasinRockall BasinThe Rockall Basin is a large sedimentary basin that lies to the west of Ireland and the United Kingdom beneath the major deepwater area known as the Rockall Trough. It is named after Rockall a rocky islet lying 301.4 km west of St Kilda...
- Porcupine Basin
The negative anomaly associated with the Worcester Basin/Graben has sharp margins and is particularly pronounced. Work by researchers, Cook and Thirlaway and also by Brooks in the 1950s and 60's resulted in estimates of the thickness of the sedimentary rock sequence in the basin. Models for the structure of the basin are in part based on their interpretation of the gravity anomaly together with data from seismic surveys and a borehole at Kempsey.
Positive Anomalies
Positive gravity anomalies are associated with areas where PrecambrianPrecambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
basement rocks are at or near the surface or where volumes of basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
or other similarly dense rocks have been put in place. Intense gravity highs coincide with a series of Palaeogene
Paleogene
The Paleogene is a geologic period and system that began 65.5 ± 0.3 and ended 23.03 ± 0.05 million years ago and comprises the first part of the Cenozoic Era...
volcanic centres associated with the opening of the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
and including those at:
- SkyeSkyeSkye or the Isle of Skye is the largest and most northerly island in the Inner Hebrides of Scotland. The island's peninsulas radiate out from a mountainous centre dominated by the Cuillin hills...
- St KildaSt Kilda, ScotlandSt Kilda is an isolated archipelago west-northwest of North Uist in the North Atlantic Ocean. It contains the westernmost islands of the Outer Hebrides of Scotland. The largest island is Hirta, whose sea cliffs are the highest in the United Kingdom and three other islands , were also used for...
- ArdnamurchanArdnamurchanArdnamurchan is a peninsula in Lochaber, Highland, Scotland, noted for being very unspoilt and undisturbed. Its remoteness is accentuated by the main access route being a single track road for much of its length.-Geography:...
- RùmRùmRùm , a Scottish Gaelic name often anglicised to Rum) is one of the Small Isles of the Inner Hebrides, in the district of Lochaber, Scotland...
- Dubh ArtachDubh ArtachDubh Artach is a remote skerry of basalt rock off the west coast of Scotland lying west of Colonsay and south-west of the Ross of Mull.A lighthouse designed by Thomas Stevenson with a tower height of was erected between 1867 and 1872 with a shore station constructed on the isle of Erraid...
- Isle of MullIsle of MullThe Isle of Mull or simply Mull is the second largest island of the Inner Hebrides, off the west coast of Scotland in the council area of Argyll and Bute....
- Isle of ArranIsle of ArranArran or the Isle of Arran is the largest island in the Firth of Clyde, Scotland, and with an area of is the seventh largest Scottish island. It is in the unitary council area of North Ayrshire and the 2001 census had a resident population of 5,058...
- LundyLundyLundy is the largest island in the Bristol Channel, lying off the coast of Devon, England, approximately one third of the distance across the channel between England and Wales. It measures about at its widest. Lundy gives its name to a British sea area and is one of the islands of England.As of...
- Anton Dohrn SeamountAnton Dohrn SeamountThe Anton Dohrn Seamount is a guyot in the Rockall Trough in the northeast Atlantic. It was named after the ship which discovered it which, in turn, had been named after the 19-century biologist Anton Dohrn....
- Rosemary BankRosemary BankRosemary Bank is a seamount in the Rockall Trough in the northeast Atlantic. It is approximately 120 km west of Scotland.The feature originated 70 million years ago as a result of volcanic activity...
The discovery of another volcanic centre, the wholly submarine Blackstones Bank Central Complex, was achieved through combining the results of magnetic, seismic and gravity surveys; its large positive gravity anomaly suggesting that it was a candidate site for further research.
In addition, intense gravity highs are associated with the Anton Dohrn
Anton Dohrn Seamount
The Anton Dohrn Seamount is a guyot in the Rockall Trough in the northeast Atlantic. It was named after the ship which discovered it which, in turn, had been named after the 19-century biologist Anton Dohrn....
and Hebrides Terrace seamounts. Gravity anomalies also accompany major lineaments, notably those associated with the Scottish Caledonides and including Great Glen Fault
Great Glen Fault
The Great Glen Fault is a long strike-slip fault that runs through its namesake the Great Glen in Scotland. However, the fault is actually much longer and over 400 million years old.-Location:...
, Walls Boundary Fault, Highland Boundary Fault
Highland Boundary Fault
The Highland Boundary Fault is a geological fault that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east...
and Southern Uplands Fault
Southern Uplands Fault
The Southern Uplands Fault is a fault in Scotland that runs from Girvan to Dunbar on the East coast. It marks the southern boundary of the Scottish Midland Valley....
.