Variations in the strength of

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

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 The British Isles are a group of islands in the North Atlantic Ocean off the north-western coast of continental Europe, consisting of the islands of Great Britain, Ireland, the Isle of Man, the Inner and Outer Hebrides, the Northern Isles, ...

and adjacent areas and they reveal aspects of these islands’ geological structure. Free-Air gravity anomaly map around Britain and Ireland

Negative anomalies

Negative gravity anomalies are associated with local deficiencies in mass due to volumes of rock which are less

dense Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematically ...

than the average. Typically these anomalies are associated with

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

plutons and with

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subside ...

s. Gravity anomaly maps of the British Isles are interpreted as indicating the presence of granite plutons in the following areas. The list is not exhaustive: *

Galloway Galloway ( ; sco, Gallowa; la, Gallovidia) is a region in southwestern Scotland comprising the historic counties of Wigtownshire and Kirkcudbrightshire. It is administered as part of the council area of Dumfries and Galloway. A native or ...

, UK *

Cheviot Hills The Cheviot Hills (), or sometimes The Cheviots, are a range of uplands straddling the Anglo-Scottish border between Northumberland and the Scottish Borders. The English section is within the Northumberland National Park. The range includes T ...

, UK *

Mountains of Mourne The Mourne Mountains ( ; ga, Beanna Boirche), also called the Mournes or Mountains of Mourne, are a granite mountain range in County Down in the south-east of Northern Ireland. They include the highest mountains in Northern Ireland, the hig ...

, UK *

Connemara Connemara (; )( ga, Conamara ) is a region on the Atlantic coast of western County Galway, in the west of Ireland. The area has a strong association with traditional Irish culture and contains much of the Connacht Irish-speaking Gaeltacht, ...

, Ireland * Donegal, Ireland *

South West England South West England, or the South West of England, is one of nine official regions of England. It consists of the counties of Bristol, Cornwall (including the Isles of Scilly), Dorset, Devon, Gloucestershire, Somerset and Wiltshire. Cities ...

(

Cornubian batholith The Cornubian batholith is a large mass of granite rock, formed about 280 million years ago, which lies beneath much of Devon and Cornwall, the south-western peninsula of Great Britain. The main exposed masses of granite are seen at Dartmoor, Bo ...

and

Haig Fras Haig 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 ...

), UK *

Askrigg Block The 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 ...

(

Yorkshire Dales The Yorkshire Dales is an upland area of the Pennines in the historic county of Yorkshire, England, most of it in the Yorkshire Dales National Park created in 1954. The Dales comprise river valleys and the hills rising from the Vale of York w ...

), UK * Alston Block (

North Pennines The 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. It is bounded to the north by the Tyne Valley and ...

), UK * Grampian Highlands (

Cairngorms The Cairngorms ( gd, Am Monadh Ruadh) are a mountain range in the eastern Highlands of Scotland closely associated with the mountain Cairn Gorm. The Cairngorms became part of Scotland's second national park (the Cairngorms National Park) on 1 S ...

etc.), UK The

Weardale Granite The North Pennine Batholith, also known as the Weardale Granite is a granitic batholith lying under northeast England, emplaced around 400 million years ago in the early Devonian.Kimbell, G.S., B. Young, D. Millward and Q. G. Crowley (2010). 'Th ...

, 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 Raydale (also known as Raydaleside) is a dale on the south side of Wensleydale in North Yorkshire, England. The northern part of the dale is the valley of the River Bain, which flows out of Semerwater, one of very few lakes in the Yorkshire Da ...

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 Moray () gd, Moireibh or ') is one of the 32 local government council areas of Scotland. It lies in the north-east of the country, with a coastline on the Moray Firth, and borders the council areas of Aberdeenshire and Highland. Between 197 ...

North Channel Basin North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography. Etymology The word ''north'' is ...

Solway Basin Solway may refer to: Places Australia *Solway, a neighbourhood of Ashburton, Victoria. a suburb of Melbourne New Zealand *Solway, New Zealand, a suburb of Masterton United Kingdom *Solway Firth, the inlet between the north west of England and so ...

Kish Bank Basin Kish may refer to: Geography * Gishi, Nagorno-Karabakh, Azerbaijan, a village also called Kish * Kiş, Shaki, Azerbaijan, a village and municipality also spelled Kish * Kish Island, an Iranian island and a city in the Persian Gulf * Kish, Iran, ...

Munster Basin ::''This the Irish'' Munster Basin ''should not be confused with the'' Münster Basin ''in northern Germany'' The Munster Basin is a late Middle to Upper Devonian age extensional (rift) sedimentary basin in the south-west of Ireland. The basin fi ...

Vale of Eden The 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 (more usually referred to as ...

Vale of Clwyd The Vale of Clwyd ( cy, Dyffryn Clwyd) is a tract of low-lying ground in the county of Denbighshire in north-east Wales. The Vale extends south-southwestwards from the coast of the Irish Sea for some 20 miles (about 30 km) forming a triangl ...

Cheshire Basin The Cheshire Basin is a late Palaeozoic and Mesozoic sedimentary basin extending under most of the county of Cheshire in northwest England. It extends northwards into the Manchester area and south into Shropshire. The basin possesses something of t ...

Worcester Basin The Worcester Basin or Worcester Graben is a sedimentary basin in central England, filled with mainly Permian and Triassic rocks. It trends roughly north-south and lies between the East Malverns Fault in the west and the Inkberrow Fault in the eas ...

Hampshire Basin The Hampshire Basin is a geological basin of Palaeogene age in southern England, underlying parts of Hampshire, the Isle of Wight, Dorset, and Sussex. Like the London Basin to the northeast, it is filled with sands and clays of Paleocene and y ...

* Cardigan Bay Basin * St George's Channel Basin *

Wessex Basin The Wessex Basin is a petroleum-bearing geological area located along the southern coast of England and extending into the English Channel. The onshore part of the basin covers approximately 20,000 km2 and the area that encompasses the Englis ...

Rockall Basin The Rockall Trough ( gd, Clais Sgeir Rocail) is a deep-water bathymetric feature to the northwest of Scotland and Ireland, running roughly from southwest to northeast, flanked on the north by the Rockall Plateau and to the south by the Porcu ...

Porcupine Basin The Porcupine Seabight or Porcupine Basin is a deep-water oceanic basin located on the continental margin in the northeastern portion of the Atlantic Ocean. It can be found in the southwestern offshore portion of Ireland and is part of a series o ...

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 '60s 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. The

Lairg Gravity Low The Lairg gravity low is a possible impact crater in Scotland about 40 kilometres in diameter, with a centre near the town of Lairg in the Scottish Highlands. Its identity as an impact crater is suspected due to the impact deposits present in the ...

is suspected to be due to a buried

impact crater An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...

40 kilometres in diameter.

Positive anomalies

Positive gravity anomalies are associated with areas where Precambrian basement rocks are at or near the surface or where volumes of

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

or other similarly dense rocks have been put in place. Intense gravity highs coincide with a series of

Palaeogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...

volcanic centres associated with the opening of the

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

and including those at: * Skye * St Kilda *

Ardnamurchan Ardnamurchan (, gd, Àird nam Murchan: headland of the great seas) is a peninsula in the ward management area of Lochaber, Highland, Scotland, noted for being very unspoiled and undisturbed. Its remoteness is accentuated by the main access ...

* Rùm *

Dubh Artach Dubh 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 ...

* Isle of Mull * Isle of Arran *

Lundy Lundy is an English island in the Bristol Channel. It was a micronation from 1925–1969. It forms part of the district of Torridge in the county of Devon. About long and wide, Lundy has had a long and turbulent history, frequently chang ...

Anton Dohrn Seamount The Anton Dohrn Seamount is a guyot in the Rockall Trough in the northeast Atlantic. It is high and is topped with pinnacles, one of which reaches a depth of . Away from the flat top upon which the pinnacles rest, the slopes fall off steeply ...

Rosemary Bank Rosemary Bank is a seamount approximately west of Scotland, located in the Rockall Trough, in the northeast Atlantic. It was discovered in 1930 by the survey vessel HMS ''Rosemary'', from which it takes its name. It is one of only three seamoun ...

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.Emeleus, CH & Bell BR 2005. British Regional Geology: the Paleogene volcanic districts of Scotland (4th Edn) British Geological Survey p134 In addition, intense gravity highs are associated with the

Anton Dohrn Felix Anton Dohrn FRS FRSE (29 December 1840 – 26 September 1909) was a prominent German Darwinist and the founder and first director of the first zoological research station in the world, the Stazione Zoologica in Naples, Italy. He worked ...

and Hebrides Terrace seamounts. Gravity anomalies also accompany major

lineaments ''See also Line (geometry)'' A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will appear as a fault-aligned valley, a series of fault or fold-ali ...

, notably those associated with the Scottish

Caledonides The Caledonian orogeny was a mountain-building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that o ...

and including Great Glen Fault, Walls Boundary Fault,

Highland Boundary Fault The Highland Boundary Fault is a major fault zone that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east. It separates two different geological terranes which give rise to two distinct physiographic terr ...

and

Southern Uplands Fault The Southern Uplands Fault (or occasionally Southern Upland Fault) is a fault in Scotland that runs from Girvan (or more specifically from the Rhins of Galloway) to Dunbar on the East coast. It marks the southern boundary of the Scottish Midland ...

References

*British Geological Survey 1996, ''Tectonic Map of Britain, Ireland and Adjacent Areas'', Pharaoh et al., 1:500K {{DEFAULTSORT:Gravity Anomalies Of Britain And Ireland Geology of England Geology of Northern Ireland Geology of Scotland Geology of Wales Geology of the Republic of Ireland Gravimetry