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 The gravity anomaly at a location on the Earth's surface is the difference between the observed value of gravity and the value predicted by a theoretical model. If the Earth were an ideal oblate spheroid of uniform density, then the gravity meas ...

have been mapped across the British Isles and adjacent areas and they reveal aspects of these islands’ geological structure. Free-Air gravity anomaly map around Britain and Ireland

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 than the average. Typically these anomalies are associated with granite

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

s and with sedimentary basins. 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, UK * Cheviot Hills, UK * Mountains of Mourne, UK * Connemara, Ireland *

Donegal Donegal may refer to: County Donegal, Ireland * County Donegal, a county in the Republic of Ireland, part of the province of Ulster * Donegal (town), a town in County Donegal in Ulster, Ireland * Donegal Bay, an inlet in the northwest of Ireland b ...

, Ireland * South West England ( Cornubian batholith and Haig Fras granite), UK * Askrigg Block ( Yorkshire Dales), UK *

Alston Block The 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 defin ...

( North Pennines), UK *

Grampian Highlands The Grampian Mountains (''Am Monadh'' in Gaelic) is one of the three major mountain ranges in Scotland, that together occupy about half of Scotland. The other two ranges are the Northwest Highlands and the Southern Uplands. The Grampian ran ...

( Cairngorms etc.), UK The Weardale Granite, a

batholith A batholith () is a large mass of intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in Earth's crust. Batholiths are almost always made mostly of felsic or intermediate rock types, such ...

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

. 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 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 ''no ...

* Solway Basin * Kish Bank Basin *

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 * Vale of Clwyd * Cheshire Basin * Worcester Basin * Hampshire Basin *

Cardigan Bay Basin Cardigan may refer to: Music * The Cardigans, a Swedish pop group * "Cardigan" (song), a 2020 song by Taylor Swift from ''Folklore'' * "Cardigan", a 2020 song by Don Toliver from '' Heaven or Hell'' Places * Cardigan, Victoria, a region in Aus ...

St George's Channel Basin ST, St, or St. may refer to: Arts and entertainment * Stanza, in poetry * Suicidal Tendencies, an American heavy metal/hardcore punk band * Star Trek, a science-fiction media franchise * Summa Theologica, a compendium of Catholic philosophy an ...

* Wessex Basin *

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 Po ...

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 ...

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 is suspected to be due to a buried impact crater 40 kilometres in diameter.

Positive anomalies

Positive gravity anomalies are associated with areas where

Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the ...

basement rocks In geology, basement and crystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The baseme ...

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: *

Skye The Isle of Skye, or simply Skye (; gd, An t-Eilean Sgitheanach or ; sco, Isle o Skye), is the largest and northernmost of the major islands in the Inner Hebrides of Scotland. The island's peninsulas radiate from a mountainous hub dominated b ...

* St Kilda * Ardnamurchan *

Rùm Rùm (), a Scottish Gaelic name often anglicised to Rum (), is one of the Small Isles of the Inner Hebrides, in the district of Lochaber, Scotland. For much of the 20th century the name became Rhum, a spelling invented by the former owner, Sir ...

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 The Isle of Mull ( gd, An t-Eilean Muileach ) or just Mull (; gd, Muile, links=no ) is the second-largest island of the Inner Hebrides (after Skye) and lies off the west coast of Scotland in the Council areas of Scotland, council area of Arg ...

Isle of Arran The Isle of Arran (; sco, Isle o Arran; gd, Eilean Arainn) or simply Arran is an island off the west coast of Scotland. It is the largest island in the Firth of Clyde and the seventh-largest Scottish island, at . Historically part of Butesh ...

* Lundy * Anton Dohrn Seamount * Rosemary Bank 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 and Hebrides Terrace seamounts. Gravity anomalies also accompany major lineaments, notably those associated with the Scottish Caledonides and including

Great Glen Fault The Great Glen Fault is a strike-slip fault that runs through the Great Glen in Scotland. The fault is mostly inactive today, but occasional moderate tremors have been recorded over the past 150 years. Location Aligned northeast to southwest, t ...

Walls Boundary Fault The Great Glen Fault is a strike-slip fault that runs through the Great Glen in Scotland. The fault is mostly inactive today, but occasional moderate tremors have been recorded over the past 150 years. Location Aligned northeast to southwest, t ...

, Highland Boundary Fault and Southern Uplands Fault.

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

Negative anomalies

Positive anomalies

See also

References