Geology of Ireland

Source: Wikipedia, the free encyclopedia.
Bedrock geological map of Ireland.
Upper Carboniferous (Namurian) sedimentary rocks, Loop Head
, County Clare

The geology of Ireland consists of the study of the rock formations on the island of Ireland. It includes rocks from every age from Proterozoic to Holocene and a large variety of different rock types is represented. The basalt columns of the Giant's Causeway together with geologically significant sections of the adjacent coast have been declared a World Heritage Site. The geological detail follows the major events in Ireland's past based on the geological timescale.

Timeline

Slieve League in western Ulster

The oldest known Irish

Inishtrahull Island off the north coast of Ulster.[1][2] Outcrops at Annagh Head on the Mullet Peninsula are almost as old.[3] In other parts of Donegal, scientists have discovered rocks that originated as glacial deposits, demonstrating that at this early period, part of what was to become Ireland was in the grip of an ice age. However, because of the effects of later upheavals, it is almost impossible to sequence these early rock layers correctly.[4]

About 600 million years ago, at the end of the Precambrian super-eon, what would later become the Irish landmass was in two parts, with one half on the north-western side of the Iapetus Ocean in Laurentia and the other on the south-eastern side in the micro-continent Avalonia, both at a latitude of around 80° South, close to what is now northwest Africa.[5] From the evidence of the Oldhamia fossils found at Bray Head in Leinster, both parts of Ireland were below sea level at this time.[6]

Over the next 150 million years, these two parts moved towards each other, eventually uniting about 440 million years ago.

freshwater mussel fossils and the footprints of a four-footed amphibian preserved in slate on Valentia Island in Munster. Old Red Sandstone also formed at this time.[7]

Between 400 million and 300 million years ago, parts of

tectonic movement which saw Ireland drift further northward. The resulting pressure created those Irish mountain and hill ranges that run in a northeast to southwest direction.[8]

Mesozoic

Karst landscape in the Burren

By 250 million years ago, Ireland was at the latitude of present-day Egypt and had a desert climate. It was at this time that most of the coal and sandstone were eroded. The thinner layers of limestone in the south of the country were also partially affected by this erosion. The limestone that was exposed by the disappearance of its sandstone mantle was subject to solution by weakly acidic water resulting in a karstic landscape that can still be seen in the Burren in County Clare.[9] Shortly after this period, organic debris in the seas around Ireland began to form the natural gas and petroleum deposits that now play a role in the economy of Ireland. Then, about 150 million years ago, Ireland was again submerged, this time in a chalky sea that resulted in the formation of chalk over large parts of the surface. Traces of this survive under the basalt lava that is found in parts of the north.[10]

About 66 million years ago, the

volcanic activity that formed this lava began.[11] The Mourne Mountains and other mountains in the northern part of the island formed as a result of this activity.[12] Climatic conditions at this time were warm and vegetation thrived. Vegetable debris in the Antrim Depression formed deposits of brown coal or lignite which remain untouched down to the present time.[7] The warm conditions produced high rainfall that accelerated processes of erosion and the formation of karstic landscape forms.[13]

Cenozoic

By 25 million years ago, Ireland was close to assuming its present position. From then on, a long period of erosion resulted in considerable

flora and fauna that would millions of years later be familiar to the first human inhabitants began to emerge. The present landscape of Ireland had more or less formed.[14] Before the Quaternary glaciations affected Ireland the landscape had a thick weathered regolith on the uplands and karst in the lowlands. Pre-Quaternary relief was more dramatic than today's smoothed landforms.[15]

Since about 1.7 million years ago, the Earth has been subjected to a cycle of warm and cold stages and these have, inevitably, affected Ireland. The earliest evidence we have for this effect comes from the period known as the Ballylinian Warm Stage, some half a million years ago. At this time, most of what are now considered to be native Irish trees were already established on the island. The action of the ice during the cold stages was the major factor in bringing the Irish landscape to its current form.[7][16]

Obvious impacts of the

glacial valleys such as Glendalough in Wicklow and of corries, or glacial lakes. The depositing of mounds of debris under the melting ice created drumlins, a common feature of the landscape across the north midlands.[17][18] Streams also formed under the ice and the material deposited by these formed eskers (Irish eiscir). The greatest of these, the Esker Riada, divides the northern and southern halves of the island and its ridge once served as the main highway connecting the east and west coasts.[19] About one half of the coastline consists of a low-lying dune pasture land known as machair
.

Rocks and soil types

Layers of siltstone, shale and sandstone can be seen in the Cliffs of Moher, near Doolin in Munster

The large central lowland is of

Thulean Plateau formed during the Paleogene period.[22]

The soils of the north and west tend to be poorly drained

gleys, including peaty podzols
. In contrast, in the south and east the soils are free-draining brown earths and brown and grey-brown podzols.

An unusual environment is present in north Clare, in

porous rock surface, and extensive cave systems have been formed in some areas. Doolin Cave is the site of one of the world's longest known free-hanging stalactites.[23]

References

  1. ^ "Site Synopsis (Inishtrahull)" (PDF). National Parks and Wildlife Service. Archived from the original (PDF) on 2009-03-27. Retrieved 30 January 2008.
  2. ISBN 0-632-03656-7
    .
  3. JSTOR 30002311
    .
  4. ^ Hull, Edward; Nolan, Joseph; Cruise, R.J.; M'Henry, Alexander (1890). Memoirs of the Geological Survey (PDF). Dublin: Lord Commissioners of Her Majesty's Treasury. pp. 15, 21. Retrieved 3 February 2016.
  5. ^ Hill, Jon; Davis, Katie (November 2007). "Precambrian History of England and Wales". GeologyRocks.com. Archived from the original on 7 December 2007. Retrieved 23 January 2008.
  6. ^ "Fossil at Bray head". Museum.ie. Retrieved 4 February 2016.
  7. ^
    ISBN 0684020947
    .
  8. ISBN 9516909213
    . Retrieved 31 August 2015.
  9. ^ "Landscapes for living!". European Landscapes. Geological Survey of Ireland. Retrieved 11 January 2008.
  10. ^ "The Ulster Chalk". Geological Survey of Ireland. Archived from the original on 5 October 2015. Retrieved 31 August 2015.
  11. ISSN 0009-2541
    .
  12. ^ "Northern Ireland – Living World". BBC. September 2004. Archived from the original on 2012-04-07. Retrieved 2013-11-22.
  13. ^ Sanders, Ian (2007). Six Common Kinds of Rock from Ireland (PDF). Department of Geology, Trinity College Dublin. p. 36.
  14. ^ Lloyd Praeger D.Sc., Robert (1934). The Botanist in Ireland. Dublin: Hodges, Figgis & Co. p. 13.
  15. ISBN 978-94-6239-219-9
    .
  16. ^ "GSI geology and cuisine". GSI. Archived from the original on 22 September 2012. Retrieved 4 February 2016.
  17. ISBN 1-85728-054-7
    .
  18. ISBN 0-19-821737-4
    .
  19. ISBN 0-7735-1817-7
    .
  20. Department of the Environment, Community and Local Government
    . Retrieved 22 November 2013.
  21. ^ "Giant's Causeway and Causeway Coast". Unesco World Heritage Sites. Retrieved 23 January 2008.
  22. ^ Brittle tectonism in relation to the Palaeogene evolution of the Thulean/NE Atlantic domain: a study in Ulster (Subscription required) Retrieved on 10 November 2007
  23. ^ Deegan, Gordon (27 May 1999). "Blasting threatens future of stalactite". Irish Examiner. Archived from the original on 2011-06-29. Retrieved 23 January 2008.

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