Harmless Auld Craters – The Story Of Ireland's Volcanoes
By Mark Dunphy - Wed Jul 20,
The Giant's Causeway on the north County Antrim coast is the best known example of past volcanic activity in Ireland, but the island has a long history of volcanic eruptions.
Ireland's now extinct volcanoes include Slieve Gullion in Co Armagh, Lambay Island in Dublin, Loch Na Fooey in Co Galway, and Croghan Hill in Co Offaly. There also are at least four known volcanic plugs located throughout Northern Ireland.
The majority of volcanic acivity in Ireland occured between 430 and 480 million years ago in the Ordovician age, the second period of the Paleozoic Era.
During the Ordovician, Western and Central Europe were separated from the rest of Eurasia, and were rotated about 90 degrees counterclockwise from their present orientation, and were in the southern tropics. The Iapetus Ocean continued to shrink as the previously passive margins of Baltica and North America converged. Where the Iapetus was, mountains were thrust up, remnant strata of which remain today in Greenland, Norway, Scotland, Ireland and north-eastern North America.
Some 50 to 60 million years ago, during the Paleogene period, County Antrim was subject to intense volcanic activity, when highly fluid molten basalt intruded through chalk beds to form an extensive lava plateau. The extensive fracture network produced the distinctive columns seen today at Giant's Causeway. The basalts were originally part of a great volcanic plateau called the Thulean Plateau which formed during the Paleogene period.
Ireland's Volcanoes
Slieve Gullion in south County Armagh is the eroded remains of a Paleocene volcanic complex. It is surrounded by a ring dyke. Slieve Gullion has been shaped by glaciation and exhibits a classic 'crag and tail' glacial feature. The 'tail', composed of glacial deposits, points south, ending at Drumintee.
Located between Finny village in Co Mayo and the Galway village of Leenane, the area around Loch Na Fooey is renowned as the site of the ancient "Finny volcano" (480 million years ago), formed as the Iapetus Ocean closed to bring the two halves of Ireland together. The volcano's landform is now gone but some of volcanic rocks are preserved in the area, including pillow lavas, and breccia. There is a small funnel-shaped island close to the south-eastern shore known as Red Island (An tOileán Rua).
Lambay Island in Dublin also is a subduction related volcano, also formed in the Ordovician age.
Describe on the Croghan National School website as a "harmless auld crater", Croghan Hill is the remains of an extinct volcano and rises from the Bog of Allen in the midlands of Ireland in County Offaly. It is said to have last erupted around 200 million years ago.
The Hill of Allen is a volcanic hill situated in the west of County Kildare, beside the village of Allen.
An Irish mountain commonly mistaken as an extinct volcano is the Great Sugar Loaf in west Co Wicklow, which is an erosion-resistant metamorphosed sedimentary deposit from the deep sea.
Evidence of volcanic activity during the Ordovician period may also be seen today in Arklow Head, Co Wicklow, and along parts of the Waterford coastline. Volcanic rocks can be found in both counties, emanating from erupting submarine volcanoes.
Volcano Plugs
There also are at least four known volcanic plugs located throughout Northern Ireland, including Scrabo near Newtownards, Co Down as well as Slieve Gallion, Slemish and Tievebulliagh.
Slieve Gallion in County Derry was formed formed from volcanic-derived igneous rock, and also granite on the southern slopes.
A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when magma hardens within a vent on an active volcano. One of the most famous volcanic plugs in Ireland and Britain is probably the one on which Edinburgh Castle is built.
Slemish is the remains of the plug of an extinct volcano. Its distinctive appearance —its upper reaches are very steep and rugged, in contrast to the tidy fields on its lower westward-facing slopes and the relatively flat bogland to the east— causes it to dominate the landscape for miles around.
Tievebulliagh in the Glens of Antrim also is formed from a volcanic plug, the intense heat generated by molten basalt has given rise to the formation of a durable flint, porcellanite, which is found at the foot of the eastern scree slope of the mountain. Three small outcrops of porcellanite can be seen on the higher south-east slope.
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