Arklow Head, County Wicklow
This breccia is formed from a combination of shattered rock fragments and ash resulting from an explosive volcanic eruption that occurred around 475 million years ago.
This part of Ireland was formerly situated on a volcanic island-arc, created by converging plate boundaries, similar to Japan today. The volcanism associated with these terranes tends to be explosive, although lava flows are not uncommon. Evidence of this volcanism can be seen today extending from County Waterford all the way to the English Lake District.
The pyroclastic rocks of Arklow Head were later intruded by a large igneous body (pyroxene diorite) which produced spectacular columnar jointing as it cooled.
The rocks from the Roadstone Wood Parnell Quarry at Arklow Head, County Wicklow are extracted for aggregate and have also been exported extensively as rock armour to protect coastlines from erosion (www.roadstone.ie).