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| Devil's Tower in Wyoming. Ben Stephenson from Cleveland, OH, CC BY 2.0 , via Wikimedia Commons |
If you were watching Close Encounters of the Third Kind this afternoon (I hadn't seen it for maybe 15 years), you would have seen the towering rock formation that's used as a plot device and features in the end of the movie when the huge spaceship appears and the aliens emerge. This is Devils Tower in Wyoming, a laccolithic butte formation, composed of igneous rock.
Examples of igneous rock are granite (which forms the Wicklow Mountains) and basalt (The Giant's Causeway). A laccolith is a type of igneous intrusion produced by a body of molten rock that pushed up through a tube in layers or strata of softer rock such as sandstone, but didn't emerge as a volcano. Instead, it formed an underground dome. Over hundreds of millions of years, the softer rock eroded away from the top and edges, to leave the harder rock at the centre. However, because the laccolith material is columnar and laminar like the grain in wood and similar to the basalt that makes up the Giant's Causeway, steep cliff like sides are left as material collapses around the perimeter. The remaining tower-like formation is known as a butte. Sloped scree accumulations from the eroded material surround the towers. In Ireland we have several features on the landscape that would originally have been volcanoes. They include the Hill of Allen (what's left of it) and Croghan Hill near Daingean in Offaly. As far as I know, andesite, the 455 million-year-old hard rock from Roadstone's Allen quarry was used for the walls and stone landscaping around St Brigid's Well in the Valley Park. Maybe someone can clarify this?
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