On The Geologic History of Ireland

My internet at home has been terrible at best; nearly every time I’ve been home it’s been out. The few times it’s been up, it’s been so slow I can’t even access email. So it’s been hard to make many posts. This afternoon I have my first exam, worth 25% of the final (or 15% of the overall grade). It’s in Sediments, and, inspired by my studying and by the material, I’ve decided to do a brief entry on the geologic history of Ireland. I now know the geologic histories in depth of both Ireland and Wisconsin! The history of Ireland, in my mind, is slightly more exciting than the history of Wisconsin. It helps that it’s not part of a craton though.

In the Precambrian, what is now Ireland was located in Pangaea, though the northern half of the country was separate from the southern half. In the Cambrian, when Pangaea began to break up, the Iapetus Ocean (the precursor to the Atlantic, hence the name) opened. The north lay to one side on Laurentia, the south to the other on Avalonia. During the Ordovician, Ireland was subject to explosive volcanism and significant erosion from huge deltas from mountain-source rivers. The Iapetus Ocean began to close, resulting in the Caledonian orogeny of the Silurian. During the Silurian the land was covered in mountains similar to today’s Himalayas – the range stretched from the southern United States, through Ireland, and finished up in Northern Scandinavia. The collision ended in the Devonian, when the climate became hot and arid. Ireland at that time was covered by expansive deserts with calcrete forming just below its surface.

In the Carboniferous Ireland experienced a range of activity. Giant delta systems once again eroded the land – which by now was flat and almost at sea level (fast uplifting à fast erosion), and there were a series of marine transgressions. The land was covered by swampy rainforests, and significant formations of limestone occurred during the closure of the Tethys Ocean. Today cyclothems can be found in the rock record from that time, either the result of the delta systems or of the glaciation of Gondwana in the south. In the Permian, the continents once again merged to form a supercontinent, and Ireland’s climate returned to that of a desert. Extreme chemical weathering occurred, and numerous evaporates such as gypsum and magnesium limestone were deposited.

Marine transgressions began again in the Triassic, adding to the evaporate deposits. During the Jurassic Pangaea began to break apart, and the marine transgressions continued. Mudstones were deposited in Ireland, and while not dinosaur skeletons have been (or likely will be) found on Ireland, the remains of Ichthyosaurs (!!) have been discovered. The marine transgressions continued into the Cretaceous, when Ireland was mostly covered by a warm, shallow sea. White chalk formed, today seen in only a few locations in Northern Ireland. The same white chalk deposits extended down through Britain, best seen in the white cliffs of Dover. Flint nodules formed in the chalk from sponge spicules, later drawing humans to those areas. In the Tertiary Ireland underwent further extreme changes; in the north volcanic eruptions created the Antrim basalts (including Giant’s Causeway) as the Atlantic began to open. The land was once again underwent an orogeny – the Alpine orogeny. A few rare sedimentary deposits formed beneath Lough Neagh in the north.

In the Quaternary, when South America separated from Antarctica, the world was thrown into a glaciation. By 1.8Ma the earth was fully cooled, and Ireland was covered by glaciers, completely changing the topography once again and almost entirely erasing the landforms of the Tertiary. Following the glaciation, Ireland was connected by a low-lying swampy land bridge to Britain for several thousand years, allowing the re-colonisation of the land. Humans arrived several thousand years after the closure of the land bridge; while they first arrived in Britain some 65,000 years ago, none came to Ireland until about 9,000 years ago.

And that is Ireland’s basic geologic history. Let’s see if I can remember all of it.