#variscan

Rocky House

This house sits along the shore of the Atlantic Ocean in Plougrescant, in the province of Brittany, France. This area contains a large, Variscan age granite pluton – the Variscan orogeny is the mountain range triggered by the collision between the continents that are today North America and Europe. This granite has been quarried in a number of places, but right here there’s a house sitting amongst the rocks.

This house was constructed back in the 19th century prior to building regulations in the area. In this spot it would otherwise be exposed to waves from the ocean, but the builder situated it so that the rocks offered protection from the crashing waves. It is still occupied as a home today.

-JBB

Image credit: Peter Hurford https://flic.kr/p/5TfNFx

THE HERCYNIAN OROGENY: The Dawn of Pangaea

Once upon a time… in the lower Paleozoic (say before around 400 million years ago), there was a scattering of continental tectonic plates upon the face of the Earth. In the mid to late Paleozoic (from ~ 400 to about 300 million years ago), these plates amalgamated to form the super-continent of Pangaea. The geologic event held responsible for this gathering of the continents into one giant continent is called The Hercynian and/or the Variscan Orogeny. An orogeny is an assortment of active geologic processes that work in tandem and result in the building of a mountain range. Today’s ongoing orogenies usually involve two tectonic plates: the plates are in collision (as is India mushing into Asia and building the Himalayas) or one plate is “subducted” beneath the other plate (as for the construction of the Andes above the zone where the Nazca plate is underriding the South American Plate). The Hercynian Orogeny apparently was a more complex affair: essentially the southern continental group (Gondwana) was heading for the northern continental group (given the name Laurussia or Euramerica), but in doing so numerous tectonic fragments were involved, all moving in such a way as to ram one into another. The result seems to be a juxtaposition of continental crusts separated by “suture zones,” essentially the mishmash of material caught up between the continental remnants.

While terminology varies between calling this orogeny the Hercynian or Variscan, the marvelous folds produced within the suture zones are famous as Variscan folding, while the continental granites of this mid-late Paleozoic age are usually referred to as “Hercynian.” (I don’t know why. Some folk are sensitive to terminology and I would rather not argue about it.) Anyway, Variscan folds can be seen in a 3000 km zone extending from Ireland, Portugal, Spain, Great Britain, France and on through the Balkans and Carpathians. Great granite massifs are found in the zone, and perhaps one of the greatest is in Bavaria…

Which brings us to the lovely photo by my colleague Bob Myhill, showing us the Hercynian granite at Plattengipfel, Germany, at sunrise, reminding us of the dawn of a former age at the birth of Pangaea…

Annie R Photo with thanks to Bob Myhill.

As part of my DPhil course we need to do outreach and communication, the blog being one form of this. It was something I was going to do any way so why not start now!  The picture on the header at the top of the page is from Cobble Hole, St Anne’s Head, Pembrokeshire, Wales. Its a pretty stunning example of an type of rock fold called a syncline. 

As ancient continents jostled for position in the rapidly assembling supercontinent, Pangea, rocks buckled and folded, sliding over each other like pages in a bending phonebook. Eventually they snapped and the top of the fold slid along the lower part creating a thrust fault. 

This all happened between 380 and 290 million years ago (Ma) during the Variscan orogeny (Oro = mountain, gen = birth). The sediments that make these rocks are much older though, from the early Devonian period around 400Ma. Wales lay in the southern desert latitudes at that time and these Old Red Sandstones represent an arid landscape threaded by seasonally flooding rivers and inhabited by giant millipedes and scorpions and other strange plants and animals.