#hercynian

Lights, Camera, Action!

As a geologist I spend a vast amount of time staring at rocks, whether it be out in the field or in the lab. Whatever I'm doing I always take pictures, as proof of what I've seen, evidence to back up my assumptions or just because a professor has demanded diagrams and I am artistically limited.

Through my education I have come to realise one thing; no matter how good the rocks look the key is lighting. When mapping in the Spanish Pyrenees the best light was at dusk or dawn, when the sun sat lowest in the sky. As you can see it beautifully highlights the cross laminations (bottom photo: laminations produced by the flow of material; the height of each set can indicate water depth and the direction in which the lines dip is the direction of flow) and slickensides (top photo: precipitation of minerals, here calcite, along a fault plane as one piece of rock slides past another) on the Buntsandstein Formation. These rocks represent an ancient braided river that swept eroded material off the Hercynian mountain range 250 million years ago.

As every good scientist knows there should always be a scale as the processes that form these structures are fractalic (they can form on a variety of scales through similar processes and it is their scale as well as their shape that define what caused the structures to form e.g. a rain drop and a river will produce similar structures during flow, the only difference being the scale of the structure formed). In this case I have used my GPS and my finger (it was very early in the morning and I had forgotten my ruler) to define the scale.

Watson

Image Credit: Watson

Cévennes National Park, France

These are the headwaters of a river in the south of France, the Chassezac. The river originates near here, in the area of Cévennes National Park and Mont Lozère.

These rocks are the last remnants of what was once a large mountain range, the Variscan (or Hercynian) Mountains, which stretched from the Appalachians in the U.S. all the way to Eastern Europe. This area in south-central France is known as the Massif Central; made up generally of uplands left over from those mountains.

These are igneous rocks. They’re granites; the weathering pattern of large, rounded boulders is a distinct characteristic of that rock type. They formed during the Carboniferous period, over 300 million years ago, underneath those mountains.

The surrounding rocks are metamorphosed sedimentary rocks which testify to the origin of the granites. As a mountain range was built, the sedimentary rocks in the area were pushed to greater and greater depths, where their temperatures increased as a consequence of the heat within the Earth. Eventually, some of those rocks reached their melting points, leading to the formation of the magmas that would one day be these granites.

The granites were somewhat able to migrate upwards into the heart of the mountains, but erosion is the process that really does the job of getting them to the surface. Similar processes are observed in actively eroding areas of the Himalayas today. After the forces building the mountains went away, erosion took over, bringing the cores of the old mountains up to the surface. As the rocks above the granites eroded, the weight above them decreased, and they began to expand and crack as a consequence.

Granites are difficult for water to erode and so erosion tends to focus along areas where cracks already exist. That’s what forms the rounded-bouldery texture you see here.

The area of Cévennes National Park has been declared a UNESCO world heritage site as an area where the types of villages and agriculture which dominated pre-industrial Europe are well represented and preserved. It contains in-tact villages built of the type of stone you see here and regular migrations of livestock herds between summer and winter pastures, as has happened in the area for over a thousand years.

-JBB

Image credit: Alain Bachellier (Creative commons license share): http://www.flickr.com/photos/alainbachellier/3476011274/

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.