Skip to main content

Geological Road Trip: Volcanic landscapes of the Massif Central

Geology and geography are fundamental to archaeologists in understanding the landscape contexts that people of the past lived within. While climate and environments have drastically altered over the time span of the Palaeolithic, the topography often, on a broad scale, remains relatively similar. Erosion can be extensive, river systems can change course (the Thames used to flow much further north than it now does for example), and the great depth of sediment accumulation in some areas changed local situations. But the big stuff made of rock like plateaux, mountains and watersheds have remained relatively static over the time hominins have been around. There are exceptions to this however, primarily in the form of volcanism and tectonic action, and the region I'm working in is a textbook example. Here in the Massif Central, there is a long history of volcanic action of many types, the most recent of which occurred less than 5000 years ago- well within the history of human settlement here.
In order to help orient ourselves within this dynamic landscape (at least, dynamic on geological timescales!), some of the team working here went for a little Rocky Road Trip, led by Jean-Paul Raynal, the research director. Here's some photos from our trip, embedded in a matrix of geological facts (yes, that was a nerdy geology joke).




The Massif Central is essentially a big uplifted region that was created along with a lot of other high areas within Europe and the Americas during what's called the Variscan Orogeny. This is a period of extensive mountain-building and other tectonic action that happened between about 380-280 million years ago during the course of the creation of Pangaea, one of the 'super-continents' that the Earth's system of tectonic plates periodically creates (over extremely long time scales). When some of the smaller plates that then existed were pushed together, the oceans separating them disappeared and they collided, forcing the crust to rise and deform, and granite from below the crust to rise up, creating highlands and mountains. Since the Variscan Orogeny, Pangaea of course split up and the separated continents we know today gradually took their current configuration. The granites and metamorphic rocks that formed the foundations of the Massif Central over about 100 million years are only the beginning however.

The Massif Central: pretty huge, it covers a large percentage of France. Image: by Technob105 (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) via Wikimedia Commons
Much later (from about 66 million years ago) the Alpine Orogeny took places when the African plate began moving into the Eurasian plate, and the huge pressures created the Alps and the Pyrenees, through massive folding and thrusting events (think of how a tablecloth concertinas when you push it over a smooth table). All this stress within the crust led to huge fissures opening up to the east of the Alps along with the formation of volcanoes, and the violent geological reaction to the Alpine Orogeny has been continuing ever since. Although quiet for the last few thousand years, this is highly likely to be a temporary phase- over geological timescales, there has essentially been no break in volcanism in this region.

Sancy Massif, part of the Massif Central to the north-east of where we are staying.
 
Puy de Dome, one of the Chaîne des Puys group of volcanoes that are further to the north-east than the region I'm in. This 500m high volcano was formed in successive stages, with a large dome forming first, which was then partially destroyed by a massive explosion. Following this, new lavas filled the blasted out flank, which you can see on the right hand side is a different, more sloping shape.
 Our field trip included looking at the broader landscape form, as well as specific interesting features related to various types of volcanism. There is a distinct split between the landscapes of the Haute Loire area, and the Ardèche further south. In the former, there are large areas of flat uplands and plateaux, many enormous lava flows. There are some valleys that have been cut down by river action, and many hills that are actual volcanoes. Additionally, there is a huge number of craters in this area, including many filled with water. The craters are called Maars, and were created during extremely violent explosions where the rising hot magma made contact with groundwater, and the resulting expanding steam was destructively released.
Following are photos of the Haute-Loire region, you can see the overall raised but not extreme relief of the landscape.

Looking north towards Le Puy Basin from near field station

Looking south towards Le Puy basin from St Pierre-Eynac

Looking west into Le Puy basin from near field station (plus adorable goats, the brown one is the herd leader with a bell, the others follow when they hear it clanging!)

Looking south-east towards the Mezenc plateau from near the field station

The Deves, largest basalt plateau formation in Europe, taken from near Rond du Barry cave

Looking south from near St Anne cave, the Deves plateau is just at right

View north-east into the Haute-Loire from the flanks of the Mezenc
 
Mont Mezenc, a huge formation of lava; on the left of the slope you can see an abrupt end to a large lava flow that stopped due to its high viscosity.

Mont Mézenc is the highest point of a large watershed that separates the high plateau of the Haute-Loire from the steeper landscape on the other side, which is the Ardèche. All the water which falls on this southern side ends up in the Mediterranean, while that which falls on the northern Haute-Loire side ends up in the Atlantic. This is also the area of the very famous source of the river Loire, at Mont Gerbier de Jonc. This is actually a huge tourist attraction, it was heaving with people when we drove through, despite being little more than a steep hill you can climb with several competing "true" sources at a fountain and farm below. I found it very interesting that there is nothing like that for any of the sources of UK rivers, there must be a very different cultural perception in France.

Mont Gerbier de Jonc, the hill with the source of the Loire beneath it (round to the left). There was a chain of people climbing it.

The amazing view from the watershed of the Mezenc massif; looking south-east towards the Rhone Valley and the Alps.
Scroll right for full length of photo above! >>>>
This very high region was a pass used in prehistoric times to access the Massif Central from the east and south; there was a Mesolithic site not far from where this photo was taken, and people in the Palaeolithic must also have been using this high access route judging by the patterns of stone tools that were transported from known sources on this side of the watershed to the other side.

Some nice volcanic formations, looking in same direction as previous photo.

Same place in landscape, looking around due south, along the flank of the Mezenc massif.

Looking north-east from the flanks of Mont Gerbier de Jonc (just left); the huge lava scree slopes on the hill in front are very clear.

Team stop by the side of the road for a quick look at phonolite section- a type of lava with vertical cracks. Hammers at the ready!

Heading back up to the Mezenc massif, with Mont  looking very high indeed.

Looking roughly south-west wards from St-Clement village, which sites on top of a massive layered lava flow with fantastic views.

Scroll right again for full image size >>>>



Another team stop for some geology- it was raining and a big thunderstorm was underway

This is what we were looking at: a 'river' of broken lava, formed in blocky shapes during cooling, and then eroded by the action of ice into this big stream down the slope of a hill. There are larger and more famous examples elsewhere in France, but this was cool.

At first glance not entirely obvious because of it's size, this photo shows a huge volcanic crater from one of the very violent phreatomagmatic explosions (where magma met water underground). You can see the slopes of the crater edge running across the centre of the image, and the very flat central area is where the vent collapsed, filling with sediments. In many craters like this one, there is a boggy area, here called "narces".


This is another Maar crater, formed from a huge explosion, but in this case there is now a lake present. It was raining so this photo isn't too great, but there was an eerie flatness and perfection to the circular margins of the lake.

The Massif Central region is therefore interesting archaeologically because it has a great diversity of topographic forms, which you need to think about for studying how people were exploiting and moving through landscapes- especially when they had to do it all by foot.
Additionally, we know that episodes of volcanism were occurring during the Pleistocene (about 1.8 million years ago to about 10,000 years ago) when Palaeolithic people were living here (and later), so this is one of the few contexts in Europe where we are certain that people experienced the effects of massive changes in landscapes caused by volcanism. This included the huge cone-like volcano of Sancy, eruptions from beneath lakes in the Le Puy basin, the formation of the vast Devès plateau from an 80 km chain of more than 100 volcanoes and vents until about 700,000 years ago, and the creation of the Chaîne des Puys around 70,000 years ago. Following this, there was yet more localised explosions and accumulations over the past tens of thousands of years, with the most recent events probably in the Bronze Age.

So I hope you enjoyed the tour, here's a final photo of some more threatening clouds from bad weather we had the past weeks, it gives a little feel of the dramatic volcanic clouds that must have been in the skies many times during the human history of the Massif Central.





Comments

Popular posts from this blog

Wherefore Art Thou, Neanderthal?

Adventures in Silcrete: "It's flint Jim, but not as you know it!"

Something that everyone who works in the archaeology of deep prehistory has to get to grips with is the technology of stone tools, or lithics. This includes thinking about the ways in which people made their tools, which techniques they chose to use, etc. It also means that Palaeolithic archaeologists, alongside needing to know stuff about climatology, palaeontology, and ecology, need to delve into the science of geology. People in prehistory might not have understood the origins of different kinds of rocks, but they certainly appreciated the diversity in stone qualities, not only between very different rock types but also within geological/mineral categories.


These two Neandertal tools that I studied for my PhD, called handaxes, are both very finely worked, but made from completely different rocks. The one on the left (Castle Lane, Bournemouth) is made from Cretaceous flint found in the south and east of Britain, and the one on the right (Coygan Cave) from rhyolite, a volcanic stone…