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176 km |
Mountains |
The Vosges a part of the Ring of Fire |
This year’s Tour de France brings the peloton back to the Vosges, one of the main crystalline massifs of France. These massifs formed the upper ‘Armorican’ plate above the subduction zone that eventually closed the Rheic ocean and that eventually led to the formation of the Ardennes mountain belt, between 335 and 300 million years ago (see yesterday’s blog). But during the oceanic subduction stage, the Vosges massif was located above a subducting oceanic plate, or ‘slab’ as we call them. And those locations are famous for their volcanism, as we see in the Ring of Fire, around the Pacific Ocean.
The Ring of Fire: How volcanoes form due to subduction
Oceanic crust forms at mid-oceanic ridges, where two plate spread apart, the Earth’s mantle below the plates rises up, melts (in part, about 20%), and the melt percolates upward to fill the gap forming new crust. During that process, the magmas interact with sea water, and that sea water gets incorporated in the minerals of the crust. Moreover during its lifetime, sediments that contain water form on the ocean floor above the crust. But over time, oceanic crust becomes denser than the underlying mantle, and sooner or later, it will sink back into the Earth’s mantle - the process we call subduction. During its descend into the mantle, the sediments and the magmatic rocks of the oceanic crust transform into denser minerals, and the water they contained is released into the mantle (a process called ‘dehydration’) above the downgoing ‘slab’.
When this happens at a depth of about 100 km or more, that water leads to (partial) melting of the mantle above the slab. At a depth of about 150 km, the last water-containing minerals release their water, and as a result, magma forms in a narrow zone where the downgoing slab is about 100-150 km deep. Those magmas arise up and form large magma chambers in the upper plate (so-called plutons, with 1-10 km diameter, and a lot of plutons together form a batholith of 10’s-100’s km dimension), and an array of volcanoes. Those volcanoes are known as a ‘volcanic arc’, and these are typically located about 150-300 km (depending on the angle at which the downgoing plate dips into the mantle) from a subduction trench, on the upper plate. Take your Atlas and have a look at the volcanoes around the Pacific, or in the Lesser Antilles, and you’ll see that these rules of thumb work pretty well.
Similar to the Andes
The Vosges are part of such a batholith, with plenty of plutons of ~350-330 million years old. Much of the Vosges contains crystallized magma chambers, which cooled slowly, 
allowing for large (cm-scale) crystals to grow, together forming granite rocks. The famous ‘Ballons’ of the Vosges are in those granites. The Planche de Belles Filles, however, is located in volcanic rocks: the volcanoes that formed the ‘Ring of Fire’ during the subduction of oceanic crust just before Avalonia collided and the Ardennes formed. Volcanoes and plutons are important sources of metal ores, and mining in the Vosges dates back as far as the Bronze Age! (Figure). Plutons and volcanoes like in the Vosges are well-known from similar settings like southern Tibet (see yesterday’s blog), and are actively formed today in places like the Andes! The Colombian and Ecuadorian riders in the peloton may feel right at home today!
I am a geologist and I study plate tectonics and the driving mechanisms in the Earth’s mantle, mountain building processes, and the geography of the geological past. I enjoy geological fieldworks all over the world, and translating the results to science and to a broad public. Read more about Douwe.
Douwe van Hinsbergen