After the adventures in Italy and Switzerland it was time to start the fieldwork part of my trip in Briancon. Part of this involved a visit to the Chenaillet Ophiolite in the French/Italian Alps.
To think about what an ophiolite is we first need to go back to some basic geology…
The outer layer of the Earth is made up of numerous large pieces which can move relative to one another in a theory known as plate tectonics, we use this to explain many of the phenomenon that we observe on the Earth. There are two types of crust making up the outer layer of the Earth; so called oceanic and continental crust. Oceanic crust is thinner, yet denser than continental crust.
Oceanic crust is created at Mid ocean ridges. These are essentially underwater volcanic mountain chains where the erupted material produces new crust. They mark the boundary between different plates, and are referred to as a divergent boundary as the plates are moving away from one another.
The difference in density between different crustal units is crucial to allow a process called subduction to take place. Subduction occurs when two plates move towards each other and the denser of the two goes underneath the less dense plate. Subduction zones are the opposite of mid-ocean ridges in that they are a convergent boundary, meaning that the plates move towards one another. As a result of subduction oceanic crust is essentially destroyed, as it is sent back down into the Earth.
When we observe rocks which have the characteristics of the oceanic crust on top of continental crust we call these abnormalities ‘ophiolites‘. The word Ophiolite apparently literally translates to ‘snake rock‘ referring to the similar aesthetics of the rocks found in ophiolites and snake skin. Ophiolites have taught us a lot about oceanic crust and tectonics but there is a catch; if normal oceanic crust is subducted back into the Earth then for ophiolites to have been preserved must mean that they do not represent “standard” oceanic crust.
So oceanic crust is created at mid ocean ridges and destroyed at subduction zones, but how did we end up with a segment of oceanic crust high up in the French Alps? Why was this segment of oceanic crust not subducted?
How do we know that the rocks here in the Alps are in fact oceanic crust?
Firstly, we have got abundant outcrops of Peridotite. Peridotite is a ultramafic igneous rock (contains less than 45% silica and solidified from melted rock). Peridotite is the main rock type in the upper mantle and it wouldn’t just form here, it needs to have been produced elsewhere and put here.
Another major piece of evidence is that the peridotite has undergone a process known as serpentinisation. This is a chemical reaction which occurs through contact with sea water, and has produced many of the minerals in the fragment shown above.
The photo above shows that some of the peridotite has been reworked and incorporated into sedimentary rocks (rocks made up of fragments of older rocks). The darker parts are the peridotite and the lighter surrounding material is limestone. Limestone of this type is produced under the sea which is further evidence towards the oceanic origin of these rocks.
Finally at the Chenaillet Ophiolite we observe large outcrops of pillow lavas. Pilow lavas are volcanic rocks which were produced underwater, providing further evidence towards the oceanic nature of these rocks.
How did these rocks get here?
These observations of rocks which seem out of place confused early geologists, and it wasn’t until the theory of plate tectonics was established that these suits of rocks were understood to represent small fragments of the oceans attached to the continents.
The process of ophiolite emplacement is termed obduction, and is defined as the overthrusting of oceanic material on top of continental. Obduction occurs at convergent tectonic boundaries (i.e. where two plates are moving towards one another), this explains why we often find ophiolites incorporated into the mountain chains produced at these boundaries.
Numerous mechanisms have been proposed to explain why obduction occurs rather than the expected subduction but they often struggle to be applied to all cases. It is therefore generally regarded that there is no singular mechanism responsible for all ophiolite occurances, and that more work is required to understand the formation of ophiolites…