The Alps come from the meeting of two converging tectonic plates, the African and the European, the latter passing under the other causing it to rise and deform. This phenomenon of plates coming together, called subduction, begins around the middle of the Cretaceous (around -100 Ma) and corresponds to the disappearance of the Alpine ocean which existed between the two continents.
The opening of the Atlantic Ocean continues and the continents collide. The subalpine ranges emerge from the water around -30 Ma. The Mont-Blanc massif then approaches that of the Aiguilles Rouges. The Alpine domain was increasingly compressed in the Oligocene (around -28 Ma) and a slow sinking of the earth's crust took place.
The subalpine mountains reached their current altitude around fifteen million years ago. Between approximately -6 and -3 Ma, the Jura folds and gains height as do the massifs of Mont-Blanc and the Aiguilles Rouges.
Magmatic intrusive rocks are formed by partial melting of the crust (granitic magma) and the deeper crust as well as the underlying upper mantle (gabbroitic magma). If the cooling takes place deep below the earth's surface, the rock is called plutonic.
These rocks consist of an interlocking mass of mineral crystals. At a depth of 65 kilometres, the temperature is almost 1,200 °C, but due to the enormous pressure, the rock does not melt. When a crack above the rock releases some of the pressure, some of the rock liquefies and the pressurised magma rises to the surface, cools and solidifies into new rock.
When the rock cools far below the surface, it cools slowly so that coarse crystals can grow during the solidification process - crystals such as light-coloured quartz and feldspar and black hornblende or mica. The slower the cooling of the magma, the larger the crystals become.
These crystals give the rock a grainy, uniform appearance. This is granitic rock - whether it is granite, granodiorite or another related rock depends on the relative proportion of the individual minerals. A good example of such a rock is the Mont Blanc granite.
Microgranular magmatic enclaves are often components of plutonic rock bodies and can be the result of mixing of mafic and felsic magma. These enclaves often have an ellipsoidal or elongated shape.
Mafic (dark) enclaves show that they have the same compositional characteristics as the plutonic bodies in which they occur, but were not formed at the same time and place beneath the Earth's surface.
The relationship between these enclaves and the plutonic bodies in which they occur is probably due to interaction, in particular differential inter- (mutual) diffusion (not mixing !) between two originally independent magmas from an original, so-called parent magma.
Mafic and felsic magmas do not mix to form magmas with an intermediate composition. Rather, they form coherent units (enclaves). Their respective mineralogical composition does not change.
Because mafic or gabbroitic magmas form at a higher temperature (~1,200 °C), they tend to be less viscous and more mobile than the felsic magma they intrude, which has a much lower temperature (900-1,000 °C).
If the cooling rate of the felsic magma is slow and the felsic magma is mobile, mafic dykes can break into small ellipsoidal enclaves or angular blocks if the mafic magma is too viscous. The mafic enclaves can line up to form a magmatic fabric. Alternatively, mafic magma can intrude and rise as a bubble through the felsic magma to form mafic enclaves. The dispersion of mafic enclaves into felsic magma is called magma mixing.
► Glaciers and erratic rocks
The current reliefs of the Alps are the result of the erosion of the glacial phases of the Quaternary era (the last two million years) which attacked a relief which had existed for several tens of millions of years (Ma). Monthey is located on the ancient Rhône glacier, which disappeared 15,000 years ago, which left behind numerous lateral moraines rich in large blocks called erratic blocks. They are very often granite and come from the Mont-Blanc massif.
The granite blocks of the Monthey moraine were exploited during the 19th and 20th centuries. The success was immense and today we find this rock in many walls, sidewalks or buildings in the city. Another amazing work of the quarrymen, the two columns of the peristyle of the church of Monthey which were cut in one piece into the rock.
SOURCES
Eglise de Monthey - Church of Monthey
Géologie Mont-Blanc - Geology Mont-Blanc
Géologie des Alpes - Geology of Alps
Granit du Mont-Blanc - Granite Mont-Blanc
Blocs erratiques de Monthey - Erratic stones in Monthey