A direct sample of the deep Earth
The Chaîne des Puys is a landscape of young volcanoes, with the most recent eruption happening less than 8,000 years ago. But hidden within the dark basaltic lavas are much older, denser, greener rocks with a far more extreme origin. These are peridotite xenoliths—fragments of the Earth's upper mantle, torn from their home and carried to the surface by ascending magma.
Peridotite is the dominant rock of the upper mantle. The examples found throughout the Massif Central are primarily composed of olivine and pyroxene, giving them a characteristic coarse-grained and greenish appearance. These xenoliths are direct physical samples of the Earth's interior, providing data otherwise inaccessible. Their composition reveals details about the mantle's structure and history deep beneath Europe. Studies show at least two distinct mantle domains under the Massif Central, likely ancient lithospheric blocks that were assembled during the Variscan orogeny, a major mountain-building event that occurred between 380 and 280 million years ago.
A two-stage journey to the surface
The formation of these mantle rocks occurred in two stages spanning hundreds of millions of years. The first stage involved the Variscan orogeny, a continental collision that sliced the Earth's crust and upper mantle, pushing slabs of peridotite up into the continental crust. There, they sat for millions of years.
The second stage is much more recent. Beginning around 65 million years ago, tectonic stretching and a rising mantle plume created the volcanic activity that defines the region today. As magma rose through the crust to feed volcanoes like the Puy de Dôme, it ripped off pieces of the embedded mantle rock and carried them to the surface. The minerals within these xenoliths, particularly certain types of spinel and garnet, are stable only under the immense pressures found at depths of 80 to 200 kilometers, confirming their deep-earth origins. The peridotites found here are mostly lherzolite and harzburgite, which represent different degrees of partial melting within the mantle.