The rare and unusual lherzolite in the Ariège Pyrenees

Geologists have a field day at the l'étang de Lherz, where this magmatic rock was pushed out of the earth's crust during the formation of the Pyrenees.

The étang (lake) de Lers, in the canton of Massat, draws many visitors by virtue of its beautiful setting, as the point of departure of numerous walks in the high mountains, for its cross-country ski domain and the mountain grazing pastures surrounding it where cows, horses and sheep spend the summer months. It is also a little paradise for geologists and mineraologists who come from far away to see the famous Lherzolite, a magmatic rock belonging to the family of peridotites.

Its characteristic colour is rust brown, mottled on the surface. But that is where one can best distinguish the three principal minerals of which it is composed:

- the emerald green pyroxene
- black spinel
- olivine which imparts the olive green tint
Freshly broken, the surface is greenish.

Lherzolite was discovered and defined in Ariège 200 years ago on the site of the etang de Lherz (today called Lers) for which it was named.

It's a granular rock that cristallised deep in the earth in the zone called the superior mantle, below the crust. Such a rock from so deep in the earth can never be observed directly; powerful techtonic movements that formed the Pyrenees pushed it up through the earth's crust to the surface in fragments.

—Didier Fert