The Ophiolites

Having taken a geological journey through the ‘big four’ rocks of the Nantais, these being granite, gabbro, schist and gneiss, there remains just one small group of rocks to cover. These are the ophiolites, a curious category of rocks which start out life on the seabed, and yet end up atop mountain ranges.

Our understanding of ophiolites is relatively recent; just a couple of centuries ago these seemingly misplaced rocks had geologists completely mystified. High up in the Alps, which are largely formed from schist, limestone and gneiss, geologists had discovered thin seams of a dark and heavy rock named peridotite, and next to it bands of shimmering green serpentinite. In 1821 French geologist and paleontologist Alexandre Brongniart (1770 – 1847) christened these strange and incongruous rocks ophiolites, derived from ophio (from the Greek for snake, which reflects the presence of many green rocks) and lithos (from the Greek for stone). Geologists thus had a name for these bands of rock, although they were no less puzzled by their origin.

It was the mid-19th century before the ophiolite enigma was decoded, largely thanks to the advent of deep-sea drilling which permitted direct examination of rock samples from the seabed. The similarities between the structure of the oceanic crust, with its layers of peridotite, gabbro and basalt, and with the Alpine ophiolites could not be ignored. And with an increasing understanding of plate tectonics geologists also had an explanation for how such materials which started out life on the seabed could end up, millions of years later, atop a mountain.

There are many different types, forms and arrangements of ophiolite, but many features are common. Peridotite is a common finding, as is serpentinite. Gabbro and metamorphosed derivatives known, helpfully, as metagabbro, are also commonly encountered features of ophiolite, but as I have already examined this ‘big four’ rock in detail I will not discuss it again here. Amphibolite, a rock with which many Muscadet drinkers will be familiar through the wines of Jo Landron (pictured above), is also documented in association with a number of ophiolites across the planet.

Here, in this final instalment of my guide to the rocks of the Nantais, I take a look at two of these ophiolites of particular relevance to the vineyards of the Nantais, amphibolite and serpentinite.

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