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SERPENTINE

Serpentine from Bonney’s Dyke

Just occasionally it happens that the crunching together of continents brings rocks from below the deep ocean floor right up to the surface. This has happened at two places in the UK – the Lizard in Cornwall and the Ayrshire coast between Girvan and Ballantrae.

One of the strange stones exposed is the pink Trondhjemite of Byne Hill. This is a primordial rock of the oceanic crust, formed at the mid-ocean ridge as the ocean on either side moved outwards. That rock is usually basalt, but can sometimes be Trondhjemite.

At Bonney’s Dyke, the turning point of this walk, appears a stone from even further down. It’s a gabbro, but with gabbro’s usual black crystals separated by a groundmass of off-white feldspar. This rock originally crystallised below the ocean floor. On the beach at Bonney’s Dyke are occasional dark, streaked pebbles, coloured greenish, reddish or yellow and slippery to the touch. This comes from even lower still. It’s the stone called serpentine (strictly, serpentinite), which is not from the earth’s crust at all but is from the next layer down, the mantle, 10km or more below the ocean floor.