Ocean Floor at the Lizard

Why this rock is strange

Most of Britain is made of rocks that formed on or near a continent. The Lizard is not. The rock here is serpentinite, and serpentinite is what you get when you take the rock of the upper mantle — peridotite — and hydrate it. Water gets into it along fractures, reacts with the olivine and pyroxene, and converts them into serpentine minerals. The result is a rock that is softer, greener, and often veined with pale material, and that has no business being on the surface of a continent at all.

The whole Lizard peninsula is a fragment of oceanic crust and upper mantle, called an ophiolite, that was caught in the collision between two continents and pushed up onto the edge of one of them. That is why the geology here is upside down in the ordinary sense: you are walking on material that should be kilometres below the sea floor.

What serpentinite looks like

It does not look like Cornwall. It looks like something between jade and soapstone, and it varies enormously over a few metres.

The pale veins are almost always the same material in different form: fluids moving through the serpentinite deposited quartz, calcite or talc in the fractures. Talc gives the soft, greasy feel that gives serpentine its soapy reputation.

Logging tasks

Send answers to the four questions below through the message centre. Do not put them in your log. A photograph of you at the headland is welcome but not required.

1. Find a piece of the true serpentinite and a piece of the pale-veined material. Describe the difference in texture with your eyes closed as well as with them open. Which feels greasier?
2. On the headland platform, find a surface showing the veins running through the green rock. Describe the angle between the veins and the general banding, and say whether the veins cut across the banding or run with it.
3. Explain in two or three sentences why rocks of the upper mantle are rarely seen at the surface of continents, and what made it possible here.
4. Serpentinite weathers to a distinctive soil. Describe the soil you can see in any hollow or path edge nearby, and give one reason why the plants growing on it are smaller than those on the surrounding schist.

Access and safety

• The headland platform is unfenced and the sea reaches it in a swell. Do not go onto the wet lower rocks.
• Serpentinite is slippery when wet even though it does not look it. The coast path here has polished surfaces on the slopes.
• The Lizard is a National Nature Reserve and a Site of Special Scientific Interest. Hammering or removing rock is prohibited; the site is regularly patrolled.
• The chough population here is re-established and nesting. Keep back from the cliffs and do not approach nests.

After the find

Kynance Cove, three miles north, is the best place on the peninsula to see the serpentine and the layered schists together, and it has the clearest view of the banding. The Lizard Point lighthouse and the Marconi museum both repay the short detour.