More on the Geological Mystery

The Diary finds it fascinating to watch scientists at work as they tease out possible answers to the mysteries that Nature sometimes throws up. Our local geologists started off by thinking that the strange golf-balls, bread rolls and coral-like structures found in an Ormsaigbeg excavation might have originated in a warm, shallow sea about 130 million years ago. But, as it became apparent that the structures were formed of silica rather than calcium carbonate, they moved towards a formation during the great Ardnamurchan volcanic eruption some 60 million years ago, and likened the place where they were formed to Yellowstone in the USA - see earlier Diary entry here.

Rob Gill has led this research. Rob runs Geosec Slides, a small, high-tech business based in the crofting township of Achnaha. He believes the structures are 'concretions' formed in hot pools of silica-rich water, where layer upon layer of silica was deposited on a 'core' of a small, gravel-sized chip of rock.

Rob has made microscope slides, and what he has found strongly supports his argument. The green in the middle of the slide is the basalt, the greeny-grey material around it is silica.

This photo shows the structure even more clearly.

Rob writes, "You can see the mass of feldspar laths, which is all that is left of the original mineralogy in the Ormsaigbeg basalt nuclei. This is what I would expect to see as the feldspar is the most stable of the minerals in the basalt, the olivine (large brown areas) has already started to break down, the pyroxene (pink) will be next, and chlorite (green), which is a secondary mineral not originally present when the basalt was erupted, is starting to develop.

"This is strong evidence that the concretions date from the Paleogene (Tertiary) volcanic era, and have nothing to do with the earlier Triassic/Jurassic/Cretaceous sedimentary rocks. I cannot think of any other process other than hydrothermal sinter deposition that could account for what we are seeing. I have also sectioned a 'pipe' from Ormsaigbeg. This does not show a core nucleus, which surely, is what one would expect."

Many thanks to Rob for slide photos and explanation.

Geosec's website is here.