A Yellowstone at Kilchoan

There's great excitement here with some real progress made in solving the mystery of the strange geological feature at Kilchoan Campsite (see earlier post here). But, if anything, the new explanation is even more amazing than those that have gone before.

The rock which contains the ball-, mushroom- and bread roll-like structures is seen at the centre of this picture, with the limestone below and the darker, rust-stained layers of igneous rock above. The structure which contains the nodules can be seen poking up into the igneous rock.

This picture shows one of the 'golf balls'. The progress in solving the mystery has come from Rob Gill, who runs a small business at Achnaha, Geosec Slides, which cuts thin sections from rocks, enabling them to be studied under a microscope. Rob has managed to section two of the 'balls'. His report follows.

"When cut and polished the 'balls' show a vague concentric structure nucleated on a dark green angular fragment. The fragment I take to be altered basalt. More tests to confirm this are in hand. The remainder of the 'ball' is nearly pure silica.

"Assuming that the core is basalt, this provides a likely date for the balls, somewhere in the volcanic era and after the Triassic/Jurassic/Cretaceous sedimentary layers in which the balls are found. The lack of any carbonate in the balls adds extra weight to this.

"It is well known that on Mull, after some initial lava eruptions, a hydrothermal episode was present. The central lavas were 'altered', so some of the minerals in them were changed into different ones by the actions of hot ground waters, the outer lavas being unaffected. It is tempting to speculate that the centre of Mull may have resembled Yellowstone Park or Rotorua. Subsequent eruptions on Mull obliterated any evidence of this as the central volcano grew.

"Water at high temperature and pressure can dissolve silica (quartz), and as these hydrothermal waters reach the surface cooling takes place and the silica is precipitated in the form of colloidal opal, also known as sinter or geyserite. As time passes the opal crystallizes into a more stable form of silica called chert. This is what the Kilchoan Balls are made of.

"In the streams and pools of a hydrothermal area there are specialized bacteria and algae that grow in the very hot mineral rich waters, forming mat-like colonies. I tentatively suggest this is the origin of the ridges and grooves in the surface of the balls. They are a form of Stromatolite.

"If this is proved to be the explanation to the Kilchoan Geological Mystery, then as far as I am aware this relic of the hydrothermal activity is unique in the Hebridean Volcanic Province. This is a challenging statement that no doubt someone will take issue with! I would, however, be interested to compare the rocks at Kilchoan with another similar site if one does exist."

Many thanks to Rob Gill for pictures and report.