Tuesday, 12 April 2016

Mystery Rock

This picture was sent by Kilchoan Early Bird, to whom many thanks for setting us a bit of a puzzle. The feature is located to the north of Glendrian, above Fascadale, and the brown-weathered, apparently inner piece is part of the main rock. The outer rock looks like a typical outcrop of the gabbroic ring dyke, but why the inner bit has weathered so differently is a bit of a mystery. Has anyone any ideas?


  1. The brown coloured rock is probably a xenolith, an inclusion in igneous rock caused when the magma carries along bits of country rock and solidifies with them held within.

  2. The xenolith is probably peridotite from lower in the magma chamber, there are some very large (100m) 'rafts' of this in the eucrite to the east of Glendrian. The brown colouration is a result of the iron in the peridotite weathering, effectively 'rusting'.

  3. A xenolith is a 'foreign rock' so I reckon Rick got it right in a word! Many thanks, Rob and Derryck, for sorting that - I think this may be an outcrop which you, Rob, wanted to show me some years ago. Typical of Kilchoan Early Bird that he should notice it. Jon