Chemical, Channel Islands

The photograph shows two sorts of igneous rock, a pale granite and a dark rock similar to gabbro. Whilst this granite is resistant to chemical attack, it is clear that groundwater has seeped into the joints (cracks) in the gabbro and has chemically attacked the rock on either side, changing it to a dark brown colour, leaving patches of intact grey gabbro in between.

The chemical process attacking the gabbro is complex, but if you want to know how it works, here is a simplified version! Acidic groundwater attacks the minerals of the gabbro in two ways: firstly, by a process called hydrolysis, in which original minerals are gradually altered to produce clay and soluble salts; secondly, by oxidation of iron in the dark coloured minerals to produce the brown iron oxide colouration seen in the photo.