Granodiorite

Granodiorite close up photos

Granodiorite is a coarse grained igneous rock that is very similar to granite but with a slightly different mineral composition. Like granite it was formed deep within the earth when molten magma was intruded at a tectonic plate margin. This happens when the plates which make up the earth collide and melt. The magma produced forms a huge mass called a Batholith and the folded rocks above become a mountain range. Very slowly the molten rock cools and crystallises. The longer a magma takes to cool the larger the resultant crystals and so a coarse grained rock such as Rannoch Moor's granodiorite is formed. The minerals within the rock include quartz, plagioclase, orthoclase, hornblende and biotite.

When the granodiorite was being intruded it, on occasion, engulfed pre-existing rocks and these may remain as visibly different rock within the granodiorite. These engulfed rocks are called Xenoliths, which is derived from Greek words meaning 'stranger rock'. These can be seen in the rocks near Loch Eigheach west of Bridge of Gaur.

Granodiorite is relatively resistant to erosion and so Rannoch Moor, which is underlain by granodiorite is high ground that formed a platform for a large ice sheet during the ice ages. The ice flowed out radially from the heights of Rannoch Moor and one glacier flowed eastwards to gouge out Strath Tummel and it was responsible for many glacial feature that we can see today.

The dark rock in the centre of photo is a Xenolith.