Getting to know metamorphic rocks

Metamorphism is the recrystallisation of sedimentary and igneous rocks (and pre-existing metamorphic rocks) under the influence of heat and pressure. Usually metamorphism occurs when the rocks are deeply buried, and the degree of metamorphism depends on what temperatures and pressures were reached. In the southwest highlands the degree of metamorphism is only moderate, and there are no samples of high grade metamorphism in this collection. Also there are no examples of metamorphosed igneous rocks (granite, gabbro, and dolerite) in this collection, as they are not common in this part of Scotland. The rocks shown here are the moderately metamorphosed sedimentary rocks that make up most of the hilly terrain in Cowal.

Metamorphism can produce alternating mica layers and quartz layers in the form of laminations, and this type of rock is called a schist.

Plate 8

PLATE 8 METAMORPHIC ROCKS
A. Mica schist, with alternating quartz-rich and white mica-rich layers B. Slate C. Grey quartzite, with a little white mica
D. Garnet schist E and F. Metamorphosed grit

Sample 8A (Plate 8, sample A) is schist derived from sedimentary shale (like Plate 7, sample B) and it contains layers rich in white mica. The sample was found on a building plot near Creggans Inn. If mica is sufficiently abundant, schist splits easily into layers. Sometimes this can be a disadvantage, as it can make the rock weak and unsuitable for building stone. When the mica content is not sufficient to deprive the stone of its robustness, schist is widely used as a building stone. Many of the old buildings in Strachur have walls built of this type of schist, and Inveraray Castle is built from a similar rock, sourced from a quarry in St Catherines.

The ability to split along micaceous layers is called schistosity. In some metamorphosed sedimentary rocks, schistosity is an advantage, as the rock splits into very thin, regular sheets to form slate (Sample 8B). Natural roofing slate, possibly sourced from Easdale, is common throughout Strachur. Note the cubic crystals of pyrite in the slate. In the graveyard of Strachur Parish Church is an extraordinary gravestone made of Easdale slate. The stone commemorates Martin Rhind, a freemason who died in 1825, and the stone is adorned with symbols of freemasonry. Cubes of pyrite have oxidised, giving a rusty stain over part of the stone. (Note: gravestones in the church yard may be unsafe, and should not be approached without proper authority).

Seil Island

Elongate slabs of slate used in jetty construction at Ellenabeich on Seil Island.
A good example of how local stone can add distinctive character.

 

Sample 8C is quartzite. It is derived from sandstone composed of pure white quartz sand. Metamorphism does not much alter the appearance because there are no elements in the rock other than silica, and no new metamorphic minerals can form. Sandstone does become much harder when it turns into quartzite, and it no longer has a ‘sandy' feel. This sample is from an outcrop on the loch shore, southwest of the village.

Quartzite

Thin bed of white quartzite within pinkish mica schist, shore of Loch Fyne just south of Creggans Inn, Strachur

Sample 8D is mica schist which contains garnets. It is therefore a garnetiferous mica-schist. Common garnet is used in the manufacture of garnet paper, because garnet sand is harder and more abrasive than quartz sand. See also sample D, Plate 4 - note how the quartz layers have no garnets, because these layers consist only of silica which cannot be metamorphosed into any other mineral. The central layer was originally clay-rich, containing iron, magnesium, sodium and other elements - ideal ingredients for the formation of garnets during metamorphism.

Samples 8E and 8F are metamorphosed coarse-grained sedimentary rocks. Rocks made of this size of particle are often called grits, and these samples are therefore metamorphosed grits. If the concrete shown as sample 7D were to be metamorphosed, it would look like these.


Schist ‘tabular stone' set into wall. Metamorphic rock.
Gravestone
Sandstone monument. Sedimentary rock
Gravestone
Monument carved from slate from Easdale.
Metamorphic rock.

Gravestone
Red granite monument. Igneous rock.
Four types of stone monument in graveyard at Strachur Parish Church
(Note: monuments may be unstable and should not be approached without proper authority)

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