Metamorphic rocks result from the alteration of pre-existing rocks caused by heat and/or pressure produced by plate movements or intrusion of magma within the Earth’s crust. Metamorphism can result in recrystallization of existing minerals, the formation of new minerals, and changes in the texture or fabric of a rock.
There are several types of metamorphism which are related to different modes of stress, of which regional metamorphism and contact metamorphism are the most important. Contact metamorphism is localized around an igneous intrusion, whereas regional metamorphism occurs on a larger scale due to processes such as mountain building and subduction.
In broad terms, metamorphism may be described as low, intermediate or high grade depending on the temperature and pressure conditions (illustrated below).
The original composition of the parent rock – the ‘protolith’ – will also control the composition of the resulting metamorphic rock. Here, for example, we illustrate the effect of different grades of metamorphism on a sedimentary protolith. Note the changes in composition and texture.
Slate is the product of low grade metamorphism of shale or siltstone. During metamorphism, tiny flakes of chlorite and white mica grow in parallel orientation with their flat surface perpendicular to the direction of regional compression. This results in a set of very closely spaced planes, called slaty cleavage, along which the slate can be easily split. Notice that in the specimen of slate displayed below, the slaty cleavage cuts across the original sedimentary bedding. Slates for roofing were quarried from various parts of Ireland including Ashford, Co. Wicklow and Valentia, Co. Kerry. The old quarry at Killaloe has recently been reopened.
Schist is a medium to coarse grained metamorphic rock which contains a significant proportion of platy or micaceous minerals in a sub-parallel orientation. Schist is formed at low to medium grades of metamorphism. Shales and slates become mica-schists like this specimen.
Gneiss is common in regions which have undergone medium to high grade metamorphism. The rock is medium or coarse grained, and has a granular texture owing to the dominance of quartz and feldspar, and the lack of a preferred orientation of its other mineral constituents. Gneisses take on many appearances. Some varieties, like the example below, have lenticular quartzo-feldspathic segregations in a darker coloured matrix. Some other varieties are banded.
Most metamorphic rocks are formed due to regional metamorphism which occurs deep in the earth’s crust, on the scale of a mountain range.
The highlands of Scotland are a classic location for the study of regional metamorphism, and the first place where the concept of a metamorphic ‘zone’ was described by George Barrow. Scotland was subjected to metamorphism during the Caledonian Orogeny 500-400 million years ago. After the closure of the Iapetus Ocean some rocks were buried to great depths while others were uplifted to form the Caledonian Mountains. Subsequent erosion of the overlying rocks has exposed the highly metamorphosed rocks in the central parts of the Scottish Highlands.
As higher temperatures and pressures were reached in the crust beneath the tectonically active zone certain minerals formed, which are stable at certain temperatures and pressures and therefore characteristic of different metamorphic zones. These so called ‘index’ minerals are featured in the gallery below (all scale bars = 1 cm).
We can also classify metamorphic rocks by ‘facies’, which is based not just on individual index minerals, but on assemblages of minerals which are stable at different temperatures and pressures. The names given to the facies are based on the metamorphism of a basic igneous rock.
Contact metamorphism occurs when an hot igneous intrusion bakes the surrounding ‘country’ rock, forming a ‘metamorphic aureole’. The most common rock produced as a result is hornfels – an extremely hard, non-foliated rock. Hornfels is typically fine-grained, but right next to an igneous contact where the highest temperatures are reached, it may have a medium or even coarse grained texture. The igneous material is the coarse-grained rock at the bottom left of the specimen on the right.
In Ireland the effects of contact metamorphism can be seen in the rocks surrounding the Leinster granite, a large igneous intrusion which stretches all the way from Dublin to Wexford, and makes up the Wicklow Mountains. The contact between igneous and metamorphic rock can be seen, for example, at Whiterock, on the north end of Killiney Beach. The cobble in the picture below has small, radiating clusters of match-stick like crystals of andulsite, which formed as a result.
The formation of a metamorphic rock therefore depends largely on pressure, temperature conditions and the composition of the protolith. This table summarizes some of the common names of metamorphic rocks and selection of metamorphic rocks from the Trinity collection is featured in the gallery below.