Granite

by Dr Tim Palmer

Granite, with its beautiful range of colours and patterns, is becoming much more familiar both for exterior work and for polished interiors. A huge range of different types is now available, with colours ranging from black and dark olive green, through speckled pinks and reds with a silver sparkle, to almost white. To a geologist, not all of these types of rock are strictly granites, but they do all have a similar geological origin, which is quite different from the sandstones and limestones which started out as soft sediments on some ancient sea or river bed.

Tens to hundreds of kilometres below the surface of the Earth, the rocks are very hot and under huge pressure from the weight of the overlying crust. Under these conditions they slowly circulate by convection like molten toffee cooking in a pan. Cracks in the rigid, cooler surface of the Earth’s crust are opened up by the force of the heaving mass beneath. In this way, the continents are slowly moved around on the surface (at a rate of a few centimetres a year) and the gaps between the plates of the moving crust are filled with upwelling material from deep in the Earth. On the opposite edges of the plates, sands and clays that have been eroded off the land and carried into the sea by rivers get pushed back deep into the Earth under the continents and via the deep trenches along the edges of the oceans.

The deep material (called the mantle) is made up of minerals that only form in the conditions of extreme high temperature and pressure found at great depths in the Earth. As convection causes it to move towards the surface, the pressure is released and this causes a partial melting, even though the temperature does not get any higher. As the so-called magma moves even closer toward the surface and starts to cool, crystals of different minerals form.

In different parts of the deep Earth, the chemical composition of this molten magma is rather different. Where sediment gets recycled back into the earth it forms magma that is less dense than the surrounding rocks, so it starts to push its way slowly back up to the surface.

Magma cools as it slowly gets closer to the surface, forming a solid interlocking 3-D mass of crystals, usually of not more than three or four main mineral types. So rocks of this type are called crystalline.

There are two main things that control what the resulting rock-type looks like. First, the chemical composition of the magma. Crystalline rocks that form under the oceans contain lots of iron and magnesium and many of the minerals that are found in them(such as pyroxene) are heavy and coloured. These black granites (strictly they should be called gabbros) are particularly familiar in graveyards.

In contrast, the rocks that ascend from the bowels of the Earth underneath continents contain much ore silica and are characterised by light coloured minerals such as quartz, potassium-containing feldspars and the sparkling micas. The feldspars are particularly variable in colour and the reds, salmons, and whites of the granites that are familiar as kerbstones, work-tops, and cladding get their colours from crystals of feldspar that differ only in the trace amounts of iron and some other minor elements.

These are the true granites, geologically speaking. The mica they contain may be silver or dark brown in colour. Both types have a characteristic flaky structure and their sparkle comes from the light reflected off the surface of each flake.

These two types of crystalline rock are the extremes. There are many that are of intermediate composition because different types of magma got mixed or sorted at depth. Some rocks get squeezed and folded during their journey upwards and concentrations of different minerals can get smeared out and stirred into one another to give streaked and swirling patterns, like chocolate sauce stirred into ice cream.

The other thing that affects the appearance of crystalline rocks is how fast they approached the surface and how quickly they cooled down. If cooling was rapid (the extreme case of this is magma that spews onto the surface of the earth as volcanic lava) the crystals did not have much time to grow before the rock became solid. In this case it is only possible to see crystalline structure in a slice of the rock seen through a microscope.

The most decorative granites are the ones where the magma cooled slowly and large crystals had time to grow. The true granites of Cornwall and Shap are of this type.

The first crystals of feldspar started to grow when the temperature of the magma was still about 8000C. As some of the chemical elements in the melted mass got locked up in the feldspars, the composition of the surrounding liquid gradually altered, and with further cooling other minerals, such as glassy quartz and mica, grew in the remaining space. If you look carefully at a polished granite counter-top, the three intergrown types of mineral crystals can easily be spotted.

Sometimes, rising magma tore off flakes of the overlying rocks as it pushed up through them, mixing and melting the fragments to form small local patches of other minerals. These fragments are discoloured nuisances to the producer, but fascinating to geologists. Occasionally they produce granites with a completely new texture, such as the famous Australian orbicular granite with golf-ball like dark masses set in a light matrix.

These coarse-crystalled rocks were still deep in the earth, sometimes several kilometres down, when they finally became completely solid. They only became exposed at the surface by the gradual erosion of overlying rock layers over millions of years.

Some of the minerals that they contain are unstable at the low temperatures and pressures of the surface, or where they meet air and moisture. Some feldspars and dark minerals like pyroxene slowly rot into clay, staining brown as iron is released. This can be a source of discoloration on granite used for exterior work.

Copyright © Dr Tim Palmer


Dr Tim Palmer (tjp@aber.ac.uk) is a Chartered Geologist and consultant on the petrography of building stones and limes with a particular interest in historic buildings and landscapes. He researches and teaches geology part-time in the Earth Sciences Institute at the University of Wales, Aberystwyth, SY23 3DB. Tel: 01970 627107.