Metamorphic rocks are probably the most interesting rocks, because they form under extreme conditions which give rise to highly unusual composition, characteristics and properties. They are also amongst the most frequently used natural materials including, slate, marble, china clay and talc. Broadly speaking, metamorphic rocks form from other types of rock, such as igneous and sedimentary rocks, when these are altered in some way, usually by the actions of extreme heat or pressure, or both.

Significantly the alteration rarely changes the composition of the rock, thereby maintaining the same chemical content as the parent material. Instead, the combined effects of pressure and heat result in the crystallization of new mineral phases. The fact that the chemical composition remains unchanged allows geologists to determine the origin of metamorphic rocks, for example, whether sedimantary or igneous, and furthermore to infer much about the origin of the material, for example, mountain ranges, volcanos or marine deposits.

Heat is generated as rocks are subducted into the crust, or as hot magma rises from the earth`s interior. Minerals in the parent rock which are normally stable at low temperature may now have their atoms rearranged, and as the rock cools, this new phase may `freeze` out as the new mineral content of the rock. By studying these mineral phases, geologists can determine the temperature at which the rock was altered and also the immediate environment in which these new rocks were created.

The pressure comes from the vast amount of material which buries the parent rock as it subducts into the crust. This pressure is called the Confining Pressure and is applied from all sides. The result of squeezing is to create denser minerals as the atoms within are forced closer together. Significantly, the heat and pressure rarely happen in equal amounts; sometimes there is more heat than pressure and vice versa. These differences result in a range of environmental conditions, with extremes where either pressure or heat dominate.

Where temperature is the dominant factor the type of metamorphism is known as Contact Metamorphism. As the name implies it happens when one rock type comes into contact, or near, a source of high temperature, usually magma. This heats the parent rock to a very high temperature and creates a ‘baked’ zone of rock around the magma that is altered by the high temperatures to form new metamorphic rocks. This ‘baked’ zone is called the Metamorphic Aureole and is usually relatively thin, typically between 1 and 50 meters wide. Pressure has little effect on contact metamorphism because it usually takes place close to the earth’s surface and also because the hot rocks become soft and can ‘flow’, easing the pressure away.

Where pressure dominates, this type of metamorphism is called Regional Metamorphism. It is important to note that heat may not be entirely absent, but it is the pressure which determines how the rocks are altered in this case. This type of metamorphism usually takes place deep underground, hence the high pressure, and is usually associated with mountain-building events. Whilst the pressure is applied from all sides, the fact that the crust is undergoing dynamic changes means that varying forces are applied to the rocks. Because these are rarely equal they apply stresses to the rocks. These stresses, called Differential Stress, deform the rocks and the minerals into stretched and flattened forms. Sometimes the stresses are applied from different directions. This creates shearing, causing different parts of the rock to move against one another in a plane.

The shearing stresses are important, influencing the texture of metamorphic rocks. They force the individual minerals within the rock to become parallel to each other. When minerals align themselves in this way it is called Foliation and such rocks are said to have a foliated texture. Geologists have identified three different types of foliated rocks and these are commonly used to classify regional metamorphic rocks into three generic groups:

• Metamorphic rocks that split easily along a flat/parallel plane; composed of microscopic platy minerals. The generic rock name for rocks with this texture is Slate.
• Rocks that have visible platy minerals (like mica) growing parallel to a plane created by differential stress. This is said to have a schistose texture and the rock with this type of texture is called Schist.
• When a metamorphic rock becomes very plastic due to heat and pressure, the minerals are often divided into distinct light and dark layers which are said to have a gneissic texture. The rock with this type of texture is called Gneiss.

Importantly, the minerals in contact metamorphic rocks do not line up in any recognisable way because differential stresses are absent. Consequently these rocks usually show no regular structure or alignment and are said to have a non-foliated texture. Examples of these metamorphic rocks are hornfels, quartzite and marble.

The classification of metamorphic rocks is therefore based on their texture and structural features. One of the first things geologists look for is whether the rock is foliated or not. This immediately determines whether it is a contact or regional metamorphic rock. If the rock is foliated they will look at the alignment of the texture and after that at the mineral content. This is usually enough to name the majority of metamorphic rocks.