Metamorphic Rocks

When rocks are subjected to different conditions from those under which they originally formed, their minerals change. This alteration can happen when rocks are exposed at the Earth's surface and their minerals react with various chemicals in the atmosphere.

Much more marked effects occur when rocks are buried deep in an emerging moun­tain range and subjected to very high tempera­tures and pressures. Under these conditions the rocks alter completely, becoming entirely differ­ent types of rocks with different mineral compo­sitions. Such transformed rocks are called metamorphic rocks.

The characteristic feature of a metamorphic rock is that its mineral composition changes without the rock itself melting. If the rock does melt and then solidify again, the result is an igneous not a metamorphic rock.

Regional Metamorphic Rocks

Regional (or dynamic) metamorphic rocks - one of the two main types - are those that have been altered by great pressure but little heat - as occurs, for example, in the heart of a fold mountain belt while it is being compressed between moving crustal plates. The effects of such a movement are usually extensive; hence regional metamorphic rocks tend to occupy large areas.

At depths in the order of tens of kilometers the weight of the overlying rocks produces suffi­ciently high pressures to alter the mineral struc­ture of the rocks beneath. For example, the minerals in shale (the black, flaky sedimentary rock that is produced by the lithification of mud) re-crystallize into the mineral mica as a result of great pressure. The flat, leaf-like mica crystals form in parallel bands (known as the rock's foliation).

Earth movements associated with metamorphic processes may then deform the mica, forcing it to distort along the lines of foliation and producing, in turn, a schist, a typical regional metamorphic rock. The mineral bands in schist are very pronounced and are often distorted and jagged in appearance - evidence of the great stresses involved in their formation.

A schist can usually be easily split along its foliation lines; this tend­ency to split along certain planes of weakness is called cleavage.

The cleavage of a regional metamorphic rock is exploited commercially in the quarrying and working of slate. Like schist, slate is formed by the metamorphism of shale, but under less extreme pressures. Compared with shale, the minerals in slate are small and are often invisible to the naked eye.

It is sometimes assumed - erroneously - that the cleavage of slate corresponds to the lines of the thin bedding in the shale from which it was originally formed. In fact the cleavage reflects the direction of the pressure to which the shale was subjected during its metamorphism rather than the original structure of the rock.

Thermal Metamorphism

In the other main type of metamorphism - ther­mal (or contact) metamorphism - rocks are changed by the effects of great heat but little pres­sure.

Thermal metamorphic rocks are formed when a hot igneous mass of magma forces its way through the Earth's crust, literally baking the rock surrounding it.

In comparison with regional metamorphism, the volume of rock affected by thermal metamorphic processes is very small; the newly-formed thermal metamorphic rock may extend for only a few centimeters around the igneous intrusion (the affected area is called an aureole) or, occasionally, the new rock may be up to about a kilometer wide around a very large batholith.

There is usually a gradation of thermal metamorphic rocks around large intrusions; near such an intrusion there are high-temperature rocks, which gradually give way with increasing distance from the intrusion to low-temperature then unmetamorphosed rocks.

Probably the most familiar thermal metamorphic rock is marble, which is produced by the metamorphism of limestone, a sedimentary rock consisting almost entirely of calcite (calcium car­bonate).

When the calcite is subjected to great heat from a nearby igneous intrusion, it first gives off carbon dioxide then recombines with this gas, thereby re-forming new calcite crystals and trans­forming the limestone to marble.

The newly-formed crystals have a regular form and grain size (as opposed to the random collection of fragments in the original limestone) which gives the marble strength and an even texture.

Usually, however, the elements in the minerals of the original rock recombine during metamor­phism to form completely different minerals, as occurs in the formation of hornfels, which often contains cordierite (a silicate mineral found only in thermal metamorphic rocks).

More Metamorphic Rocks

Dislocation metamorphism is a relatively rare type that occurs when the rocks on each side of a major fault move against each other as the fault slips. In this situation the stresses can be so great that the minerals in the rocks at the fault break down and re-crystallize, thereby giving rise to a hard, flinty metamorphic rock called mylonite.

Metasomatism is similar to - and often asso­ciated with - thermal metamorphism. As an igne­ous mass cools, it gives off hot liquids and gases, which may percolate through cracks and cavities in the surrounding rock. The hot fluids may then alter the surrounding rock by a combination of heat and deposition of minerals dissolved in the fluids.