Metamorphic Rock Facts: Types of Metamorphic Rocks

Metamorphic rocks are created when other types of rocks are transformed due to great pressure and heat within the earth. The meaning of the term ‘metamorphic’, in fact, is ‘changed’. When igneous rocks, or sedimentary rocks, or even metamorphic rocks get buried very deep under the earth’s surface, a process that takes millions of years, they get changed into something else by the enormous pressure and heat inside the earth.

Some examples of metamorphic rocks are:

• Limestone being changed into marble
• Shale turning into slate
• Granite being changed into gneiss
• Sandstone turning into quartzite

Since metamorphic rocks are formed from previously existing rocks, they can vary in appearance from each other. The texture and the kinds of minerals they consist of are how metamorphic rocks are classified. Most classification of metamorphic rock involves first separating the rocks into two categories by their texture: the ones that are layered, are known as foliated metamorphic rocks, and those that are not layered, are known as non-foliated metamorphic rocks.

Foliated Metamorphic Rocks: The foliation or layering that occurs in many types of metamorphic rocks is because of the immense directional pressure they undergo deep within the earth, usually along the boundary of a converging plate tectonic. In this environment, over an enormous time period, flat crystals of minerals as well as fragments, like stream pebbles and volcanic pyroclasts, progressively become perpendicular to the direction of the pressure. Apart from the effects of the pressure, the increase of the temperature can also result in minerals such as chlorite and mica re-crystallizing into bigger crystals that are more visible. Some examples of foliated rocks are: schist, gneiss and slate. And some examples of non-foliated metamorphic rocks are: marble and quartzite.

Types of Metamorphic Rocks

Marble: This is formed due to the metamorphosis of dolomite or limestone. Both dolomite and limestone contain calcium carbonate in large concentrations. Compared to its parent rock, marble is much harder. This enables it to be polished, which is why it is such a popular material used in buildings, such as bathtubs, floor tiling, sink tops, kitchen counter tops, and so on. Artists also use it as a carving material. Marble is made up of various sized crystals and also has many variances in color because of the impurities that are present during the formation. Hence, marble can be white, gray, black, red, green, pink, banded and mottled.

Slate: This metamorphic rock is fine-grained, containing perfect cleavage, which enables it to be split into fine sheets. Slate generally contains dark to light brown streaks. Slate is formed by comparatively low pressures and temperatures, which is referred to as low-grade metamorphism. Not being very hard, slate can be engraved quite easily. It has been used in various ways over the years, for example as grave markers or headstones. However, one of the problems with slate is the perfect cleavage it has, which resulted in grave stones splitting and cracking along the cleavage lines. It is also commonly used for chalkboards. These days, due to its cracking and splitting, and its weight, slate is not used much.

Schist: This is a metamorphic rock that is classified as medium grade, which means it has been formed by more pressure and heat compared to slate. This rock is coarse grained with the individual grains of the minerals that it is made up of being visible to the naked eye. Most of the original minerals are transformed into flakes, and since it has experienced far more pressure, it is usually found crumpled or folded. Usually, schists are named according to the main mineral they have been formed from. For example: talc schist, garnet mica schist, hornblende schist, and bitotite mica schist.

Gneiss: This metamorphic rock is classified as high grade, which means that compared to schist it has been subjected to more pressure and heat. Gneiss is distinctly banded and is coarser than schist. The banding comprises of alternating layers, which are made up of different minerals. One of the most important minerals that gneiss is made up of is feldspar, along with quartz and mica. Gneiss can form from the metamorphosis of sedimentary rock like shale or sandstone, or from igneous rock like granite. Gneiss is used as a building stone and for paving.

Quartzite: When sandstone is metamorphosed it turns into quartzite, which is far more harder than its parent rock. It forms when sandstone comes into contact with magma that is deeply buried. Quartzite appears quite similar to sandstone, and the best way to tell the difference is to break both the rocks. While sandstone shatters into many grains, quartzite breaks across the grains.