What Are the Characteristics of Metamorphic Rocks?

Anne Worner/CC-BY-SA 2.0

The two characteristics used to classify metamorphic rocks are foliation and lineation. These rocks are identified by the presence of certain mineral types and specific textures.

Metamorphic rocks are those formed by other types of rocks that have been exposed to heat, pressure and time, which change them into a different type of rock. Metamorphic rocks can be formed from sedimentary, igneous and even other metamorphic rocks. The composition of the rocks as well as the temperature and amount of pressure placed on them all play a role in the type of metamorphic rock formed. For this reason, metamorphic rocks can take on all types of colors and textures. However, foliation and lineation are two characteristics commonly seen in metamorphic rocks and are used to help identify and classify the rocks. By studying metamorphic rocks, scientists can gain insight into the conditions inside the Earth during the metamorphic process.

Foliation Foliation is an arrangement of flaky layers along the rock that break off easily. Also known as salty cleavage, this characteristic is often seen in low-grade metamorphic rocks. Foliation occurs when uneven pressure acts on the parent rock and is accompanied by a change in temperature. It is the result of pressure only acting in one direction. However, it is important to note that not all metamorphic rocks exhibit foliation nor does all foliation present in the same manner. Some examples of foliated metamorphic rocks are slate, mica, schist and gneiss.

Foliated rocks are named after the type of foliation they exhibit. Since non-foliated rocks lack this feature, they are named for their mineral compositions instead. Examples of non-foliated metamorphic rocks are marble, quartzite and hornfels.

Lineation Lineation is another characteristic commonly seen in metamorphic rocks. Intersection lineation is the most common type of lineation seen in metamorphic rocks. This is formed by the intersection of any two foliations, such as bedding and cleavage or cleavage to a second cleavage. Lineation can run parallel or perpendicular to foliations. This feature forms mostly due to a drastic change in pressure and is less dependent on temperature changes.

Types of Metamorphism Metamorphism can occur in several ways. However, scientists have identified three main processes that lead to metamorphism: thermal, dynamic and metasomatic. Thermal metamorphism involves the structural and chemical alteration of rocks through the exposure of intense heat. A subcategory of this type is regional metamorphism which covers rock over a large area. A second subcategory is contact metamorphism, which refers to the small-scale heating of a localized portion of rock.

Dynamic metamorphism, sometimes referred to as burial metamorphism, does not alter the chemical composition of the rock. Instead, the extreme pressure imposed upon the rock causes it to change the physical structure. As rocks get buried, the weight of the material on top increases the pressure exerted on the rock, leading to a physical change.

Metasomatic metamorphism occurs when some of the elements in the rock minerals are replaced with others. This occurs when liquids and gases permeate the bedrock during the metamorphosis process. The resulting rock has undergone both physical and chemical changes.