Texture Groups of Metamorphic Rocks

Metamorphic rocks are all created by the forces of pressure and heat. Generally they are working together altering the chemical composition of rocks. Despite the same forces at work not every rock ends up with the same texture. Instead metamorphic rocks have two texture groups one being foliated and the other non-foliated. The reason for this has to deal with the amount of heat and pressure each rock was exposed to as well as the composition of the rock.

Foliated rocks have an obvious layered appearance to them. Foliated rocks are exposed to significant heat and pressure. Foliated rocks are formed through regional metamorphism most commonly associated with mountain building. Pressure, heat, and composition determine what type of foliation occurs. The foliation that rocks exhibit comes in three distinct patterns. They are slate, schist, and gneiss.

In slate the patterns line up perpendicular to each other indicating where exactly on the rock the stress took place. Grains in slate tend to have a very fine texture making them difficult to see with the naked eye. Slate is considered to be a low grade metamorphic rock indicating that it was exposed to less pressure and heat those rocks that are a higher grade. Slate is widely used for building and decorative purposes.

Rocks that exhibit schist  foliation have less to do with stress and more to do with the composition of the rock. The material is these rocks are quartz, mica, and feldspar blended together well. As a result these rocks will not exhibit the patterns associated with slates. Instead they have a pattern where minerals alternate through the rock. Particles in the rock are larger and far more course. Considered to be a medium grade rock is has particles that can be seen with the naked eye. This rock type is weaker and it relatively easy to break.

Gneiss patterns in the rock are clearly defined bands with alternating color. These bands are the result of heavier and lighter material gathering together. Rocks with signs of gneiss foliation are high grade and have undergone the most intense forces of pressure and heat. The particles in the rock are large and course. Gneiss is a stronger rock and is great for construction and is frequently used in both roads and buildings. Foliated rocks are usually associated with mountain building activity and can be found over large geographic areas. These rocks are very common with worldwide distribution.

Non-foliated rocks do not show signs of layering. While foliated rocks are the result of more than one mineral coming together non-foliated have a much more pure mineral composition. Marble, quartz, and basalts, are examples of rocks that lack foliations. These rocks are created when parent rocks are exposed to extreme heat from magma in the earth’s surface. These rocks maintain most of the chemical composition of their parent rock. The process of reformation and the purity of the rock is what prevent foliation from occurring. As a result looking at them they often have a smooth slick appearance. These rocks also have great strength and durability.