Foliated rocks are a type of metamorphic rock formed under intense pressure that is greater in one direction. This pressure squeezes the flat or long minerals within the rock, causing the minerals to fold or align perpendicularly to the direction of the greatest pressure.
The alignment of the longer minerals relieves some of the pressure on the rock and also gives foliated rocks a distinctive banded or layered appearance. On the other hand, non-foliated metamorphic rocks have a uniform appearance; this is because they are formed when the force of pressure is equal on all sides.
Foliated rocks are separated into four basic categories based on their appearance: slate, phyllite, schist and gneiss. Slate is formed under very low pressure and heat, while gneiss is formed under the most intense pressure and highest temperatures. Due to the low pressure, slate breaks into thin, even sheets, which are often used in roofing and other construction projects.
Both foliated and non-foliated rocks are formed deep within the Earth. Over long periods of time, heat and pressure change the chemical composition of other rocks and minerals to transform them into metamorphic rocks. These base rocks can be igneous, sedimentary or other metamorphic rocks.