Metamorphic slates tend to be formed by the action of heat as well as force acting over hundreds of millions of years on consolidated clays (muds) which were deposited under liquid. Rarely they can be created by the same process acting on volcanic ash similarly laid straight down and compacted. The process of metamorphism involves heat and pressure and is quite moderate or low grade but provides place over a millions to years. heat and force combined result in the clay nutrients to recrystalise and to align themselves with their long axis perpendicular to your direction to force. It is this mineral alignment which imparts the belongings of ‘slatey cleavage’ and makes it possible for the rock towards stay divide into thin, strong sheets.
The predominant and essential clay minerals are phyllosilicates including muscovite and chlorite, that are stable. Slate may, nevertheless, contain other minerals which are unstable and which can seriously lessen the durability of roofing slates.