The protolith of slate is shale. Slate is a metamorphic rock that forms when shale is subjected to low-grade metamorphism. This process involves changes in mineralogy, texture, and chemical composition due to heat and pressure over time.
When shale, which is composed mainly of clay minerals and silt, undergoes metamorphism, the fine-grained materials realign and recrystallize, leading to the development of a dense, finely layered rock known as slate. This transformation enhances the rock’s durability and alters its physical properties, allowing slate to be used for various applications, including roofing and flooring.
In summary, slate originates from shale through metamorphic processes, making shale its protolith.