Shale doesn't directly change into slate. It's the other way around! Slate forms from metamorphism of shale. Here's how it works:

* Shale: Shale is a sedimentary rock, meaning it's formed from the accumulation and cementation of sediments, primarily clay minerals. Think of it as hardened mud.

* Metamorphism: This is a process where existing rocks are transformed by heat and pressure. The changes happen deep underground, often due to tectonic activity.

* Slate Formation: When shale is subjected to moderate heat and pressure, the clay minerals within it recrystallize and align themselves. This process, known as foliation, creates the characteristic flat layers or planes seen in slate.

In summary:

1. Starting point: Shale (sedimentary rock)

2. Transforming process: Metamorphism (heat and pressure)

3. Result: Slate (metamorphic rock)

Slate is essentially a "cooked" version of shale, with its structure and properties altered by geological forces.