What is slate?

• A. When shale is exposed to heat and pressure without melting, this type of rock is formed.
• B. When sandstone is cemented.
• C. When limestone is dissolved by acid.
• D. When granite forms at high temperature.

Answer: (A)

Slate is a fine-grained metamorphic rock formed when shale is subjected to heat and directed pressure while remaining solid, so it doesn’t melt. This low-grade metamorphism reorients the minerals in shale into a flat, platy habit, creating slaty cleavage that lets the rock split easily into thin sheets. That combination of solid-state metamorphism and foliation is what distinguishes slate from other rock types. It isn’t produced by cementing sandstone (lithification of sediment), nor by chemical dissolution of limestone, nor by high-temperature melting to form granite.