Metamorphic rock Metamorphic rock Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat (temperatures greater than 150 to 200 °C) and pressure ( or more), causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock. Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface. They are classified by texture and by chemical and mineral assemblage (metamorphic facies). They may be formed simply by being deep beneath
Aberllefenni Slate Quarry slates. Foel Grochan mine consists of eight near-horizontal tunnels at approximately vertical separation. These were bored into the valley side just to the north of the near-vertical narrow vein. Each tunnel connected to a large chamber from which the rock was extracted. These chambers ranged from to in length with of rock left between the bottom of one chamber and the top of the next lower chamber. As more slate was extracted, several of the upper chambers were joined vertically to form an extremely large cavern known as Twll Golau which is open at the top. From the mid-1850s the